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.     

Effects of dissolved ammonium addition and host feeding with Artemia salina on photoacclimation of the hermatypic coral Stylophora pistillata

 Effects of nutrient treatments on photoacclimation of the hermatypic coral Stylophora pistillata (Esper) were studied. Studies on photoacclimation of colonies from different light regimes in the field were evaluated and used to design laboratory experiments. Coral colonies were collected in the Gulf of Eilat (Israel) from January to March 1993. Exterior branches of colonies from different depths (1 to 40 m) displayed different trends in production characteristics at reduced and very low levels of illumination. From 24 ± 3% to 12 ± 2% of incident surface photosynthetic active radiation (PAR_o), zooxanthella population density and chlorophyll a+c per 10⁶ zooxanthellae increased, a trend seen in the range of light levels optimal for coral growth (90 to 30% PAR_o). The P _max of CO₂ per 10⁶ zooxanthellae decreased, while P _max of CO₂ per 10³ polyps increased, indicating an increase in zooxanthella population density at low light levels. Proliferous zooxanthella frequency (PZF, a measure of zooxanthella division) declined significantly at light levels <18 ± 3% PAR_o. At the lowest levels of illumination (<5% PAR_o), zooxanthella population density decreased, as did the PZF; chl a+c per 10⁶ zooxanthellae was unchanged. In 28-d experiments, exterior coral branches from the upper surfaces of colonies from 3 m depth (65 ± 4% PAR_o) were incubated in aquaria under bright (80 to 90% PAR_o), reduced (20 to 30% PAR_o), and extremely low (2 to 4% PAR_o) light intensities. At each light intensity, the corals were maintained in three feeding treatments: sea water (SW); ammonium enriched SW (SW + N); SW with Artemia salina nauplii (SW + A). An increase in P _max of CO₂ per 10³ polyps was found in corals acclimated to reduced light (20 to 30% PAR_o) in nutrient-enriched SW, while in SW, where the increase in zooxanthella population density was smaller, it did not occur. Nutrient enrichments (SW + N at 2 to 4% PAR_o and SW + A at 20 to 30% PAR_o) increased zooxanthella population density, but had no effect on chl a+c per 10⁶ zooxanthellae. Acclimation for 14 d to reduced (10 to 20% PAR_o) and extremely low (1 to 3% PAR_o) light intensities shifted ¹⁴C photoassimilation into glycerol and other compounds (probably glycerides), rather than sugars. Both ammonium addition and feeding with Artemia salina nauplii resulted in an increase in photosynthetic assimilation of ¹⁴C into amino acids. We conclude that acclimation to reduced light consists of two processes: an increase in photosynthetic pigments and in zooxanthella population density. Both processes require nitrogen, the increase in zooxanthella population density needing more; this adaptation is therefore limited in nitrogen-poor sea water. Received: 19 June 1998 / Accepted: 13 June 2000     

Light absorption and utilization among hermatypic corals: a study in Jamaica,West Indies

The chlorophyll specific absorption coefficient ( _c) was measured for zooxanthellae from six hermatypic coral species obtained, where possible, from four depths (1, 10, 30, 50 m) on reef sites near Discovery Bay, Jamaica in February and March 1983. Measurements of photosynthetic rates versus irradiance, as well as cellular and areal chlorophyll a, were also performed on these colonies or sister colonies. Together the data were used to compare minimum quantum requirements (1/Φ m) among species and depths and to assess the importance of light utilization to the growth and depth distribution of these corals. Our data suggest that, although _c was found to decrease with depth, interspecific differences in _c do not occur for zooxanthellae from the corals investigated. Minimum quantum requirements (1/Φ m) decreased significantly with depth, thereby reflecting an increase in photosynthetic light utilization efficiency with decreasing irradiance. Interspecific differences in 1/Φ m determinations were suggested but not statistically conclusive. We conclude that interspecific differences in gross photosynthesis, and perhaps growth and depth distribution, are primarily attributable to differences in the light utilization capacity of the whole coral, as reflected by the product of _c ^′ and chlorophyll per unit surface area, and in-situ quantum efficiencies. This research was performed under the auspices of the US Department of Energy under Contract No. DE-AC02-76CH00016     

Interplay of host morphology and symbiont microhabitat in coral aggregations

The Belizean reef coral Agaricia tenuifolia Dana forms aggregations in which rows of thin, upright blades line up behind each other. On average, the spacing between blades increases with depth and hence with decreasing ambient irradiance. We designed and built a small, inexpensive light meter and used it to quantify the effect of branch spacing on light levels within colonies at varying distances from branch tips. Concurrently, we measured photosynthetic pigment concentrations and population densities of symbiotic dinoflagellates (zooxanthellae) extracted from coral branches of colonies with tight (≤3 cm) vs wide (≥6 cm) branch spacing, collected at 15 to 17 m and from colonies with tight branch spacing collected at 1 to 2 m. Light levels decreased significantly with tighter branch spacing and with distance from the branch tips. Total cellular pigment concentrations (chlorophylls a, c ₂ and peridinin) as well as chlorophyll a:c ₂ and chlorophyll a: peridinin ratios all increased significantly with distance from the branch tip, indicating very localized differences in photoacclimation within individual branches. Zooxanthellae from colonies with widely-spaced branches displayed significantly lower chlorophyll a:c ₂ and chlorophyll a:peridinin ratios, and were present at significantly higher population densities than those from colonies with tightly-spaced branches collected at the same depth (15 m). Tightly-spaced colonies collected from shallow environments (1 to 2 m) displayed pigment ratios similar to those from widely-spaced colonies from deeper water (15 m), but maintained zooxanthellae populations at levels similar to those in tightly-branched colonies from deeper water. Thus, variation in colony morphology (branch spacing and distance from branch tip) can affect symbiont physiology in a manner comparable to an increase of over 15 m of water depth. These results show that a host's morphology can strongly determine the microhabitat of its symbionts over very small spatial scales, and that zooxanthellae can in turn display steep gradients in concordance with these altered physical conditions. Received: 12 June 1997 / Accepted: 24 June 1997     

Floristic and physiological differences between the shallow and the deep nanophytoplankton community in the euphotic zone of the open tropical Atlantic revealed by HPLC analysis of pigments

Suspended matter sampled in 1982 in the North Equatorial Current, in the open Atlantic to the west of West Africa, was analyzed by high performance liquid chromatography. The pigment fingerprint of samples taken in the surface mixed layer was dominated by zeaxanthin and chlorophyll a, in agreement with observed dominance of coccoid cyanobacteria. Near the bottom of the euphotic zone the fingerprint was more complicated, with a sharp transition at the depth of the deep chlorophyll maximum layer to dominance of chlorophyll b, 19-hexanoyloxyfucoxanthin and an unknown fucoxanthin derivative in the lower part of this layer; this fingerprint suggests dominance of eukaryotes (green algae, Prymnesiophyceae and Chrysophyceae) at depth. Up to 90% of the chl a was contained in particles smaller than 8 m, and in the surface mixed layer even more than 50% in particles smaller than 1 m. The high concentration of zeaxanthin relative to chl a near the surface suggests adaptation of the cyanobacteria to exposure to high irradiance. Evidence of this adaptation was the very high specific phytoplankton growth rate between sunrise and sunset (=0.16 h^-1), measured by recording ¹⁴C incorporation into organic carbon and into chl a carbon after isolation of the latter by HPLC. The high concentration of chl b relative to chl a at depth was possibly caused by shade-adapted green algae containing more chl b than chl a. The specific growth rate of the deep shade community was low (<0.04 h^-1), yet net primary production, calculated on the basis of chl a increase during incubation, was greatest at depth.     

Daily rhythm of O2-evolution in the cyanobacterium Trichodesmium thiebautii under natural and constant conditions

The rate of oxygen evolution by the tropical marine cyanobacterium Trichodesmium thiebautii was recorded at different times during the day in samples collected in 1992 from the Bahama Islands and the NE Caribbean Sea. This cyanobacterium is unique in that it is the only non-heterocystous diazotroph capable of N₂-fixation in daylight. Oxygen evolution was measured under conditions of natural day/night (LD, N=50), constant light (LL, N=14), and constant dark (DD, N=2×14). Photosynthesis vs intensity (P-I) relationships were calculated at various times of day, and the following parameters were used for further evaluation: photosynthesic capacity (P _max, 66 to 91 mg O₂ mg chl a ^-1 h^-1), initial slope of the P-I curve (, 0.23 to 0.27), dark respiration (R, 12 to 27 mg O₂ mg chl a ^-1 h^-1), and the intensity at which O₂ consumption is compensated by O₂ production (I _c, 78 to 160 Em^-2 s^-1). All means showed large standard deviations (for some parameters more than 200%). In some cases, these variations could be explained with a sinusoidal 24-h time course, but only the compensation point showed a significant daily variation (p0.001) in both LD and DD. The fact that the time course of I _c typical for natural conditions remains rhythmic under constant dark conditions strongly suggests a circadian regulation. Few circadian rhythms have been observed in prokaryotes, and this appears to be the first observation of such a rhythm in a cyanobacterium which fixes N₂ in daytime.     

Light harvesting by wavelength transformation in a symbiotic coral of the Red Sea twilight zone

We report an extraordinary depth range for Leptoseris fragilis (Milne Edwards and Haime), a reef building coral of the Red Sea living in cytosymbiosis with zooxanthellae. The coral harbours an as yet unknown pigment system. We suggest that the heterotrophic host — the coral — provides its photoautotrophic symbionts with additional light. The supplementary light is provided by host pigments which transform light of short wavelengths into suitable wavelengths for photosynthesis, thus amplifying and increasing the transfer of photoassimilates from the zooxanthellae to the host.     

Light absorption,photosynthesis and growth of Nannochloris atomus in nutrient-saturated cultures

Nannochloris atomus was maintained in exponential growth at photon flux densities (PFD) from 400 to 700 nm, ranging from 10 to 200 mol m^-2 s^-1. Growth was lightsaturated at PFDs in excess of 100 mol m^-2 s^-1, with a mean light-saturated growth rate at 23 °C of 1.5×10^-5s^-1 (1.2 d^-1). The light-limited growth rates extrapolated to a compensation PFD for growth that was not significantly different from zero, although no changes in cell numbers were observed in a single culture incubated at a PFD of 1.0 mol m^-2s^-1. Dark-respiration rates were independent of PFD, averaging 1.7×10^-6 mol O₂ mol^-1 C s^-1 (0.14 mol O₂ mol^-1 C d^-1). The maximum photon (quantum) efficiency of photosynthesis was also independent of PFD, with a mean value of 0.12 mol O₂ mol^-1 photon. The chlorophyll a-specific light absorption cross-section ranged from 3 to 6×10^-3 m² mg^-1 chl a and was lowest at low PFDs due to intracellular self-shading of pigments associated with high cell-chlorophyll a contents. The C:chl a ratio increased from 10 to 40 mg C mg^-1 chl a between PFDs of 14 and 200 mol m^-2 s^-1. These new observations for N. atomus are compared with our previous observations for the diatom Phaeodactylum tricornutum in terms of an energy budget for microalgal growth.     

Pigment content and photosynthetic rate of the fronds of Macrocystis pyrifera

The Macrocystis pyrifera (L.) C. Ag. frond is here described in terms of chlorophyll a, fucoxanthin, chlorophyll c and photosynthetic rate. Pigment concentrations increased back from the apical meristem reaching a maximum after 2 to 3 m. Pigment concentrations were then generally constant throughout most of the length of the frond, finally decreasing again in the oldest parts of the frond with the exception of the sporophylls. Pigment ratios remained relatively constant throughout. Maximum net photosynthetic rates on a given frond showed a decrease with tissue age on both an area basis (1040 down to 463 nmol O₂ cm^-2 h^-1) and on a chlorophyll a basis, which was shown as half-saturation constants (quantum irradiance) which dropped on an area basis from 85 mol m^-2 sec^-1 at 4.5 m above the holdfast to 26 mol m^-2 sec^-1 at 15.5 m. Young sporophytes transplanted from the sea floor to the surface (12 m) tended to decrease pigment content, while those transplanted to the bottom tended to increase all pigments, but especially fucoxanthin. Photosynthetic rates, however, changed little on a unit area basis. The results of these data are considered in the light of recent work on photosynthetic units, tissue age effects and general adaptations of the M. pyrifera frond to its light environment.     

A chromatophore system in the hermatypic,deep-water coral Leptoseris fragilis (Anthozoa: Hexacorallia)

Ultrastructural evidence is presented of a chromatophoresystem in the zooxanthellae containing hermatypic, deep-water coral Leptoseris fragilis (Milne Edwards and Haime). It consists of multilobed cells which mainly occupy the intercellular space of the oral gastrodermis. The cellular processes are filled with electron-dense granules up to 1-m-long and 0.5-m-wide. Within the cytoplasm an elaborate system of microtubules is established. The ramifications of the pigment cells, containing the pigment granules, form a dense and nearly continuous layer close to the overlying zooxanthellae. It is speculated that host pigments may transform the violet portion of the incident light into longer wavelengths, thus increasing the photosynthetic efficiency of the zooxanthellae.     

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.     

The ammonium/methylammonium uptake system of Symbiodinium microadriaticum

The substrate analogue [¹⁴C]-methylammonium was used to study ammonium/methylammonium uptake by Symbiodinium microadriaticum (zooxanthellae). The value of the Michaelis constant (K _m) for the uptake system was approximately 35 M with methylammonium as substrate; ammonium was a competitive inhibitor of methylammonium uptake, and the K _m for ammonium uptake (determined as the inhibition constant, K _i, for methylammonium) was 6.6 M. Methylammonium uptake by zooxanthellae was light-dependent. Methylammonium uptake rates of zooxanthellae which had been freshly isolated from the hermatypic coral Acropora formosa (0.85±0.05x10^-10 mol min^-1 cell^-1) were lower than those of axenic cultures of the zooxanthellae from Montipora verrucosa (Acroporidae) grown under various nitrogen regimes (1.6 to 12x10^-10 mol min^-1 cell^-1). Maximum uptake rates were found for ammonium-starved cultured M. verrucosa zooxanthellae (10.2 to 12x10^-10 mol min^-1 cell^-1); M. verrucosa zooxanthellae growing with ammonium as nitrogen source and zooxanthellae which had been freshly isolated from A. formosa gave similar and considerably lower uptake rates (0.85 to 1.6x10^-1 mol min^-1 cell^-1). These results suggest that either coral tissue contains sufficient ammonium to repress synthesis of the uptake system of the algal symbionts or, alternatively, there are additional barriers to ammonium transport for zooxanthellae in vivo.     

Nutrient limitation in the giant clam-zooxanthellae symbiosis: effects of nutrient supplements on growth of the symbiotic partners

The effect of ammonium (5, 10 M N) and phosphate (2, 5, 10 M P) on the growth of the giant clam Tridacna gigas and its symbiotic dinoflagellate Symbiodinium sp. was examined. A 3 mo exposure to these nutritients significantly increased the N or P composition of the soft tissues, as reflected in a corresponding change in C:N:P ratio. Furthermore, exposure to N or N+P markedly increased the amount of soft tissue, but P alone did not, demonstrating that increased availability of inorganic nitrogen enhances tissue growth of the clam host. With addition of N, or N+P, there was a significant increase in the total number of zooxanthellae per clam, with a corresponding decrease in chlorophyll a (chl a) content per zooxanthella. However, only with N+P was there an increase in the zooxanthellae mitotic index. The inverse relationship between zooxanthellae number and chl a per zooxanthella is consistent with phytoplankton studies indicating conditions of nutrient-limitation. Furthermore, the unaffected C:N:P composition of the zooxanthellae and their relatively low specific-growth rates (4 to 10%) also suggest that they are nutrient-limited in vivo. In particular, their high mean C:N:P ratio of 303:52:1 indicates that, relative to C, they are much more depleted in P and less in N than are free-living phytoplankton. Furthermore, polyphosphates (phosphate reserves) were undetectable, and the activity levels of acid phosphatase in the zooxanthellae were relatively high and not influenced by the host's exposure to increased P concentrations in the sea water, implicating the clam host in active regulation of P availability to its symbiotic algae. This is strong evidence that N-limitation of clam zooxanthellae is a function of the availability of ammonium to the symbiosis while, irrespective of nutrient levels in sea water, clam zooxanthellae still show characteristics of P-limitation.     

A comparative study of photoadaptation in four diatoms isolated as endosymbionts from larger foraminifera

Four endosymbiotic diatoms were isolated from 2 species of larger foraminifera collected in the Red Sea and Hawaii. The photoadaptive responses of the cultured diatoms were measured at 312, 19 and 7 W cm^-2. Two of the diatoms (Fragilaria shiloi and Nitzschia laevis), both isolated from Amphistegina lessonii, grew fastest at 312 W cm^-2. The other two diatoms (N. valdestriata and N. panduriformis) which were isolated from Heterostegina depressa, grew best at 19 W cm^-2. Of the four diatoms, F. shiloi grew best at high light levels. Also in F. shiloi, chlorophyll c content per cell was directly proportional to light intensity; in contrast chlorophyll a and carotenoids increased to maxima at 19 W cm^-2. The chlorophyll a and c and carotenoid content of N. valdestriata were also maximal at 19 W cm^-2. Photosynthetic rates, measured by respirometry, suggested that the diatoms were photoinhibited at higher light intensities and did well at moderately low light intensities (175W cm^-2). The photocompensation points of all 4 diatoms were about 2% of the light available in the spring at 1-m depth at Elat on the Red Sea. At Elat the photocompensation point would lie between 40 and 50 m if the algae were free in nature. The amount of attenuation of light by the shells of the host has not yet been measured. Presumably photocompensation of the algae within hosts is reached at depths less than 40 m.     

Enhancement of chlorophyll a production in Gulf Stream surface seawater by synthetic versus natural rain

Rainwater concentrations of either ammonium or nitrate were sufficient to stimulate chlorophyll a (chl a) production in bioassay experiments using Gulf Stream surface water collected off North Carolina during the summer of 1991. Previous studies primarily examined inshore waters and did not address the impact of rainwater ammonium. An increase in chl a occurred within 1 d of the addition of synthetic rainwater (2 or 5% rainwater, 98 or 95% seawater) containing up to 10 M ammonium; this increase was followed by a decrease in chl a the following day. A similar response to nitrate addition (5% addition of 20 M nitrate rain) was observed. In separate experiments, natural rainwater having nitrate and ammonium concentrations less than those in the experimental synthetic rain yielded a greater chl a response than synthetic rain when added at similar dilutions (0.5 to 5.0% rain). The maximum dissolved inorganic nitrogen concentration in the enriched seawater in these bioassays was 1.8 M; prior to enrichment the maximum was < 0.4 M. Bioassay experiments begun 2 d after a major storm event (sustained NE winds with gusts to 13 m s^-1 and ca. 390 mol m^-2 inorganic nitrogen deposition from rain) showed a chl a increase in response to addition of natural rainwater, but not to synthetic rainwater with similar dissolved inorganic nitrogen concentration. These results suggest that phytoplankton stimulants, in addition to nitrate and ammonium, exist in natural rain but not in the synthetic rain used in these experiments.     

Resource partitioning by reef corals as determined from stable isotope composition

The pattern of resource partitioning vs depth by corals collected in February 1983 from Jamaica and the Red Sea was determined from their stable carbon isotope composition. Observations were made on isolated zooxanthellae and corresponding algae-free animal tissue from eight species at four depths over a 50 m bathymetric range. Zooxanthellae ¹³C was high in shallow water and became lower as depth increased. This trend correlated significantly with the anual integrated photosynthetic rate. The trend is interpreted according to a depletion-diffusion hypothesis; in shallow water, at high rates of photosynthesis, metabolic CO₂ is nearly depleted and the supply of CO₂ from seawater bicarbonate is limited by diffusion. Since most of the available CO₂ is fixed, isotope fractionation is minimal. In deeper water, at lower rates of photosynthesis, metabolic CO₂ is ample, and isotope fractionation is greater. Animal tissue ¹³C was slightly lower than corresponding zooxanthellae values in shallow water. As depth increased the difference between zooxanthellae and animal tissue ¹³C increased and the latter approached the ¹³C of oceanic particulate organic carbon. These data suggest that carbon is translocated at all depths and that deep-water corals draw significantly on allocthonous sources of carbon.Contribution No. 436 of the Discovery Bay Marine Laboratory of the University of the West Indies     

Ecotypic differentiation in the marine diatom Skeletonema costatum: influence of light intensity on the photosynthetic apparatus

Three genetically distinct clones of Skeletonema costatum (Grev.) Cleve were grown at 20°C under high (274 E m^-2 s^-1) and low (27 E m^-2 s^-1) light conditions and their photoadaptive photosynthetic responses compared. When all three clones were grown under low light, pigment analyses and fluorescence excitation spectra demonstrated that the accessory pigments, chlorophyll c and fucoxanthin, became more important in light-harvesting compared to chlorophyll a. Photosynthetic unit sizes increased for Photosystems I and II in low light, but photosynthesis vs irradiance characteristics were not reliable predictors of photosynthetic unit features. Fluorescence excitation spectra and photosynthesis vs irradiance (P-I) relationships indicated that changes in energy transfer occurred independent of changes in pigment content. Large increases in accessory pigment content were not accompanied by large increases in excitation from these pigments. Changes in energy transfer properties were as important as changes in PSU size in governing the photoadaptive responses of S. costatum. When the three clones were grown under identical conditions, each had a separate and distinct pattern of photoadaptation. Significant differences among clones were found for pigment ratios, photosynthetic unit sizes for Photosystems I and II and efficiency of energy transfer between pigments. These strikingly different photoadaptive strategies among clones may partially account for the great ecological success of the diatom species. This is the first quantitative investigation of the importance of both chlorophyll c and fucoxanthin to the adaptive responses of diatoms to light intensity, and represents the most complete characterization of the photoadaptive responses of a single species of marine phytoplankter to differences in light environment.     

Seasonal study of phytoplankton pigments and species at a coastal station off Sydney: Importance of diatoms and the nanoplankton

Phytoplankton pigments and species were studied at a coastal station off Sydney (New South Wales, Australia) over one annual cycle. Sudden increases in chlorophyll a (up to 280 mg m^-2), due to short-lived diatom blooms, were found in May, July, September, January and February. These were superimposed upon background levels of chlorophyll a (20 to 50 mg m^-2), due mostly to nanoplankton flagellates, which occurred throughout the year. The nanoplankton (<15 m) accounted for 50 to 80% of the total phytoplankton chlorophyll, except when the diatom peaks occurred (10 to 20%). The annual cycle of populations of 16 dominant species-groups was followed. Possible explanations as to alternation of diatom-dominated and nanoplankton-dominated floras are discussed. Thin-layer chromatography of phytoplankton pigments was used to determine the distribution of algal types, grazing activity, and phytoplankton senescence in the water column. Chlorophyll c and fucoxanthin (diatoms and coccolithophorids) and chlorophyll b (green flagellates) were the major accessory pigments throughout the year, with peridinin (photosynthetic dinoflagellates) being less important. Grazing activity by salps and copepods was apparent from the abundance of the chlorophyll degradation products pheophytin a (20 to 45% of the total chlorophyll a) and pheophorbide a (10 to 30%). Chlorophyllide a (20 to 45%) was associated with blooms of Skeletonema costatum and Chaetoceros spp. Small amounts of other unidentified chlorophyll a derivatives (5 to 20%) were frequently observed.     

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.     

Photosynthesis-irradiance responses and photosynthetic periodicity in the sea anemone Aiptasia pulchella and its zooxanthellae

Sea anemones (Aiptasia pulchella) containing zooxanthellae (Symbiodinium microadriaticum) were maintained in a long-term laboratory culture on a 12 h light (100 E m^-2 s^-1):12 h dark cycle. Photosynthetic oxygen production was measured for the symbiotic association and for freshlyisolated zooxanthellae. Light utilization efficiencies () were similar for both sets of zooxanthellae, suggesting negligible shading of zooxanthellae by animal tissue in this association. Whereas freshly-isolated zooxanthellae were photoinhibited at high irradiances (800 to 1 800 E m^-2 s^-1), zooxanthellae in the host continued to function at photosynthetic capacity. Time of day may influence photosynthetic measurements in symbiotic organisms, as it was found that photosynthesis in A. pulchella followed a diel periodicity at both light-saturating (1 200 E m^-2 s^-1) and subsaturating (150 E m^-2 s^-1) irradiances. There was a peak period of photosynthesis between 12.00 and 14.00 hrs. Light stimulated dark respiration rates of A. pulchella. Dark respiration of sea anemones increased somewhat towards the end of the light cycle and was always greater after exposure to high irradiances.