Diurnal photosynthetic performance of seaweeds measured under natural conditions

Statements comparing photosynthetic performance characteristics of species rely upon empirical data, usually light-saturation curves (photosynthesis, P, versus incidentlight flux-density, I _o, relationships) derived from instantaneous measurements. The specific comparative parameters are initial slope and maximum photosynthesis, P _max. For phytoplankton, diurnal variation in specific productivity at maximum incident light, I _max is typically asymmetrical, i.e., there is a morning maximum followed by an afternoon depression. Five seaweed species, numerical dominants from the Outer Banks of North Carolina, were examined for patterns of diurnal photosynthetic performance in sunlight of habitat equivalence. These were Codium decorticatum (Woodw.) Howe and Ulva curvata (Kütz.) De Toni in the Chlorophyceae, Dictyota dichotoma (Huds.) Lamour. and Petalonia fascia (O.F. Müll.) Küntze in the Phaeophyceae, and Gracilaria foliifera (Forssk.) Børg. in the Rhodophyceae. Diurnal patterns of oxygen exchange were varied, some symmetrical about the midday axis, others asymmetrical, and were specific for (1) species, (2) derived habitat, (3) thallus absorptance (1-I/I _o, where I is the transmitted light), (4) developmental stage, and (5) diurnal photosynthetically active radiation (PAR) history. All species show a depression in oxygen exchange rates at less than 0.1 I _max, and show varying degrees of recovery when I _o decreases from that value. Diurnal photosynthetic performance of some species at 0.03 I _o (total diurnal maximum) exceeds several times that at 0.70 I _o (total diurnal maximum), an observation not predicted by instantaneous measurements. Specific day-rate integrals of I _o vary, producing transient initial slope and P _max values. Thus, initial slope and P _max values derived from instantaneous measurements in the laboratory bear little relationship to actual diurnal production. At this time there appears to be no substitute for direct measurement of diurnal photosynthesis.     

Diurnal rhythms in netplankton and nannoplankton assimilation ratios

Surface nannoplankton and netplankton photosynthetic rates and chlorophyll a concentrations were measured 3 h before and 3 h after local apparent noon at 36 stations in the eastern Pacific. Morning and afternoon nannoplankton and netplankton photosynthetic capacities were plotted against chlorophyll a concentrations. Comparison of am (morning) and pm (afternoon) regression coefficients revealed that neither netplankton nor nannoplankton photosynthetic efficiencies varied diurnally in temperate California Current waters. However, in oligotrophic tropical surface waters where the photosynthetic rates of both fractions were nitrogen limited, netplankton assimilation ratios exhibited an afternoon maximum, while nannoplankton efficiencies exhibited a morning maximum. The netplankton followed the same pattern in eutrophic tropical surface waters with high nitrate concentrations, but nannoplankton assimilation ratios were highest in the afternoon. Assuming that midday light intensities inhibit surface photosynthesis, it was concluded that assimilation ratios will reach a mid-morning or mid-afternoon maximum depending upon the relative importance of netplankton and nannoplankton productivity and the degree to which nutrients are limiting.     

Diurnal cycle and kinetics of ammonium assimilation in the green alga <Emphasis Type="Italic">Ulva pertusa</Emphasis>

The kinetics of ammonium assimilation was investigated in Ulva pertusa (Chlorophyceae, Ulvales) from northeastern New Zealand. Ammonium assimilation exhibited Michaelis–Menten kinetics with a maximum rate of assimilation (V _max) of 54 ± 5 μmol g⁻¹ dry weight h⁻¹ and half-saturation constant (K _m) of 23 ± 8 μM. In contrast, values for ammonium uptake were considerably higher with a V _max of 316 ± 59 μmol g⁻¹ dry weight h⁻¹ and K _m of 135 ± 46 μM. At environmentally relevant ammonium concentrations (5 μM), assimilation accounted for most (70%) of the ammonium taken up. Darkness decreased the maximum rate of ammonium assimilation by 83%. We investigated the hypothesis that rates of biosynthetic processes are greater in the early part of the day in Ulva. Consistent with this hypothesis, the maximum rate of ammonium assimilation in U. pertusa peaked in the morning and coincided with low levels of the photosynthetic product sucrose, which peaked in the afternoon. There was a diurnal cycle in the rate of ammonium uptake and assimilation in light and dark, but the amplitude was much greater for assimilation than uptake. Moreover, our data suggest that net ammonium assimilation only occurs during the day in U. pertusa. We suggest that two major roles for diurnal cycles are minimisation of interspecific competition for resources and metabolic costs.     

Diurnal lipid and mucus production in the staghorn coral Acropora acuminata

Net ¹⁴C-accumulation into lipids of Acropora acuminata was rapid and increased with light intensity. Dark ¹⁴C-incorporation was less than 1% noon maximum. Structural lipids were the first radioactively labelled lipid types showing linear ¹⁴C-uptake kinetics. Storage lipids showed non-linear, power-curve kinetics for ¹⁴C-uptake. The rate of ¹⁴C-incorporation into triglycerides and wax esters was maximal during early afternoon and at midday, respectively. Electron microscopic evidence is given for zooxanthellae being primary sites for synthesis of lipids which are exuded from chloroplasts and transferred to animal tissues. Free lipid droplets and crystalline inclusions (wax ester) were common in animal tissues, the inclusions being often associated with mucus-producing cells. The diurnal rate of mucus production was constant. However, ¹⁴C-mucus-lipid production showed a light-dependent diurnal pattern and accounted for 60 to 90% total ¹⁴C of mucus during periods of photosynthetically-saturating light. Here, ¹⁴C was primarily associated with wax esters which were always present in the mucus-lipid. ¹⁴C-triglycerides occur in mucus released only during the day. Lipid and mucus synthesis is discussed in relation to the carbon budget of A. acuminata, in which mucus represented a loss of 40% net C fixation.     

Time-course of photoadaptation in the photosynthesis-irradiance relationship of a dinoflagellate exhibiting photosynthetic periodicity

Cultures of the marine dinoflagellate Glenodinium sp. were light-shifted and rates of photoadaptation determined by monitoring changes in cell volume, growth rate, pigmentation, parameters of the photosynthesisirradiance (P-I) curves and respiration. To approximate physiological conditions of field populations, cells were cultured on an alternating light-dark cycle of 12hL:12hD, which introduced a daily periodicity of photosynthesis. One result of the present study was to demonstrate how specific parameters of the P-I relationship influenced by periodicity of the light: dark cycle are distinguished from photosynthetic parameters influenced by changes in light level. Under steady-state conditions, rates of both light-saturated (P_max) and light-limited photosynthesis changed in unison over the day; these changes were not related to pigmentation, and displayed their maxima midday. This close relationship between P_max and the slope (a) of the cellular P-I curves in steadystate conditions was quickly adjusted when growth illumination was altered. Rates of light-limited photosynthesis were increased under low light conditions and the periodicity of cellular photosynthesis was maintained. The short-term responses of the P-I relationship to changing light level was different, depending on (1) whether the light shift was from high to low light or vice versa, and (2) whether the high light levels were sufficient to promote maximal photosynthesis rates. Major increases in the photosynthetic carotenoid peridinin, associated with a single type of light-harvesting chromo protein in the chloroplast, was observed immediately upon shifting high light cultures to low light conditions. Following pigment synthesis, significant increases in rates of light-limited photosynthesis were observed in about one-tenth the generation time, while cellular photosynthetic potential was unaffected. it is suggested that general results were consistent with suggested that general results were consistent with earlier reports that the major photoadaptive strategy of Glenodinium sp. is to alter photosynthetic unit (PSU) size. Photoadaptive response times to high light were light-dependent, but appeared to be shower than photoadaptive responses to low light. If light intensities were bright enough to maximize growth rates, photosynthetic response times were on the order of a generation period and pigmentation fell quickly as cells divided at a faster rate. If light-intensities were not sufficient to maximize growth rates, then pigment content did not decline, while rates of light-limited photosynthesis declined quickly. In all cases, photoadaptation was followed best by monitoring fast changes in half saturation constants for photosynthesis, rather than fluctuating changes in pigmentation. Results compared well with time-course phenomena reported for other groups of phytoplankton. Overall, results suggest phytoplankton can bring about photo-induced changes in photosynthesis very quickly and thus accommodate widely fluctuating light regimes over short periods of time.     

Obligate mixotrophy inLaboea strobila,a ciliate which retains chloroplasts

The planktonic ciliateLaboea strobila Lohmann sequesters photosynthetically functional chloroplasts derived from ingested algae. The chloroplasts lie free in the cytoplasm and are most abundant just under the pellicle of the ciliate. The maximum rate of photosynthesis (P_max) was 925 pg C ciliate^-1h^-1 (3.7 pg C pg chl.a ^-1h^-1). At saturating irradiance, the amount of carbon fixed h^-1 equaled 12.6% of the body carbon of the ciliate. To grow,L. strobila requires both light and algal food. In the absence of food, survival ofL. strobila is significantly longer in the light than in the dark. Based on ingestion rate and photosynthetic rate, we calculate that photosynthesis can make an important contribution to this ciliate's carbon budget even when algal food is plentiful.     

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     

Photosynthesis in whole plants of Chondrus crispus

Photosynthesis in whole plants of Chondrus crispus Stackhouse was measured by ¹⁴C uptake under various light intensities and temperature regimes. In plants collected from waters of 6° to 7°C, photosynthesis did not increase with light intensity over a range of 200 to 2400 foot candles (ft-c). However, in plants collected from waters of 10° to 12°C, the rate of photosynthesis was lower over the range 200 to 1400 ft-c but, at 2000 ft-c, increased to a level similar to that of the 6° to 7°C plants. Ethanol-soluble and insoluble fractions showed similar patterns.     

Phytoplankton photosynthetic adaptation to high frequency light fluctuations simulating those induced by sea surface waves

In order to test the ability of phytoplankton to adapt to the high frequency light fluctuations induced by sea surface waves, the green alga Dunaliella tertiolecta was grown under both steady and fluctuating (0.1, 1.0 and 10 Hz) illuminations. The latter conditions reproduced those fluctuations experienced by phytoplankton in the upper photic layer. For each culture, photosynthesis versus irradiance were measured under four incubation frequencies (steady, 0.1, 1.0 and 10 Hz fluctuating illuminations). Results indicated that growth rates were similar for algae grown under steady light and 10 Hz fluctuating light (0.26–0.33 d^–1). Cells grown at 0.1 and 1.0 Hz showed lower growth rates (0.17–0.26 d^–1). Chlorophyll a and b were significantly higher under 0.1 and 10 Hz frequencies than under steady illumination; at 1.0 Hz, there were no significant differences with steady light. No changes in carotenoids were evidenced at any frequency tested. Photosynthetic measurements showed that algae grown under steady illumination had higher photosynthetic efficiency and capacity when incubated under steady and 0.1 Hz fluctuating light. Photosynthetic characteristics of algae grown under 0.1 Hz illumination did not show any clear responses to fluctuating light. Algae grown under 1.0 or 10 Hz had higher photosynthetic efficiency and capacity than those grown under steady illumination, when incubated under 1.0 and 10 Hz light. This suggests that microalgae grown under high frequency illumination (1.0 and 10 Hz) can adapt their photosynthetic characteristics to the rapidly fluctuating light regime experienced during growth, and that algae grown under steady conditions respond better to steady or slowly fluctuating (0.1 Hz) light. Such an adaptation provides a means of probing the photosynthetic responses of phytoplankton to vertical mixing.Contribution to the program of GIROQ (Groupe interuniversitaire de recherches océanographiques du Québec)     

Effects of silicate depletion on photosynthesis by diatoms in the plume of the Hudson River

Silicate depletion was observed during a bloom of netplankton diatoms. Netplankton chlorophyll a increased over the same salinity range and at the same rate that silicate decreased. Silicate depletion coincided with a decrease in the apparent Si:N uptake ratio as well as a change in the magnitude and diurnal phasing of light saturated photosynthesis (P _m ^B ) by netplankton diatoms. Nanoplankton P _m ^B was unaffected by silicate depletion and increased with temperature. Consequently, nanoplankton P _m ^B eventually exceeded netplankton P _m ^B while netplankton biomass was still increasing relative to nanoplankton biomass.     

Interactive effects of nitrogen and dissolved inorganic carbon on photosynthesis,growth, and ammonium uptake of the macroalgae Cladophora vagabunda and Gracilaria tikvahiae

Dissolved inorganic carbon (DIC) is rarely considered limiting for macroalgae, but some research suggests that under conditions of N sufficiency, photosynthetic capacity is enhanced with DIC enrichment. During spring (April–May) and summer (July–August) 1993, we investigated the interactive effects of nitrogen (N) and DIC on photosynthetic capacity, growth, and nutrient uptake rates of the macroalgae, Cladophora vagabunda (L.) van den Hoek and Gracilaria tikvahiae (McLachlan), dominant species in a temperate eutrophic estuary (Cape Cod, Massachusetts, USA). Water-column CO₂ concentrations showed significant diurnal fluctuations, ranging from a morning CO₂ peak (21 M) to an afternoon low (13 M) during summer, probably associated with metabolic activities in a thick algal mat. Results from instantaneous photosynthesis measurements and microcosm experiments indicate that DIC limits photosynthetic capacity and growth rates of C. vagabunda during summer, perhaps related to tissue N sufficiency and low water-column CO₂ concentrations. For example, this species showed enhanced growth (F=8.69, P<0.02) under DIC but not N enrichment. G. tikvahiae showed marginal DIC enhancement of maximum photosynthetic rate, while growth was significantly stimulated by addition of N. Reduced thallus N of this species during the summer further identifies N as the primary factor limiting growth. In addition, G. tikvahiae has the ability to use DIC in its several forms, while C. vagabunda primarily uses dissolved CO₂. DIC enrichment resulted in a depression of NH₄ ⁺ uptake rates for both species, particularly during summer at saturating (60 M) ammonium levels, suggesting competition between NH₄ ⁺ uptake and DIC acquisition under conditions of N sufficiency. Dominance of C. vagabunda and G. tikvahiae in areas undergoing eutrophication has been attributed to their successful procurement and storage of N as well as to high growth rates. The present study revealed that under conditions of N sufficiency during summer, DIC may control rates of production of these opportunistic macroalgae.     

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.     

Effects of diurnal variations in phytoplankton photosynthesis obtained from natural fluorescence

Effects of diurnal variation in phytoplankton photosynthesis on estimating daily primary production (DPP) were examined using field data from Sagami Bay, Japan. DPP at 5 m depth was calculated from the continuous data of chlorophyll a (Chl a) and light intensity monitored by a natural fluorescence sensor with and without considering time-dependent changes in the photosynthesis–irradiance (P–E) relationship. Chl a could be estimated from natural fluorescence examining the variations in the quantum yield of fluorescence (φ _f) and Chl a-specific light absorption coefficient (a*_ph), and relating them to Chl a. The P–E relationship was determined by water sampling three times daily. A distinct diurnal pattern was observed for the maximum photosynthetic rate (P*_max), being maximal at noon, while periodicity of the maximum light utilization coefficient (α*) was less obvious. The actual DPP was calculated by interpolating the P–E parameters from those obtained at dawn, noon, and dusk. For comparison, DPP was calculated by fixing the P–E parameters as the constants measured at dawn, noon or dusk for a day. The difference from the actual DPP was small when the P–E parameters measured at dawn (3% on average) and noon (5%) were used as the constants for a day. The difference was largest when the values at dusk were used (−43%). The medium values of P*_max at dawn, its low values at dusk, and the fact that a major part of the DPP was produced around noon were responsible for these results. The present study demonstrates that measurement of the P–E parameters at dawn or noon can give a good estimation of DPP from natural fluorescence.     

Experimental ecology of the temperate scleractinian coral Astrangia danae I. Partition of respiration,photosynthesis and calcification between host and symbionts

Calcification, photosynthesis and respiration of the scleractinian coral Astrangia danae were calculated from the changes in total alkalinity, pH, calculated total CO₂, and oxygen concentration produced by colonies incubated in glass jars. A correction for changes in ammonia, nitrate and nitrite was taken into account and the method evaluated. The fluxes of oxygen and CO₂ were highly correlated (r=0.99). The statistical error of alkalinity determinations was less than 10% of the changes observed in the slowest calcifying samples. Metabolism of polyparium alone was estimated by difference after removal of tissue and reincubation of bare corallum. Zooxanthellae concentration in the polyps was obtained from cell counts made on homogenates of polyp tissue. The calculated photosynthetic rate of the zooxanthellae in vivo was 25 mol O₂ (10⁸ cell)^-1 h^-1 at a light intensity of 120 Ein m^-2 s^-1. In corals having 0.5x10⁹ zooxanthellae/dm² of colony area up to 8% of the total photosynthesis was attributed to the corallum microcosm. Polyp respiration, photosynthesis, and CaCO₃ uptake rates were all much higher than rates previously reported from A. danae, apparently because in these experiments the organisms were better fed. This increased photosynthesis in turn enhanced calcification still further. The symbiosis therefore appears to provide a growth advantage even to fed corals, under the conditions of these experiments.     

Distribution and maximum growth depth of Fucus vesiculosus along the Gulf of Finland

 A survey of the distribution and maximum depth of a continuous Fucus vesiculosus belt was carried out in the Gulf of Finland in 1991. F. vesiculosus is widely distributed throughout the Gulf of Finland, including the vicinity of Vyborg Bay, Russia in the east. The maximum growth depth of F. vesiculosus in the Gulf of Finland reflects two different patterns according to the exposure to wave action. The most robust and continuous F. vesiculosus belt is observed on exposed shores, where the maximum growth depth is 5 to 6 m, with the optimum at 2 to 3 m. On moderately exposed shores the maximum growth depth is 3 m, with an optimum growth depth of <2 m. The maximum growth depth also varies geographically, with a decreasing trend towards the east. Maximum growth depth of F. vesiculosus correlates with light intensity. The compensation point for F. vesiculosus photosynthesis is about 25 μmol m⁻² s⁻¹, and photosynthesis is saturated at a light intensity of 300 μmol m⁻² s⁻¹. Vertical irradiance attenuation measurements in situ in summer revealed that for F. vesiculosus photosynthesis the quantity of light is optimal (200 to 300 μmol m⁻² s⁻¹) at <3 m depth. At depths >5 m the quantity of light is near or below the photosynthesis compensation point and insufficient for growth. These depth limits of light penetration coincide with measured growth depths of F. vesiculosus in the Gulf of Finland. Received: 7 May 1999 / Accepted: 18 November 1999     

Effect of diurnal variations in natural irradiance on the apical length growth and light saturation of growth in five species of benthic macroalgae

The diurnal variation in apical length growth rate of five benthic macroalgae from the Swedish West Coast (Fucus spiralis L., F. vesiculosus L., F. serratus L., Ascophyllum nodosum (L.) Le Jolis, and Chondrus crispus Stackhouse) was measured during 1.5 to 2.0-h intervals for 3 to 5 d in September 1984. All species showed a maximum in growth rate in the morning, followed by a continuous decline throughout the rest of the day (most pronounced in F. serratus, A. nodosum and C. crispus). By comparing the growth rates with the average irradiance during the same interval, saturation irradiances for length growth in natural daylight regime were estimated; 60 to 70 Wm^-2 for F. spiralis and F. vesiculosus, 90 to 100 Wm^-2 for A. nodosum, 30 to 40 Wm^-2 for F. serratus and 10 to 20 Wm^-2 for C. crispus. Average growth rates during the dark hours ranged from 33% of average day growth rates in A. nodosum to 63% in C. crispus, respectively.     

Adaptation of solitary corals and their zooxanthellae to low light and UV radiation

Adaptation of solitary corals, Fungia repanda and F. echinata, and their zooxanthellae to low light and ultraviolet light B (UV-B) was studied with respect to changes in their protein contents, photosynthetic pigment contents and the photosynthesis-irradiance (P-I) curves. The corals were collected from 1 to 50 m depths in the Republic of Belau (Paulau) in 1990 and 1991. The chlorophyll a content in a unit surface area of the coral did not change significantly with the depth of the habitat, whereas cellular chlorophyll a in the algae increased with the depth. Zooxanthellae density and protein content in a unit surface area of Fungia spp. decreased with the depth. Photosynthetic parameters normalized by a unit surface area of the Fungia spp., maximum gross photosynthetic rate (P _gmax area^-1) and dark respiration rate (R area^-1), were negatively correlated with the depth, while initial slope of the P-I curve () did not show significant correlation with the depth. Compensation light intensity (Ic) decreased with the depth. In isolated zooxanthellae, P _max chl a ^-1, and R chl a ^-1 decreased with the depth, while _{chl a} was constant. P _gmax cell^-1 and R cell^-1 did not change significantly but _cell increased with the depth. Ic decreased with the depth as in the intact corals. Reduction of protein content in a unit area of the coral from deeper habitat implies decrease of host animal tissues. Reduction of Ic can be explained by decrease of R area^-1, which may be due to the diminution of animal tissues. The photoadaptational response to low light intensity of intact Fungia spp. was found to be a combination of the photoadaptation of symbiotic algae and the decrease of host animal tissue. In order to study their adaptation to ultraviolet (UV) radiation, P-I curves of Fungia spp. and isolated zooxanthellae were analyzed before and after UV-B irradiation. 1 h UV-B irradiation showed no effect on the photosynthetic rate of the shallow water (1 m) corals, while it inhibited the photosynthesis of the deep water (30 m) corals and zooxanthellae isolated from both shallow and deep water corals. These results indicate that the host, Fungia spp., in shallow water have protective mechanism for intense UV-B in their habitat. These photoadaptational mechanisms seem to allow the Fungia spp. to have wide vertical distribution where light intensity spans more than two orders of magnitude.     

Light-dependency of nitrate uptake by phytoplankton over the spring bloom in Auke Bay,Alaska

The effect of light intensity on nitrate uptake by natural populations of phytoplankton was examined by ¹⁵N traceruptake experiments during the spring (March–May 1987) in Auke Bay, Alaska. The data were fit to a rectangular hyperbolic model which included a term for dark uptake. Three types of curves described nitrate uptake as a function of light intensity. The first (Type I) had a low half-saturation light intensity (K _I), low chlorophyll-specific uptakes rates, no dark uptake and occasional photoinhibition. These were observed during a period of biomass decrease, accompanied by low daily light and strong wind, prior to the major bloom. The second type (Type II) had relatively high K _I, high chlorophyll-specific uptake rates, and no dark uptake. Type II curves were observed during most of the period prior to nitrate depletion in the surface waters. Types I and II both appeared prior to nitrate depletion in the water and reflected variations in the light history of the phytoplankton population. The third type (Type III) occurred in nitrate-deplete conditions, when nitrate uptake was less dependent on light intensity (i.e., high rates of dark uptake and lower K _I). Decreased light-dependency during this period was coupled with physiological nitrogen deficiency in the population. Comparing these parameters to those of photosynthetic carbon fixation, K _Ivalues of nitrate uptake were generally higher than those of photosynthesis prior to nitrate depletion, and lower during nutrient-deplete conditions.     

Kinetics of light-intensity adaptation in a marine planktonic diatom

The marine planktonic diatom Thalassiosira weisflogii was grown in turbidostat culture under both continuous and 12 hL: 12 hD illumination regimes in order to study the kinetics of adaptation to growth-irradiance levels. In both illumination regimes adaptation to a higher growth-irradiance level was accompanied by an increase in cell division rates and a decrease in chlorophyll a cell^-1. The rates of adaptation for both processes, derived from first order kinetic analysis, equaled each other in each experiment. The results suggest that during the transition from low-to-high growth-irradiance levels chlorophyll a is diluted by cell division and is not actively degraded. Introduction of a light/dark cycle lowered the rate of adaptation. In transitions from high-to-low growth-irradiance levels there was a sharp drop in growth rates and a slow increase in chlorophyll a cell^-1 under both continuous and intermittent illumination. In the 12 hL:12hD cycle there was a circadian rhythm in chlorophyll a cell^-1, where cellular chlorophyll contents increased during the light cycle and decreased during the dark cycle. This circadian rhythm was distinctly different from light intensity adaptation. For kinetic analysis of light intensity adaptation in a 12 hL: 12 hD cycle, the circadian periodicity was separated from the light intensity response by subjecting the data to a Kaiser window optimization digital filter. Kinetic parameters for light-intensity adaptation were resolved from the filtered data. The kinetics of lightintensity adaptation of marine phytoplankton are discussed in relation to their spatial variations and time scales of mixing.This research was performed at Brookhaven National Laboratory under the auspices of the United States Department of Energy under Contract No. DE-AC02-76 CH00016     

Effects of simulated current on the growth rate and nitrogen metabolism of Gracilaria tikvahiae (Rhodophyta)

In a series of multifactorial laboratory experiments, Gracilaria tikvahiae apical segments were grown in an apparatus in which they were exposed simultaneously to 3 simulated current speeds (7.5, 15, 22.5 cm s^-1) and a still control, and either 3 ammonium concentrations (<6, 37–39, and 119–136 M) under ample uniform light (ca 200 E m^-2 s^-1) or 3 light intensities (ca 35, 90, and 270 E m^-2 s^-1) with uniform surplus ammonium. Growth rates of apical segments were determined in each experiment as well as nitrogen and carbon composition of tissues and fluxes of NH₄, NO₃/NO₂, and PO₄ in media. In a supplementary series of field experiments, apical segments of G. tikvahiae weresimultaneously exposed to 2 different regimes of water motion in adjacent chambers at several sites characterized by widely different ammonium regimes. The application of simulated current significantly enhanced growth rates in all experiments which utilized recently collected plants. Generally, this enhancement was fully realized at 7.5 cm s^-1, with growth rates tending to plateau above that speed. Growth enhancement by simulated current was independent of ammonium concentration and was considerably reduced at the lowest light intensity. In experiments conducted with plants that had been maintained for several months in aquaria, simulated current failed to enhance growth rates. This suggested that growth responses were at least partly a function of prior conditioning. Growth rates were a direct function of light intensity and an inverse function of ammonium concentration, indicating ammonium inhibition at the higher applied concentrations. Simulated current slightly enhanced rates of ammonium uptake but this did not consistently result in reduced C:N ratios, suggesting that the growth-stimulating effect of relative water motion was attributable to factors other than N uptake. There was evidence of luxury consumption of ammonium. In field experiments, growth rates were not significantly related to exposure to water motion.