Relationship between photosynthesis and non-photochemical quenching of chlorophyll fluorescence in two red algae with different carotenoid compositions

Algae are continuously exposed to short-term fluctuations in irradiance. We investigate how two red algae species regulate photosynthetic efficiency to cope with such changes and identify some strategies that differ from higher plants. Two red algae, Gracilaria domingensis and Kappaphycus alvarezii, with antheraxanthin and lutein as major xanthophylls, respectively, reacted to the onset of low light (below E _k) with a substantial decrease of NPQ. This is different from higher plants, but similar to previous observations in, e.g. cyanobacteria where it indicates an increase in the effective absorbance cross-section of Photosystem II (PSII) by state transition. Kinetic studies in continuous light revealed a high susceptibility of PSII to light stress ((1-q _P)/NPQ) in K. alvarezii immediately after the sudden onset of high light, followed by a decrease. This was caused by a slower onset of NPQ in K. alvarezii, followed by acclimation. In G. domingensis, susceptibility of PSII to light stress was stable with time, but absolute values of (1-q _P)/NPQ were higher than in K. alvarezii. These observations suggest that K. alvarezii may be better adapted to high light levels, but is less well prepared for large sudden changes in irradiation. In K. alvarezii, photosynthesis continued to increase with increasing irradiation when NPQ was saturated. As (1-q _P) and NPQ were still balanced in this situation, most likely, processes other than photosynthetic oxygen release are responsible for the increasing net O₂ production observed.     

Photosynthetic utilisation of carbon and light by two tropical seagrass species as measured in situ

In situ measurements of seagrass photosynthesis in relation to inorganic carbon (Ci) availability, increased pH and an inhibitor of extracellular carbonic anhydrase were made using an underwater pulse amplitude modulated (PAM) fluorometer. By combining the instrument with a specially designed Perspex chamber, we were able to alter the water surrounding a leaf without removing it from the growing plant. Responses to Ci within the chamber showed that subtidal plants of the seagrasses Cymodocea serrulata and Halophila ovalis had photosynthetic rates that were limited by the ambient Ci concentration depending on the irradiance that was available during short-term photosynthesis–irradiance trials. Relative electron transport rates (RETRs) at light saturation (up to 500 μ mol photons m⁻² s⁻¹) increased by 66–100% when the Ci concentration was increased from ca. 2.2 to 6.2 mM. On the other hand, intertidal plants of the same species exhibited a much lesser limitation of photosynthesis by Ci at any irradiance (up to 1500 μ mol photons m⁻² s⁻¹). Both species were able to use HCO⁻ ₃ efficiently, and there was stronger evidence for direct uptake of HCO⁻ ₃ rather than extracellular dehydration of HCO⁻ ₃ to CO₂ prior to Ci uptake. Subtidally, H. ovalis and C. serrulata grew to 10 and 12 m, respectively, where ambient irradiances were approximately 16 and 11% of those at the surface. Maximum RETRs (at light saturation) were lower for these deep-growing plants than for the intertidally growing ones. For both species, the onset of light saturation of photosynthesis (E _k) occurred at approximately 100 μ mol photons m⁻² s⁻¹ for the deep water populations, which was four and two times lower than for the shallow populations of C. serrulata and H. ovalis, respectively. This, and the differences in maximal photosynthetic rates (RETR _max), reflects an acclimation of the deep-growing populations to the lower light environment. The results presented here show that photosynthesis, as measured in situ, was limited by the availability of Ci for the deeper growing plants in Zanzibar, while the intertidally growing plants photosynthesised at close to Ci saturation. The latter result is contrary to previous conclusions regarding Ci limitations for these intertidal plants, and, in general, our findings highlight the need for performing similar experiments in situ rather than under laboratory conditions. Received: 4 April 2000 / Accepted: 31 August 2000     

Modification of light utilization for skeletal growth by water flow in the scleractinian coral <Emphasis Type="Italic">Galaxea fascicularis</Emphasis>

In this study, we tested the hypothesis that the importance of water flow for skeletal growth (rate) becomes higher with increasing irradiance levels (i.e. a synergistic effect) and that such effect is mediated by a water flow modulated effect on net photosynthesis. Four series of nine nubbins of G. fascicularis were grown at either high (600 μE m⁻² s⁻¹) or intermediate (300 μE m⁻² s⁻¹) irradiance in combination with either high (15–25 cm s⁻¹) or low (5–10 cm s⁻¹) flow. Growth was measured as buoyant weight and surface area. Photosynthetic rates were measured at each coral’s specific experimental irradiance and flow speed. Additionally, the instantaneous effect of water flow on net photosynthetic rate was determined in short-term incubations in a respirometric flowcell. A significant interaction was found between irradiance and water flow for the increase in buoyant weight, the increase in surface area, and specific skeletal growth rate, indicating that flow velocity becomes more important for coral growth with increasing irradiance levels. Enhancement of coral growth with increasing water flow can be explained by increased net photosynthetic rates. Additionally, the need for costly photo-protective mechanisms at low flow regimes could explain the differences in growth with flow.     

Intra-colonial variability in light acclimation of zooxanthellae in coral tissues of Pocillopora damicornis

We investigated heterogeneity of light acclimation of photosynthesis in sun- and shade-adapted coenosarc and polyp tissues of Pocillopora damicornis. The zooxanthellar community within P. damicornis colonies at Heron Island is genetically uniform, yet they showed a large degree of plasticity in their photo-physiological acclimation linked to light microclimates characterised by fibre-optic microprobes. Microscale scalar irradiance measurements showed higher absorption in polyp than coenosarc tissues and higher absorption in the more densely pigmented shade-adapted polyps than in sun-adapted polyps. The combination of an O₂ microelectrode with a fibre-optic microprobe (combined sensor diameter 50–100 μm) enabled parallel measurements of O₂ concentration, gross photosynthesis rate and photosystem II (PSII) quantum yield at the coral surface under steady-state conditions as a function of increasing irradiances. Lower O₂ levels at the tissue surface and higher compensation irradiance indicated a higher respiration activity in sun-adapted polyp tissue as compared to shade-adapted polyps. Shade-adapted coenosarc and polyp tissues exhibited lower maxima of relative electron transport rates (rETR_max) (84±15 and 41±10, respectively) than sun-adapted coenosarc and polyp tissues (136±14 and 77±13, respectively). Shade-adapted tissues showed stronger decrease of rETR at high scalar irradiances as compared to sun-adapted tissues. The relationship between the relative PSII electron transport and the rate of gross photosynthesis, as well as O₂ concentration, was non-linear in sun-adapted tissues over the entire irradiance range, whereas for shade-adapted tissues the relationship became non-linear at medium to high scalar irradiances >200 μmol photons m⁻² s⁻¹. This suggests that rETR measurements should be used with caution in corals as a proxy for photosynthesis rates. The apparently high rates of photosynthesis (oxygen evolution rates) suggest that there must be a considerable electron transport rate through the photosystems that is not observed by the rETR measurements. This may be accounted for by vertical heterogeneity of zooxanthellae in the tissue and the operation of an alternative electron pathway such as cyclic electron flow around PSII.     

Photoadaptation of photosynthesis in Gonyaulax polyedra

Gonyaulax polyedra Stein exhibited a combination of photoadaptive strategies of photosynthesis when only a single environmental variable, the light intensity during growth, was altered. Which of several biochemical/physiological adjustments to the light environment were employed depended on the level of growth irradiance. The photoadaptive strategies employed over any small range of light levels appeared to be those best suited for optimizing photosynthetic performance and not photosynthetic capacity. (Photosynthetic performance, P _i, is defined as the rate of photosynthesis occurring at the level of growth irradiance.) Among all photosynthetic parameters examined, only photosynthetic performance showed a consistent correspondence to growth rates of G. polyedra. Above 3500 to 4000 W cm^-2, where photosynthetic performance was equal to photosynthetic capacity, cells were not considered light-limited in either photosynthesis or growth. At these higher light levels, photosynthetic perfomance, cell volume, growth rates and respiration rates remained maximal; photosynthetic pigment content varied only slightly, while the photosynthetic capacity of the cells declined. At intermediate light levels (3000 to 1500 W cm^-2), photosynthesis, not growth, was light-limited, and photoadaptive strategies were induced which enhance absorption capabilities and energy transfer efficiencies of chlorophyll a to the reaction centers of G. polyedra. Photosynthetic capacity remained constant at about 280 mol O₂ cm^-3 h^-1, while photosynthetic performance ranged from 100 to 130 mol O₂ cm^-3 h^-1. Major increases in photosynthetic pigments, especially peridinin-chlorophyll a-proteins and an unidentified chlorophyll c component, accompanied photoadaptation to low irradiances. Maximal growth rates of 0.3 divisions day^-1 were maintained, as were respiration rates of about-80 mol O₂ cm^-3 h^-1 and cell volumes of about 5.4×10^-8 cm^-3 cell^-1. Below about 1250 W cm^-2, photosynthesis in G. polyedra was so light-limited that photosynthetic performance was unable to support maximal growth rates. Under these conditions, G. polyedra displayed photostress responses rather than photoadaptive strategies. Photostress was manifested as reduced cell volumes, slower growth, and drastic reductions in pigmentation, photosynthetic capacity, and rates of dark respiration.     

Photosynthetic activity of the non-dormant <Emphasis Type="Italic">Posidonia oceanica</Emphasis> seed

The photosynthetic adaptive features of non-dormant seeds in Posidonia oceanica were studied in order to evaluate the effects of light on germination success. Transmission electron micrographs showed the presence of chloroplasts in the epidermal cells, close to the nucleus at the periphery of the cytoplasm. The well-developed thylakoid membranes and the presence of starch granules indicated that the chloroplasts were photosynthetically active. The relationship between photosynthesis versus irradiance in P. oceanica seeds incubated at 15 and 21°C was analysed. The net photosynthesis in the non-dormant seed of P. oceanica was positive and compensated its respiration demand (90 μmol quanta m⁻² s⁻¹) at both temperatures. Net photosynthesis was negative at the other irradiance values. To test the effects of light on germination success, seeds were placed both in dark and light conditions. Germination success was significantly higher in light rather than in dark condition. The characteristics observed in the photosynthesis in P. oceanica seed could be a mechanism to guarantee seedling survival in temperate waters, demonstrating though the specialized nature of this species.     

Microsensor studies of photosynthesis and respiration in larger symbiotic foraminifera. I The physico-chemical microenvironment of Marginopora vertebralis, Amphistegina lobifera and Amphisorus hemprichii

 The physico-chemical microenvironment of larger benthic foraminifera was studied with microsensors for O₂, CO₂, pH, Ca²⁺ and scalar irradiance. Under saturating light conditions, the photosynthetic activity of the endosymbiotic algae increased the O₂ up to 183% air saturation and a pH of up to 8.6 was measured at the foraminiferal shell surface. The photosynthetic CO₂ fixation decreased the CO₂ at the shell down to 4.7 μM. In the dark, the respiration of host and symbionts decreased the O₂ level to 91% air saturation and the CO₂ concentration reached up to 12 μM. pH was lowered relative to the ambient seawater pH of 8.2. The endosymbionts responded immediately to changing light conditions, resulting in dynamic changes of O₂, CO₂ and pH at the foraminiferal shell surface during experimentally imposed light–dark cycles. The dynamic concentration changes demonstrated for the first time a fast exchange of metabolic gases through the perforate, hyaline shell of Amphistegina lobifera. A diffusive boundary layer (DBL) limited the solute exchange between the foraminifera and the surrounding water. The DBL reached a thickness of 400–700 μm in stagnant water and was reduced to 100–300 μm under flow conditions. Gross photosynthesis rates were significantly higher under flow conditions (4.7 nmol O₂ cm⁻³ s⁻¹) than in stagnant water (1.6 nmol O₂ cm ⁻³ s⁻¹), whereas net photosynthesis rates were unaffected by flow conditions. The Ca²⁺ microprofiles demonstrated a spatial variation in sites of calcium uptake over the foraminiferal shells. Ca²⁺ gradients at the shell surface showed total Ca²⁺ uptake rates of 0.6 to 4.2 nmol cm⁻² h⁻¹ in A. lobifera and 1.7 to 3.6 nmol cm⁻² h⁻¹ in Marginopora vertebralis. The scattering and reflection of the foraminiferal calcite shell increased the scalar irradiance at the surface up to 205% of the incident irradiance. Transmittance measurements across the calcite shell suggest that the symbionts are shielded from higher light levels, receiving approximately 30% of the incident light for photosynthesis. Received: 6 July 1999 / Accepted: 28 April 2000     

Microsensor studies of photosynthesis and respiration in the symbiotic foraminifer Orbulina universa

Oxygen and pH microelectrodes were used to investigate the microenvironment of the planktonic foraminifer Orbulina universa and its dinoflagellate endosymbionts. A diffusive boundary layer surrounds the foraminiferal shell and limits the O₂ and proton transport from the shell to the ambient seawater and vice versa. Due to symbiont photosynthesis, high O₂ concentrations of up to 206% air saturation and a pH of up to 8.8, i.e. 0.5 pH units above ambient seawater, were measured at the shell surface of the foraminifer at saturating irradiances. The respiration of the host–symbiont system in darkness decreased the O₂ concentration at the shell surface to <70% of the oxygen content in the surrounding air-saturated water. The pH at the shell surface dropped to 7.9 in darkness. We measured a mean gross photosynthetic rate of 8.5 ± 4.0 nmol O₂ h⁻¹ foraminifer⁻¹. The net photosynthesis averaged 5.3 ± 2.7 nmol O₂ h⁻¹. In the light, the calculated respiration rates reached 3.9 ± 1.9 nmol O₂h⁻¹, whereas the dark respiration rates were significantly lower (1.7 ± 0.7 nmol O₂ h⁻¹). Experimental light–dark cycles demonstrated a very dynamic response of the symbionts to changing light conditions. Gross photosynthesis versus scalar irradiance curves (P vs E _o curves) showed light saturation irradiances (E _k) of 75 and 137 μmol photons m⁻² s⁻¹ in two O. universa specimens, respectively. No inhibition of photosynthesis was observed at irradiance levels up to 700 μmol photons m⁻² s⁻¹. The light compensation point of the symbiotic association was 50 μmol photons m⁻² s⁻¹. Radial profile measurements of scalar irradiance (E _o) inside the foraminifera showed a slight increase at the shell surface up to 105% of the incident irradiance (E _d). Received: 26 January 1998 / Accepted: 11 April 1998     

Photosynthetic characteristics of giant kelp (Macrocystis pyrifera) determined in situ

Rates of net photosynthesis and nocturnal respiration by individual blades of the giant kelp Macrocystis pyrifera (L.) C. Agardh in southern California, were determined in situ by measuring oxygen production in polyethylene bags during spring/summer of 1983. Mature blades from different depths in the water column exhibited different photosynthetic characteristics. Blades from the surface canopy (0 to 1 m depth) exhibited higher photosynthetic capacity under saturating irradiance and higher photosynthetic efficiency at low irradiances than blades from 3 to 5 or 7 to 9 m depths. Saturating irradiance was lower for canopy blades than for deeper blades. Canopy blades showed no short-term photoinhibition, but photosynthetic rates of deeper blades were significantly reduced during 1 to 2 h incubations at high irradiances. Results of 1 to 2 wk acclimation experiments indicated that differences between photosynthetic characteristics of blades from different depths were primarily attributable to acclimation light conditions. Vertical displacement of blades within the kelp canopy occurred on a time-scale of 1 min to 1 h. Blades continually moved between the unshaded surface layer and deeper, shaded layers. Vertical movement did not maximize photosynthesis by individual blades; only a small proportion of blades making up a dense surface canopy maintained light-saturated photosynthetic rates during midday incubations. The relatively high photosynthetic rates exhibited by canopy blades over the entire range of light conditions probably resulted from acclimation to intermittent high and low irradiances, a consequence of vertical displacement. Vertical displacement also reduced the afternoon depression in photosynthesis of individual canopy blades. The overall effect of vertical displacement was optimization of total net photosynthesis by the kelp canopy and, therefore, optimization of whole-plant production.     

Irradiance and temperature regulation of oxygenic photosynthesis and O2 consumption in a hypersaline cyanobacterial mat (Solar Lake, Egypt)

 Short-term effects of temperature and irradiance on oxygenic photosynthesis and O₂ consumption in a hypersaline cyanobacterial mat were investigated with O₂ microsensors in a laboratory. The effect of temperature on O₂ fluxes across the mat–water interface was studied in the dark and at a saturating high surface irradiance (2162 μmol photons m⁻² s⁻¹) in the temperature range from 15 to 45 °C. Areal rates of dark O₂ consumption increased almost linearly with temperature. The apparent activation energy of 18 kJ mol⁻¹ and the corresponding Q ₁₀ value (25 to 35 °C) of 1.3 indicated a relative low temperature dependence of dark O₂ consumption due to mass transfer limitations imposed by the diffusive boundary layer at all temperatures. Areal rates of net photosynthesis increased with temperature up to 40 °C and exhibited a Q ₁₀ value (20 to 30 °C) of 2.8. Both O₂ dynamics and rates of gross photosynthesis at the mat surface increased with temperature up to 40 °C, with the most pronounced increase of gross photosynthesis at the mat surface between 25 and 35 °C (Q ₁₀ of 3.1). In another mat sample, measurements at increasing surface irradiances (0 to 2319 μmol photons m⁻² s⁻¹) were performed at 25, 33 (the in situ temperature) and 40 °C. At all temperatures, areal rates of gross photosynthesis saturated with no significant reduction due to photoinhibition at high irradiances. The initial slope and the onset of saturation (E _k = 148 to 185 μmol photons m⁻² s⁻¹) estimated from P versus E _d curves showed no clear trend with temperature, while maximal photosynthesis increased with temperature. Gross photosynthesis was stimulated by temperature at each irradiance except at the lowest irradiance of 54 μmol photons m⁻² s⁻¹, where oxygenic gross photosynthesis and also the thickness of the photic zone was significantly reduced at 40 °C. The compensation irradiance increased with temperature, from 32 μmol photons m⁻² s⁻¹ at 25 °C to 77 μmol photons m⁻² s⁻¹ at 40 °C, due to increased rates of O₂ consumption relative to gross photosynthesis. Areal rates of O₂ consumption in the illuminated mat were higher than dark O₂ consumption at corresponding temperatures, due to an increasing O₂ consumption in the photic zone with increasing irradiance. Both light and temperature enhanced the internal O₂ cycling within hypersaline cyanobacterial mats. Received: 30 November 1999 / Accepted: 11 April 2000     

Effects of salinity and possible interactions with temperature and pH on growth and photosynthesis of <Emphasis Type="Italic">Halophila johnsonii</Emphasis> Eiseman

The effects of salinity, temperature, and pH variations on growth, survival, and photosynthetic rates of the seagrass Halophila johnsonii Eiseman were examined. Growth and survival responses to salinity were characterized by aquarium experiments in which plants were exposed to seven different salinity treatments (0, 10, 20, 30, 40, 50, and 60 psu) during 15 days. Photosynthetic behavior was assessed for short-term salinity exposures (1 or 20 h) by incubation experiments in biological oxygen demand (BOD) bottles and by measuring photosynthesis versus irradiance (PI) responses in an oxygen electrode chamber. In the bottle experiments the possible effects of interactions between salinity and temperature (15, 25, and 35°C) or pH (5, 6, 7, and 8.2) were also examined. Growth and survival of H. johnsonii were significantly affected by salinity, with maximum rates obtained at 30 psu. Salinity also altered the parameters of the PI curves. Light-saturated photosynthesis (P _max) and the photosynthetic efficiency at subsaturating light (α) increased significantly up to an optimum of 40 psu, decreasing again at the highest salinities. Dark respiration rates and compensating irradiance (I _c) showed minimum values at 40 and 50 psu, while light-saturation point (I _k) was maximum at 30–50 psu. An interaction between salinity and temperature was not found although an increase of temperature alone produced an increase in α, P _max, respiration rates, and I _k. An interaction between salinity and pH was only found in the P _max response: P _max increased with pH=5 at 30 psu. In addition, reducing the pH increased α significantly. In the BOD bottles experiment a significant reduction in the dark respiration with decreasing pH was observed, but the opposite trend was observed in the photosynthetic rate. These results suggest that the endemic seagrass H. johnsonii could be negatively affected by hypo- or hypersalinity conditions, although salinity changes did not seem to alter the tolerance of this species to other environmental factors, such as temperature or pH.     

Acclimation responses to salinity of three estuarine red algae from New Jersey

The effects of salinity and acclimation time on the net photosynthetic responses of 3 estuarine red algae, Bostrychia radicans Mont., Caloglossa leprieurii (Mont.) J. Ag., and Polysiphonia subtilissima Mont., from Great Bay Estuary, New Jersey, USA, were investigated. The algae were cultured in a series of synthetic seawater media of 5, 15, 25 and 35% S for acclimation periods of 0, 2, 4, 8, and 16 days prior to determining their photosynthetic responses. All species were euryhaline, and demonstrated photosynthesis at all the above salinities. B. radicans, which was more common towards the mouth of the estuary, had a maximum photosynthetic rate at 25% S, whilst C. leprieurii and P. subtilissima, which were more common towards the head of the estuary, had photosynthetic maxima between 15 and 25%, and at 15%, respectively. The curves relating net photosynthesis to salinity were usually similar within a species at different acclimation periods, although statistically significant differences were sometimes noted. The acclimation periods producing maximal net photosynthesis were 0, 2 and 4 days for B. radicans, and 4 days for C. leprieurii, whilst for P. subtilissima there was no significant difference in response for any acclimation period over the range of salinities studied.     

Effect of light quality on photosynthesis of the reef coral Montipora verrucosa

Pieces of the reef coral Montipora verrucosa (Lam.), collected from Kaneohe Bay, Oahu, Hawaii in 1982, were grown in four low-light treatments (11% sunlight): blue, green, red and the full spectrum of photosynthetically active radiation (PAR); and at high-intensity full PAR (90% sunlight). These acclimated corals were then tested for photosynthetic ability in blue, green, red, and white light. The photosynthetic parameters that were measured were; ligh-saturated photosynthetic rate, the initial slope of the photosynthesis/irradiance curve, the light intensity where these two lines crossed, and dark respiration. While acclimation intensity had a pronounced effect, the results also showed that the color of the acclimation treatment influenced the photosynthetic responses of the corals. The color of the light used in the measurements of photosynthesis had much less effect on the photosynthetic responses of the corals.Contribution No. 729 of the Hawaii Institute of Marine Biology     

In situ seagrass photosynthesis measured using a submersible, pulse-amplitude modulated fluorometer

Assessments of photosynthetic activity in marine plants can now be made in situ using a newly developed, submersible, pulse-amplitude modulated (PAM) fluorometer: Diving-PAM. PAM fluorometry provides a measure of chlorophyll a fluorescence using rapid-light curves in which the electron-transport rate can be determined for plants exposed to ambient light conditions. This technique was used to compare the photosynthetic responses of seagrasses near Rottnest Island, Western Australia. Several fluorescence parameters were measured as a function of time of day and water depth; electron-transport rate (ETR), quantum yield, photochemical quenching and non-photochemical quenching and Photosystem II (PSII) photochemical efficiency (F _v :F _m ratio) were measured. Results indicate that recent light-history plays a crucial role in seagrass photosynthetic responses. Maximum ETR of Posidonia australis, Amphibolis antarctica and Halophila ovalis is influenced by the irradiance during the diurnal cycle, with low rates at dawn and dusk (<10 μmol electron m⁻² s⁻¹), highest rates in late morning (40 to 60 μmol electron m⁻² s⁻¹) and a mid-day depression. Maximum ETR and PSII photochemical efficiency varied widely between seagrass species and were not correlated. A comparison of photochemical to non-photochemical quenching indicated that seagrasses in shallow water receiving high light have a high capacity for non-photochemical quenching (e.g. light protection) compared to seagrasses in deep water. These results indicate that in situ measurements of photosynthesis will provide new insights into the mechanisms and adaptive responses of marine plants. Received: 26 May 1997 / Accepted: 27 May 1998     

Chlorophyll fluorescence measures of seagrasses <Emphasis Type="Italic">Halophila ovalis</Emphasis> and <Emphasis Type="Italic">Zostera capricorni</Emphasis> reveal differences in response to experimental shading

In coastal waters and estuaries, seagrass meadows are often subject to light deprivation over short time scales (days to weeks) in response to increased turbidity from anthropogenic disturbances. Seagrasses may exhibit negative physiological responses to light deprivation and suffer stress, or tolerate such stresses through photo-adaptation of physiological processes allowing more efficient use of low light. Pulse Amplitude Modulated (PAM) fluorometery has been used to rapidly assess changes in photosynthetic responses along in situ gradients in light. In this study, however, light is experimentally manipulated in the field to examine the photosynthesis of Halophila ovalis and Zostera capricorni. We aimed to evaluate the tolerance of these seagrasses to short-term light reductions. The seagrasses were subject to four light treatments, 0, 5, 60, and 90% shading, for a period of 14 days. In both species, as shading increased the photosynthetic variables significantly (P < 0.05) decreased by up to 40% for maximum electron transport rates (ETR_max) and 70% for saturating irradiances (E_k). Photosynthetic efficiencies (α) and effective quantum yields (ΔF/Fm′) increased significantly (P < 0.05), in both species, for 90% shaded plants compared with 0% shaded plants. H. ovalis was more sensitive to 90% shading than Z. capricorni, showing greater reductions in ETR_max, indicative of a reduced photosynthetic capacity. An increase in E_k, Fm′ and ΔF/Fm′ for H. ovalis and Z. capricorni under 90% shading suggested an increase in photochemical efficiency and a more efficient use of low-photon flux, consistent with photo-acclimation to shading. Similar responses were found along a depth gradient from 0 to10 m, where depth related changes in ETR_max and E_k in H. ovalis implied a strong difference of irradiance history between depths of 0 and 5–10 m. The results suggest that H. ovalis is more vulnerable to light deprivation than Z. capricorni and that H. ovalis, at depths of 5–10 m, would be more vulnerable to light deprivation than intertidal populations. Both species showed a strong degree of photo-adaptation to light manipulation that may enable them to tolerate and adapt to short-term reductions in light. These consistent responses to changes in light suggest that photosynthetic variables can be used to rapidly assess the status of seagrasses when subjected to sudden and prolonged periods of reduced light.     

Photoinhibition and recovery of the kelp Laminaria saccharina at optimal and superoptimal temperatures

Effects of high irradiance on photosynthetic characteristics were examined in sporophytes of the kelp Laminaria saccharina Lamour. from 1992 to 1994. Exposure to high irradiance (700 mol photons m^-2s^-1) for 1 h at optimal temperature (12°C) caused a 40 to 60% decline in photosynthetic efficiency (alpha), quantum yield, and the ratio of variable to maximum chlorophyll fluorescence (F_v/F_m), an indicator of Photosystem II efficiency. Although the photoinhibition effects were partly attributable to protective mechanisms, a concurrent increase in minimal fluorescence (F_o) indicated damage to Photosystem II reaction centers. The magnitude of photoinhibition was proportional to irradiance and duration; however, F_v/F_m was significantly reduced after exposure to irradiances as low as 40 to 50 mol photons m^-2s^-1 for 1 h, or to 700 mol photons m^-2s^-1 for only 5 min. In contrast, photosynthetic capacity (P_max) was affected only at much higher irradiance. Superoptimal temperatures up to 24°C did not exacerbate high-light effects. At 25°C, however, alpha and P_max were more susceptible to photoinhibition than at lower temperatures. Recovery from photoinhibition was examined by following F_v/F_m and F_o for 24 h after exposure to high light. Recovery of F_v/F_m was fastest during the first 1 to 3 h, and slowed or ceased after 6 to 12 h, while recovery of F_o was relatively constant over 12 h. Dithiothreitol, which blocks formation of energy-dissipating xanthophylls, reduced both the initial rate and extent of recovery. Chloramphenicol, which blocks chloroplast-encoded protein synthesis, had little effect on initial rates of recovery, but stopped recovery after 3 h. Thus, L. saccharina appears to rely on the xanthophyll cycle to protect the photosynthetic apparatus, and reversal of this protective mechanism causes the rapid initial recovery in F_v/F_m. Longterm recovery depends on repair of damaged reaction centers. Both the rate and extent of recovery were temperature-dependent. The initial rate was higher at 18 to 22°C than at 12°C, but the extent of recovery over 24 h declined with increasing temperature. High temperatures, therefore, appear to enhance protective mechanisms, but disrupt repair processes. L. saccharina from Long Island Sound, an ecotype adapted to low light and high temperature, showed slightly but consistently greater effects of photoinhibition than plants from the Atlantic coast of Maine, but exhibited faster recovery at superoptimal temperatures.     

Light and carbon balance in the seagrass Thalassia testudinum : evaluation of current production models

The production dynamics and carbon balance of Thalassia testudinum in the lower Laguna Madre, Texas, USA, were examined during the 1995 summer period based on in situ photosynthesis vs irradiance (PI) measurements and continuous measurements of underwater photon-flux density (PFD). The validity of applying the H _sat model, used to calculate production for Zostera marina as the product of the maximum rate of photosynthesis (P _max) and daily hours of saturating irradiance (H _sat) was assessed for T. testudinum by comparison with integrated production estimates derived through numerical integration. Gross integrated production values were combined with dark-respiration measurements of photosynthetic (PS) and non-photosynthetic (NPS) tissues and areal biomass to generate daily whole-plant carbon balance. Production and whole-plant carbon balance are discussed in relation to surface and underwater PFD measurements, biomass and other physical and chemical parameters collected during a 1 yr period from January to December 1995. The H _sat model significantly underestimated production during all summer months, averaging 70% of integrated production over the entire study period. Gross integrated production ranged between 11.5 mg C g⁻¹ leaf dry wt d⁻¹ in June (during a period of unseasonably low PFDs caused by a drift-alga mat covering the seagrass bed) to 26.7 mg C g⁻¹ leaf dry wt d⁻¹ in July. Modeled net carbon gain was highest in July at 454 mg C m⁻² d⁻¹ (1.4 g dry wt m⁻² d⁻¹), sufficient to account for measured rates of leaf production in the study area and representative of T. testudinum populations of low productivity. During part of the summer period, however, the population was in negative carbon balance. The relatively low productivity of this population and the periods of negative carbon balance are attributed to low net photosynthesis:dark respiration (P _net:R _d) ratios, sporadic low-light periods, the small fraction of PS tissue relative to whole-plant biomass (5 to 13%) and nutrient limitation. Production models are sensitive to both light availability and the proportion of PS tissue supporting NPS biomass as reflected in whole-plant P _net:R _d ratios. Received: 13 August 1997 / Accepted: 6 March 1998     

Photosynthesis and growth of red and green morphotypes of Kappaphycus alvarezii (Rhodophyta) from the Philippines

The effect of photosynthetic available radiation (PAR) levels, light quality, ultraviolet (UV) radiation, and temperature on photosynthesis, growth, and chlorophyll fluorescence was evaluated in red and green morphotypes of the rhodophyte Kappaphycus alvarezii (Doty) Doty under controlled conditions. Chlorophyll a and phycoerythrin (PE) levels were similar in the red and green morphotypes cultured under the same conditions, but phycocyanin (PC) and allophycocyanin (APC) levels were 2-fold greater in the green than in the red morphotype. Pigment characterization indicated that the overexpression of PC and APC masked the red pigmentation in the green morphotype. Maximum photosynthesis and photosynthetic efficiency were similar between the two morphotypes assayed at a wide temperature range, which was reflected in the similar growth rates observed in outdoor culture systems. In the green morphotype, photosynthetic efficiency increased 2-fold relative to the red morphotype when assayed with red light (λ > 600 nm), indicating that photosynthetic characteristics are modified as a result of pigment variation in these morphotypes. Such increase in photosynthetic efficiency in the green morphotype, however, did not result in greater growth rates when cultured under white light. Short exposure to high levels of solar radiation (UV-A + UV-B + PAR), and filtered solar radiation (UV-A + PAR or PAR) decreased effective quantum yield (ΔF/F _m′) in both morphotypes. The reduction of ΔF/F _m′ values in the red and green morphotypes was accounted for by high levels of PAR and not by the UV-A + UV-B + PAR and UV-A + PAR treatments. Photoinhibition caused by UV-A, UV-B, or PAR was completely reversed within 30 h after incubations. Recovery rates from photoinhibition, however, were significantly reduced in the green morphotype when incubated with UV-B radiation. The results here suggest that the overexpression of pigments do not necessarily increase photosynthesis and growth in these morphotypes. Received: 19 June 2000 / Accepted: 28 November 2000     

Effect of feeding regime and irradiance on the photophysiology of the symbiotic sea anemone Aiptasia pulchella

To determine how the animal and algal components of the symbiotic sea anemone Aiptasia pulchella respond to changes in food availability and culture irradiance, sea anemones from a single clone were maintained at four irradiance levels (320, 185, 115, and 45 E m^-2 s^-1) and either starved or fed for 5 wk. Changes in protein biomass of sea anemones maintained under these conditions were not related to the productivity of zooxanthellae, since the protein biomass of fed A. pulchella decreased with increase in irradiance and there was no difference in protein biomass among starved sea anemones at the four irradiance levels. Except for the starved high-light sea anemones, the density of symbiotic zooxanthellae was independent of culture irradiance within both starved and fed. A. pulchella. Starved sea anemones contained over twice the density of zooxanthellae as fed sea anemones. Within both starved and fed individuals, chlorophyll per zooxanthella increased with decreasing culture irradiance while algal size remained constant (in fed sea anemones) at about 8.80 m diameter. Chlorophyll a: c ₂ ratios of zooxanthellae increased with decreasing culture irradiance in zooxanthellae from starved sea anemones but remained constant in zooxanthellae from fed sea anemones. As estimated from mitotic index data, the in situ growth rates of zooxanthellae averaged 0.007 d^-1 and did not vary with irradiance or feeding regime. Photosynthesis-irradiance (P-I) responses of fed A. pulchella indicated an increase in photosynthetic efficiency with decreasing culture irradiance. But there was no consistent pattern in photosynthetic capacity with culture irradiance. Respiration rates of fed sea anemones also did not vary in relation to culture irradiance. The parameter I _k , defined as the irradiance at which light-saturated rates of photosynthesis are first attained, was the only parameter from the P-I curves which increased linearly with increasing culture irradiance. The daily ratio of net photosynthesis to respiration for A. pulchella ranged from 1.6 to 2.8 for sea anemones maintained at the three higher irradiances, but was negative for those maintained at 45 E m^-2 s^-1. Since the final protein biomass was greatest for sea anemones maintained at the lowest irradiance, these results indicate that sea anemone growth cannot be directly related to productivity of zooxanthellae in this symbiotic association.     

Effects of irradiance and ultraviolet radiation on photoadaptation in the zooxanthellae of Aiptasia pallida: primary production,photoinhibition, and enzymic defenses against oxygen toxicity

Cnidarians which contain symbiotic algae are constantly faced with the challenges of a changing photic regime and a hyperoxic environment. Zooxanthellae (Symbiodinium sp.) from the sea anemone Aiptasia pallida (Verrill), collected and cultured at Bermuda Biological Station in 1986, exhibit a suite of compensatory responses to changes in irradiance, ultraviolet radiation (UV), and to the toxicity resulting from their interaction with photosynthetically produced oxygen. Superoxide dismutase (SOD) and catalase inactivate superoxide radicals (O₂ ^-) and hydrogen peroxide (H₂O₂), which are mediators of oxygen toxicity, show an increase in specific activity with irradiance and in response to UV, both in cultured zooxanthellae (CZ) and freshly isolated zooxanthellae (FIZ) from acclimated anemones. CZ and FIZ exposed to environmentally realistic UV levels show a 30 to 40% increase in SOD activities compared with zooxanthellae exposed to similar irradiances without UV. CZ consistently show higher activities of both SOD and catalase compared to FIZ. Both CZ and FIZ exhibit changes in chlorophyll content and in the relationship between photosynthesis and irradiance which suggest photoadaptive changes in CO₂-fixing enzymes, the photosynthetic-electron transport system, or in photosynthetic unit size (PSU). UV has a greater effect on the photosynthetic capacity (P _max) of FIZ when compared to CZ acclimated at an equivalent irradiance with or without a UV component. UV also enhances the photoinhibition observed at high irradiance in both CZ and FIZ. Differences in enzyme activity between CZ and FIZ suggest an important role for the host in the protection of zooxanthellae against the direct effects of environmentally realistic UV while the photosynthetic performance of zooxanthellae in situ may not be as well protected.