CHANGES IN THE ULTRASTRUCTURE OF SYMBIOTIC ZOOXANTHELLAE (SYMBIODINIUM SP., DINOPHYCEAE) IN FED AND STARVED SEA ANEMONES MAINTAINED UNDER HIGH AND LOW LIGHT1

The ultrastructure of symbiotic dinoflagellates (Symbiodinium sp., zooxanthellae) in the sea anemone Aiptasia pallida Verrill was examined in well-fed or starved (up to 120 days) anemones maintained under two light levels (5 and 50 μmol · m^?2· s^?1). Cell size of zooxanthellae was not affected by feeding history; however, both light and feeding history affected the relative cell volume of chloroplasts, lipids, and vacuoles. Stereological analysis of transmission electron micrographs showed that algae in low-light starved anemones had 10 times as much lipid (17.4% of cell volume) as those in well-fed anemones under the same light conditions (1.8%). The lipid content of algae from anemones in high light increased from 15.4% in well-fed anemones to 30.1% in starved anemones. The starch content of zooxanthellae in low-light anemones was law (4.1%) and not affected by feeding history, while the starch content of zooxanthellae in high-light anemones was greater (10.7%), with some differences among groups. Algal photoacclimation to low light included an increase in chloroplast relative volume from 17% (in well-fed high-light anemones) to 33% in well-fed low-light anemones. Starvation of the host resulted in a significant decrease in chloroplast volume in zooxanthellae in anemones at both light levels. Morphometry provides quantitative confirmation of biochemical and physiological data on zooxanthellae, because the changes in zooxanthellae with starvation of the host are consistent with other indicators of nutrient limitation of zooxanthellae of A. pallida held without food for long periods of time.     

Temperate and tropical algal-sea anemone symbioses

Abstract. In this review, we seek to develop new insights about the nature of algal‐sea anemone symbioses by comparing such associations in temperate and tropical seas. Temperate seas undergo pronounced seasonal cycles in irradiance, temperature, and nutrients, while high irradiance, high temperature, and low nutrients are seasonally far less variable in tropical seas. We compare the nature of symbiosis between sea anemones (= actinians) and zooxanthellae (Symbiodinium spp.) in both regions to test tropical paradigms against temperate examples and to identify directions for future research. Although fewer anemone species are symbiotic in temperate regions, they are locally dominant and ecologically important members of the benthic community compared to the tropics. Zooxanthella densities tend to be lower in temperate anemones, but data are limited to a few species in both temperate and tropical seas. Zooxanthella densities are far more stable over time in temperate anemones than in tropical anemones, suggesting that temperate symbioses are more resistant to fluctuations in environmental parameters such as irradiance and temperature. Light‐saturated photosynthetic rates of temperate and tropical zooxanthellae are similar, but temperate anemone hosts receive severely reduced carbon supplies from zooxanthellae during winter months when light is reduced. Symbiont transmission modes and specificity do not show any trends among anemones in tropical vs. temperate seas. Our review indicates the need for the following: (1) Investigations of other temperate and tropical symbiotic anemone species to assess the generality of trends seen in a few “model’ anemones. (2) Attention to the field ecology of temperate and tropical algal‐anemone symbioses, for example, how symbioses function under seasonally variable environmental factors and how zooxanthellae persist at high densities in darkness and winter. The greater stability of zooxanthella populations in temperate hosts may be useful to understanding tropical symbioses in which bleaching (loss of zooxanthellae) is of major concern. (3) Study of the evolutionary history of symbiosis in both temperate and tropical seas. Continued exploration of the phylogenetic relationships between host anemones and zooxanthella strains may show how and why zooxanthellae differ in anemone hosts in both environments.     

Chemosensory and feeding responses of the nudibranch Aeolidia papillosa to the symbiotic sea anemone Anthopleura elegantissima

Abstract. The aeolid nudibranch Aeolidia papillosa is an important predator on the sea anemone Anthopleura elegantissima , a host to two kinds of endosymbiotic algae: zooxanthellae and zoochlorellae. The possible influence of the algae on the nudibranch's predatory response to this anemone was examined in a laboratory study. In chemosensory experiments, the nudibranch detected and chose anemone scent over a seawater control, but in both chemosensory and feeding experiments showed no preference for zooxanthellate or zoochlorellate anemones. Ingestive conditioning on zooxanthellate or zoochlorellate anemones had no effect on choice of these two anemone types in chemosensory experiments. Comparisons of the productivity and photosynthetic pigments of algae obtained from nudibranch feces and from anemones show that both algae survive passage through the nudibranch gut. The productivity of fecal zooxanthellae was 1.6X greater than that of zooxanthellae freshly isolated from anemones, although the chlorophyll a content of fecal zooxanthellae was reduced. The productivity and amount of pigments were the same for zoochlorellae in nudibranch feces and freshly isolated from anemones. Comparing fecal and isolated algae, there was no significant difference in the percentage of zooxanthellae in the process of cell division. However, the percentage of dividing cells was 2.6X higher in fecal than in freshly isolated zoochlorellae (18% and 6.9% respectively). Although the endosymbiotic algae do not make their host more or less attractive to the nudibranch, this predator may play an important role in maintaining the symbiotic relationship of Anthopleura elegantissima with zooxanthellae and zoochlorellae by providing viable algae in its feces as a source for the anemone host.     

The role of symbiotic dinoflagellates in the temperature-induced bleaching response of the subtropical sea anemone Aiptasia pallida

Coral bleaching involves the loss of symbiotic dinoflagellates (zooxanthellae) from reef corals and other cnidarians and may be a stress response of the host, algae or both. To determine the role of zooxanthellae in the bleaching process, aposymbiotic sea anemones from Bermuda (Aiptasia pallida) were infected with symbionts from other sea anemones (Aiptasia pallida from Florida, Bartholomea annulata and Condylactis gigantea). The expulsion of algae was measured during 24-h incubations at 25, 32 and 34 degrees C. Photosynthetic rates of freshly isolated zooxanthellae were also measured at these temperatures. The C. gigantea (Cg) symbionts were expelled in higher numbers than the other algae at 32 degrees C. Photosynthesis by the Cg algae was completely inhibited at this temperature, in contrast to the other symbionts. At 34 degrees all of the symbionts had increased expulsion rates, and at this temperature only the symbionts from Florida A. pallida exhibited any photosynthesis. These results provide the first evidence that the differential release of symbionts from the same host species is related to decreased photosynthesis at elevated temperatures, and support other findings suggesting that zooxanthellae are directly affected by elevated temperatures during bleaching events.     

The influence of "host release factor" on carbon release by zooxanthellae isolated from fed and starved Aiptasia pallida (Verrill).

Symbiotic dinoflagellates (zooxanthellae) typically respond to extracts of host tissue with enhanced release of short-term photosynthetic products. We examined this "host release factor" (HRF) response using freshly isolated zooxanthellae of differing nutritional status. The nutritional status was manipulated by either feeding or starving the sea anemone Aiptasia pallida (Verrill). The release of fixed carbon from isolated zooxanthellae was measured using 14C in 30 min experiments. Zooxanthellae in filtered seawater alone released approximately 5% of photosynthate irrespective of host feeding history. When we used a 10-kDa ultrafiltrate of A. pallida host tissue as a source of HRF, approximately 14% of photosynthate was released to the medium. This increased to over 25% for zooxanthellae from anemones starved for 29 days or more. The cell-specific photosynthetic rate declined with starvation in these filtrate experiments, but the decline was offset by the increased percentage release. Indeed, the total amount of released photosynthate remained unchanged, or even increased, as zooxanthellae became more nutrient deficient. Similar trends were also observed when zooxanthellae from A. pallida were incubated in a 3-kDa ultrafiltrate of the coral Montastraea annularis, suggesting that HRF in the different filtrates operated in a similar manner. Our results support the suggestion that HRF diverts surplus carbon away from storage compounds to translocated compounds such as glycerol.     

The effects of feeding frequency and symbiosis with zooxanthellae on the biochemical composition of Astrangia Danae Milne Edwards & Haime 1849

The coral Astrangia danae Milne Edwards & Haime 1849 occurs naturally with and without symbiotic algae and thus may have two sources of nourishment: (1) particles captured by the coral polyps, and (2) photosynthetic products translocated from their zooxanthellae. Symbiotic colonies may have both sources, and nonsymbiotic ones certainly have only the former. The relative importance of these two food sources was studied in the laboratory by examining the tissues of corals fed with frozen brine shrimp. Stock corals were fed once per week. Two to three weeks prior to each experiment, selected corals were placed on one of three feeding schedules: starved (S), fed once per week (1/wk), and fed three times per week (3/wk). The coral tissues were analyzed for protein, lipid, carbohydrate, and zooxanthellae content. Increased feeding frequency (1/wk → 3/wk) resulted in an increased tissue biomass and lipid to protein (L/P) ratio; starvation (1/wk → S) caused a decrease in these parameters. Symbiosis with zooxanthellae had an effect similar to increased feeding frequency in that the S and 1/wk symbiotic corals had a higher L/P ratio than comparable nonsymbiotic ones. There were no significant differences in L/P ratios between the 3/wk symbiotic and nonsymbiotic corals. Freshly collected colonies had a tissue composition most similar to the laboratory animals fed 3/wk. This result is consistent with the hypothesis that ingestion of solid food is the major nutritional source for A. danae in Narragansett Bay, Rhode Island, but our experiments suggest that the algae can have an important effect on tissue L/P ratios during times of food scarcity.     

Seasonal variation in light-shade adaptation of natural populations of the symbiotic sea anemone Aiptasiapulchella (Carlgren, 1943) in Hawaii

The productivity and biomass parameters of the symbiotic anemone Aiptasia pulchella (Carlgren, 1943) from a shaded mangrove lagoon (maximum summer irradiance of 100 μE m⁻² · s⁻¹) and a sunlit reef flat (maximum summer irradiance of 1400 μE · m⁻² · s⁻¹) were examined in Hawaii. Light-shade adaptation was evident in the summer populations (1981) but not observed during the fall (1982). In the summer, zooxanthellae from the lagoon A. pulchella (shade anemones) contained 2.97 pg Chl a cell ⁻¹ and those from the reef flat (sun anemones) contained 1.70 pg Chl a · cell⁻¹; but Chl a : c₂ ratios were 2.5 in zooxanthellae from both shade and sun anemones. During the fall, there were no significant differences in Chl a and c₂ of zooxanthellae (2.25 pg Chl a · cell⁻¹) in shade and sun anemones, but Chl a : c₂ ratios averaged 3.9. During both seasons, shade anemones were larger and contained higher densities of zooxanthellae than sun anemones. In addition to differences between shade and sun habitats, there was localized photoadaptation of zooxanthellae within individual anemones due to microhabitat variations in ambient irradiance. Growth rates of zooxanthellae in A. pulchella differed in shade and sun anemones. Specific growth rates for zooxanthellae in situ were the same for shade populations in both summer and fall (0.016 day⁻¹). However, zooxanthellae in sun anemones grew four times faster in the fall (0.033 day⁻¹) than during the summer (0.008 day⁻¹). These results suggest that growth of zooxanthellae in these anemones was independent of ambient irradiance. Photosynthesis-irradiance (P-I) responses of shade and sun anemones during the summer showed that shade anemones had greater photosynthetic efficiencies (α) but lower photosynthetic capacities (P_max) than sun anemones. Dark-respiration rates of sun anemones were twice those obtained with shade anemones. In the fall, these populations of anemones did not exhibit P-I responses characteristic of light-shade adaptation. Both α and P_max of shade and sun anemones were higher in the fall, indicating that zooxanthellae in A. pulchella adapted to seasonal reduction in irradiance.     

BROAD THERMAL TOLERANCE OF THE SYMBIOTIC DINOFLAGELLATE SYMBIODINIUM MUSCATINEI (DINOPHYTA) IN THE SEA ANEMONE ANTHOPLEURA ELEGANTISSIMA (CNIDARIA) FROM NORTHERN LATITUDES1

The sea anemone Anthopleura elegantissima (Brandt) hosts two species of symbiotic dinoflagellates, known as zooxanthellae, which coexist within the host at southern latitudes only. One of these species, Symbiodinium muscatinei LaJeunesse et Trench, has a broad latitudinal distribution, occurring in intertidal anemones from Washington state to Southern California. To investigate whether high thermal tolerance contributes to the ability of S. muscatinei to inhabit anemones from northern and southern regions, the upper thermal tolerance limit for photosynthesis of symbionts in northern (48°24′ N) populations of A. elegantissima was determined by subjecting anemones to a gradual increase in temperature from 12°C to 30°C over a 10‐week period. Light‐saturated photosynthetic rates of isolated zooxanthellae were the same over the range of 12°C–24°C and declined significantly at 26°C, which is 14°C and 5°C above average summertime seawater temperatures in northern Puget Sound and Southern California, respectively. At 28°C, zooxanthellae isolated from the anemones, and those expelled by their hosts, exhibited extremely low rates of photosynthesis and highly reduced chl content. The photosynthetic rates and chl content of expelled zooxanthellae were lower than those of retained zooxanthellae. The high thermal tolerance of S. muscatinei isolated from northern populations of anemones supports the broad latitudinal distribution of this symbiont, allowing it to coexist with S. californium (#383, Banaszak et al. 1993 ) in southern populations of anemones.     

Sedimentary nitrogen uptake and assimilation in the temperate zooxanthellate sea anemone Anthopleura aureoradiata

The sea anemone Anthopleura aureoradiata (Carlgren), which harbours symbiotic dinoflagellates (zooxanthellae), is abundant on mudflats and rocky shores around New Zealand. We measured the potential for particulate nitrogen uptake from sediment by A. aureoradiata and the subsequent consequences of this uptake on the nitrogen status of its zooxanthellae. Sediment was rinsed, labelled with (¹⁵NH₄)₂SO₄, and provided to anemones at low (0.23 g ml^− 1) and high (1.33 g ml^− 1) sediment loads for 6 h. Both anemone tissues and zooxanthellae became enriched with ¹⁵N. Enrichment of anemone tissues was similar at both high and low sediment loads, but the zooxanthellae became more enriched at the lower load. This was presumably because the uptake of ammonium, arising from host catabolism, by zooxanthellae is light driven and because the anemones at the lower load were able to extend their tentacles into the light while those at the higher load were not. The influence of sediment uptake on the nitrogen status of the zooxanthellae was determined by measuring the extent to which 20 μM NH₄⁺ enhanced the rate of zooxanthellar dark carbon fixation above that seen in filtered seawater (FSW) alone; the ammonium enhancement ratio (AER) was expressed as [dark NH₄⁺ rate/dark FSW rate], where ‘rate’ refers to C fixation and a ratio of 1.0 or less indicates nitrogen sufficiency. When anemones were starved with and without rinsed sediment in nitrogen-free artificial seawater for 8 weeks, zooxanthellar nitrogen deficiency became apparent at 2-4 weeks and reached similar levels in both treatments (AER = ~ 2). In contrast, anemones fed 5 times per week for 8 weeks with Artemia nauplii were nitrogen sufficient (AER = 1.03). In the field, zooxanthellae from mudflat anemones were largely nitrogen sufficient (AER = 1.26), while nitrogen deficient zooxanthellae were present in anemones from a rocky intertidal site (AER = 2.93). These results suggest that, while there was evidence for particulate nitrogen uptake, dissolved inorganic nitrogen (especially ammonium) in interstitial pore water may be a more important source of nitrogen for the zooxanthellae in mudflat anemones, and may explain the marked difference in nitrogen status between the mudflat and rocky shore populations.     

Ammonium metabolism in the symbiotic sea anemone Anemonia viridis

The temperate sea anemone Anemonia viridis (Forskål) forms an endosymbiotic association with dinoflagellate algae commonly referred to as zooxanthellae. It is now well established that under appropriate environmental conditions, these associations can be autotrophic for carbon. Under such conditions, many of these symbioses, including A. viridis, not only retain excretory ammonium, but can take up ammonium added to the surrounding seawater. The flux from inorganic to organic nitrogen will be via the free amino acid pools and in A. viridis these were found to be markedly different between zooxanthellae and host with glycine and taurine dominant in the latter. When anemones were maintained with 20 M ammonium, the concentration of free amino groups increased in the zooxanthellae but appeared not to change in the host. There was no evidence that the ratio of glutamine – glutamate in zooxanthellae changed when anemones were maintained with 20 M ammonium for 47 days. These ratios imply that zooxanthellae from this temperate symbiosis may not be nitrogen-limited. GDH was detected in both zooxanthellae and host where it was most active with the coenzyme NADPH. In addition, GDH showed activity when glutamine replaced ammonium as the substrate, indicating that the host may have alternative means to assimilate ammonium. Zooxanthellae were shown to possess GOGAT activity in the presence of a ferredoxin analogue. This suggests that in vivo zooxanthellae could assimilate ammonium via the activity of GS linked with ferredoxin-dependent GOGAT. Given evidence from other studies of rapid ammonium assimilation and essential amino acid synthesis in symbiotic host tissue, it appears that the capacity of cnidarians to metabolise nitrogen may at present be underestimated.     

Comparative gene expression in the symbiotic and aposymbiotic Aiptasia pulchella by expressed sequence tag analysis

Intracellular symbiotic relationships are prevalent between cnidarians, such as corals and sea anemones, and the photosynthetic dinoflagellate symbionts. However, there is little understanding about how the genes express when the symbiotic relationship is set up. To characterize genes involved in this association, the endosymbiosis between sea anemone, Aiptasia pulchella, and dinoflagellate zooxanthellae, Symbiodinium spp., was employed as a model. Two complementary DNA (cDNA) libraries were constructed from RNA isolated from symbiotic and aposymbiotic A. pulchella. Using single-pass sequencing of cDNA clones, a total of 870 expressed sequence tags (ESTs) clones were generated from the two libraries: 474 from symbiotic animal and 396 from aposymbiotic animal. The initial ESTs consisted of 143 clusters and 231 singletons. A BLASTX search revealed that 147 unique genes had similarities with protein sequences available from databases; 120 of these clones were categorized according to their putative function. However, many ESTs could not assign functionally. The putative roles of some of the identified genes relative to endosymbiosis were discussed. This is the first report of the use of EST analysis to examine the gene expression in symbiotic and aposymbiotic states of the cnidarians. The systematic analysis of EST from this study provides a useful database for future investigations of the molecular mechanisms involved in algal-cnidarian symbiosis.     

Nitrogen and photosynthetic function of hermatypic corals. Oxygen exchange of Stylophora pistillata coral under artificial feeding

The change of Stylophora pistillata coral photosynthetic function (oxygen exchange and biomass of symbionts) under starvation and food enrichment was studied to understand the role of heterotrophy in nitrogen supplements of zooxanthellae. The starvation caused the decrease of frequency of zooxanthellae cells division in 7-10 times. The number of degraded algae cells increased in same proportion and, as a result, the density of zooxanthellae in corals decreased about two times during one-two weeks. Under starvation corals kept their photosynthetic capacity at the level of corals in situ by means of enhancing the zooxanthellae gross photosynthesis. The respiration rate of coral had tendency to increase and the dry mass of polyp tissue to decrease. Under artificial feeding which was following starvation the zooxanthellae density increased in 1.5-2 times, and particular food caused more intensive accumulation of zooxanthellae comparing to dissolved inorganic ammonium. The feeding regime did not affect dry mass of polyp tissue and chlorophyll content as well as respiration and gross productivity of the corals. The conclusion about high effectiveness of particular feeding for supplying symbiotic algae with nitrogen was made and trophic status of zooxanthellae in hospite was determined as unlimited by nitrogen.     

Molecular cloning of Rab5 (ApRab5) in Aiptasia pulchella and its retention in phagosomes harboring live zooxanthellae

The intracellular association of symbiotic dinoflagellates (zooxanthellae) with marine cnidarians is the very foundation of the highly productive and diversified coral reef ecosystems. To reveal its underlying molecular mechanisms, we previously cloned ApRab7, a Rab7 homologue of the sea anemone Aiptasia pulchella, and demonstrated its selective exclusion from phagosomes containing live zooxanthellae, but not from those containing either dead or photosynthesis-impaired algae. In this study, Rab5 was characterized, due to its key role in endocytosis and phagocytosis acting upstream of Rab7. The Aiptasia Rab5 homologue (ApRab5) is 79.5% identical to human Rab5C and contains all Rab-specific signature motifs. Subcellular fractionation study showed that ApRab5 is mainly cytosolic. EGFP reporter and phagocytosis studies indicated that membrane-associated ApRab5 is present in early endocytic and phagocytic compartments, and is able to promote their fusion. Significantly, immunofluorescence study showed that the majority of phagosomes containing either resident or newly internalized live zooxanthellae were labeled with ApRab5, while those containing either heat-killed or photosynthesis-impaired algae were mostly negative for ApRab5 staining whereas the opposite was observed for ApRab7. We propose that active phagosomal retention of ApRab5 is part of the mechanisms employed by live zooxanthellae to: (1) persist inside their host cells and (2) exclude ApRab7 from their phagosomes, thereby, establishing and/or maintaining an endosymbiotic relationship with their cnidarian hosts.     

Biogeography and microhabitat variation in temperate algal-invertebrate symbioses: zooxanthellae and zoochlorellae in two Pacific intertidal sea anemones, Anthopleura elegantissima and A. xanthogrammica

Abstract. Temperate sea anemones in the genus Anthopleura are unique among cnidarians in harboring two phylogenetically distinct symbiotic algae, zooxanthellae (golden-brown dinophytes, Symbiodinium ) and zoochlorellae (green chlorophytes). To determine whether their physiological differences generate patterns in anemone habitat and biogeographic distribution, we sampled symbiotic algae in the small clonal A. elegantissima and the large solitary A. xanthogrammica at 8 field sites (and the other large solitary Anthopleura species at one site) spanning 18° of latitude along 2500 km of the Pacific coast of North America. We found that zoochlorellae predominate in low intertidal habitats and northerly latitudes and in A. xanthogrammica , while zooxanthellae constitute the majority of symbionts in high intertidal habitats and more southerly latitudes and in A. elegantissima. These data are consistent with published predictions based on photosynthetic efficiency of the two algae under varied temperature and light regimes in the laboratory. This anemone-algal system provides a potential biological signal of benthic intertidal communities' responses to El Niño events and long-term climate changes in the Pacific.     

Light intensity as a factor in the regulation of the density of symbiotic zooxanthellae in Aiptasia tagetes (Coelenterata, Anthozoa)

Experiments designed to investigate the effect of different levels of illumination on the density of symbiotic zooxanthellae in the anemone Aiptasia tagetes (Duch. & Mich.) are described.
The anemones are found to regulate the densities of their zooxanthella populations to fixed levels which are dependent upon ambient light intensity.
Regulation is continuous and results from the interaction of increase in algal numbers, growth of host tissues, population of new host tissues by symbionts and extrusion of zooxanthellae by the host.
The nature of the process by which zooxanthellae are selected for extrusion is discussed and a scheme is outlined indicating the pathways through which the level of the algal population in Aiptasia , and other coelenterates, is believed to be controlled.     

Individual and population growth in the asexually reproducing anemone Aiptasia pallida Verrill

The effects of feeding regime and zooxanthellae on individual and population growth of Aiptasia pallida (Verrill) were studied in a series of laboratory experiments. Individual and population growth were measured as biomass increase. Both individual and population growth were significantly affected by feeding regime while zooxanthellae enhanced growth only at the most infrequent feeding regime. Feeding regime had no significant effect on the number of individual anemones in the population produced asexually by pedal laceration after 8 wk, although there were temporal differences in pedal laceration between feeding regimes during this 8-wk period. Anemones fed three times per wk underwent little pedal laceration prior to week 4 with most individuals produced after week 4. In contrast, anemones fed once per 4 wk produced almost all anemones prior to week 4 with little subsequent pedal laceration. Zooxanthellae significantly increased the number of anemones produced by pedal laceration only among individuals fed at 4-wk intervals. Zooxanthellae had no effect on pedal laceration among individuals fed three times per week.

Population growth, measured as weight change, after 4 wk was greater than individual growth for all treatments except aposymbiotic anemones fed at 4-wk intervals for which there was no significant difference between individual and population percent weight change. These results suggest that zooxanthellae enhance growth only during periods of prey scarcity and that asexual reproduction by symbiotic individuals increases biomass at a greater rate than does individual anemone growth.     

Photosynthetic activity of a temperate coral Acropora pruinosa (Scleractinia, Anthozoa) with symbiotic algae in Japan

Physiological properties of the temperate hermatypic coral Acropora pruinosa Brook with symbiotic algae (zooxanthellae) on the southern coast of the Izu Peninsula, Shizuoka Prefecture, central Japan, were compared between summer and winter. Photosynthesis and respiration rates of the coral with symbiotic zooxanthellae were measured in summer and winter under controlled temperatures and irradiances with a differential gasvolumeter (Productmeter). Net photosynthetic rate under all irradiances was higher in winter than in summer at the lower range of temperature (12–20°C), while lower than in summer at the higher range of temperature (20–30°C). The optimum temperature for net photosynthesis was apt to fall with the decrease of irradiance both in summer and winter, whereas it was higher in summer than in winter under each irradiance. At 25/ 50/100 μmol photons nr² s^?1, it was nearly the sea‐water temperature in each season. Dark respiration rate was higher in winter than in summer, especially in the range from 20–30°C. In both seasons the optimum temperature for gross photosynthesis was 28°C under 400 μmol photons nr² s^?1 and lowered with decreasing irradiance up to 22°C under 25 μmol photons nr² s^?1 in summer, while 20°C under the same irradiance in winter. The optimum temperature for production/respiration (P/R) ratio was higher in summer than in winter under each irradiance. Results indicated that metabolism of coral and zooxanthellae is adapted to ambient temperature condition under nearly natural irradiance in each season.     

Growth at Low Temperature Mimics High-Light Acclimation in Chlorella vulgaris

Structural and functional alterations to the photosynthetic apparatus after growth at low temperature (5[deg]C) were investigated in the green alga Chlorella vulgaris Beijer. Cells grown at 5[deg]C had a 2-fold higher ratio of chlorophyll a/b, 5-fold lower chlorophyll content, and an increased xanthophyll content compared to cells grown at 27[deg]C even though growth irradiance was kept constant at 150 [mu]mol m-2 s-1. Concomitant with the increase in the chlorophyll a/b ratio was a lower abundance of light-harvesting polypeptides in 5[deg]C-grown cells as observed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and confirmed by western blotting.The differences in pigment composition were found to be alleviated within 12 h of transferring 5[deg]C-grown cells to 27[deg]C. Furthermore, exposure of 5[deg]C-grown cells to a 30-fold lower growth irradiance (5 [mu]mol m-2 s-1) resulted in pigment content and composition similar to that in cells grown at 27[deg]C and 150 [mu]mol m-2 s-1. Although both cell types exhibited similar measuring-temperature effects on CO2-saturated O2 evolution, 5[deg]C-grown cells exhibited light-saturated rates of O2 evolution that were 2.8-and 3.9-fold higher than 27[deg]C-grown cells measured at 27[deg]C and 5[deg]C, respectively. Steady-state chlorophyll a fluorescence indicated that the yield of photosystem II electron transport of 5[deg]C-grown cells was less temperature sensitive than that of 27[deg]C-grown cells. This appears to be due to an increased capacity to keep the primary, stable quinone electron acceptor of photosystem II (QA) oxidized at low temperature in 5[deg]C- compared with 27[deg]C-grown cells regardless of irradiance. We conclude that Chlorella acclimated to low temperature adjusts its photosynthetic apparatus in response to the excitation pressure on photosystem II and not to the absolute external irradiance. We suggest that the redox state of QA may act as a signal for this photosynthetic acclimation to low temperature in Chlorella.     

Ability of Monaco shrimp Lysmata seticaudata (Decapoda: Hippolytidae) to control the pest glass anemone Aiptasia pallida (Actiniaria: Aiptasidae)

The present work highlights the ability of wild and cultured Monaco shrimp Lysmata seticaudata to control the glass anemone Aiptasia pallida. Starved shrimp ingested the highest percentages of glass anemones [85.7% and 89.3% for wild (W) and cultured (C) shrimp, respectively]. The absence of symbiotic zooxanthellae in glass anemones did not influence the shrimps feeding rate, with shrimp offered aposymbiotic anemones displaying the same feeding percentages (67.3% and 70.7% for W and C shrimp, respectively) as those offered sea anemones with symbiotic zooxanthellae (70.0% and 74.4% for W and C shrimp, respectively). Shrimp offered larger sea anemones had the lowest feeding percentages (33.0% and 36.3% for W and C shrimp, respectively), along with shrimp offered an alternative food (27.3% and 36.0% for W and C shrimp, respectively). There were no significant differences in the percentage of glass anemones ingested by cultured and wild Monaco shrimp in the same feeding trial.     

Possible Role of Cbr,an Algal Early-Light-Induced Protein,in Nonphotochemical Quenching of Chlorophyll Fluorescence

The unicellular green alga Dunaliella bardawil exhibits typical responses to excessive light when starved for sulfate under normal light (60 [mu]E m-2 s-1) but not under low light (14 [mu]E m-2 s-1). Algae were analyzed during several days of sulfate starvation for nonphotochemical quenching of chlorophyll fluorescence in the absence or presence of the uncouplers SF-6847 (SF) or carbonyl cyanide p- trifluoromethoxyphenyl hydrazone. Parallel analyses followed two light-stress responses: (a) violaxanthin conversion to zeaxanthin and (b) accumulation of Cbr, a protein analogous to plant early-light-induced proteins and implicated in zeaxanthin binding. In cells starved under normal light SF inhibited nonphotochemical quenching during the first 24 h, but not from 40 h onward. In cells starved under low light SF inhibited nonphotochemical quenching throughout the starvation period. Under normal light accumulation of zeaxanthin was nearly maximal by 24 h, but Cbr was fully induced only by 40h. Under low light zeaxanthin accumulated slowly but no Cbr was evident. These results suggest that during exposure to excessive light, the initial pH gradient-dependent, Cbr-independent mode of nonphotochemical quenching is modified to become less dependent on pH gradient and requires Cbr.