Adaptation strategies of the corallimorpharian <Emphasis Type="Italic">Rhodactis rhodostoma</Emphasis> to irradiance and temperature

Corallimorpharians may dominate some habitats on coral reefs and compete with stony corals for access to light, yet little is known concerning their photosynthetic traits. At Eilat in the northern Red Sea, we observed that the abundance of individuals of the corallimorpharian Rhodactis rhodostoma decreased significantly with depth on the reef slope. Field and laboratory experiments revealed that they employ several mechanisms of photoadaptation to high irradiance on the shallow reef flat. Their endosymbiotic microalgae (zooxanthellae) varied significantly in both abundance and chlorophyll content with level of irradiance. Use of a diving pulse amplitude modulated fluorometer revealed that the zooxanthellae of R. rhodostoma effectively disperse excess light energy by expressing significantly higher values of non-photochemical quenching and maximum excitation pressure on photosystem II when experimentally exposed to high light (HL) versus low light (LL). Host corallimorpharian tissues mediated this response by shielding the algal symbionts from high irradiance. The endoderm of host tentacles thickened significantly and microalgal cells were located further from the mesoglea in HL than in LL. The clades of zooxanthellae hosted by the corallimorpharians also varied with depth. In shallow water, all sampled individuals hosted clade C zooxanthellae, while in deep water the majority hosted clade D. The photosynthetic output of individuals of R. rhodostoma was less affected by HL than was that of a stony coral examined. When exposed to both high temperature (HT) and HL, individuals of R. rhodostoma reduced their maximum quantum yield, but not when exposed to HL at low temperature (LT). In contrast, colonies of the scleractinian coral Favia favus reduced their photosynthetic output when exposed to HL in both temperature regimes. After 2 weeks of HT stress, R. rhodostoma polyps appeared to bleach completely but re-established their zooxanthella populations upon return to ambient temperature. We conclude that mechanisms of photoadaptation to high irradiance employed by both the endosymbiotic zooxanthellae and host corallimorpharians may explain in part the abundance of R. rhodostoma on some shallow reef flats. The ability to survive for weeks at HT while bleached also may allow corallimorpharians to repopulate shallow reef areas where scleractinians have been killed by thermal stress. B. Kuguru and G. Winters contributed equally to this work.     

Gas-exchange studies with the staghorn coral Acropora acuminata and its zooxanthellae

Compensation point and light-saturation values were determined from oxygen-exchange experiments with branches and isolated zooxanthellae from the staghorn coral Acropora acuminata. Branches and dense suspensions of zooxanthellae showed similar lightresponse curves for oxygen exchange, with light saturation at about 23 Klux (300 W. m^-2) and compensation point occurring between 4 and 6 Klux (60–80 W. m^-2). Zooxanthellae appear to be mutually shaded in dense suspensions and coral tissues. The effects of metabolic inhibitors, including photosynthetic and respiratory inhibitiors, on oxygen exchange in coral branches and isolated zooxanthellae are presented. Bubbles formed on coral tissues and on several macroalgae under conditions of high illumination contained large amounts of oxygen, suggesting that a high oxygen tension may occur in coral tissues during the day. Photorespiration and dissolved organic carbon production by suspensions of zooxanthellae are discussed in relation to a high oxygen tension which probably occurs in coral tissues during daylight.     

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.     

Monthly skeletal extension rates of the hermatypic corals<Emphasis Type="Italic"> Montastraea annularis</Emphasis> and<Emphasis Type="Italic"> Montastraea faveolata</Emphasis>: biological and environmental controls

Monthly skeletal extension rates were measured in colonies of Montastraea annularis and M. faveolata growing at Mahahual and Chinchorro Bank, in the Mexican Caribbean. Temperature, light extinction coefficient (k_d), sedimentation rate, dissolved nutrients and wave energy were used as indicators of environmental conditions for coral growth. Zooxanthella density and mitotic index, nitrogen, phosphorous and protein in coral tissue, and living tissue thickness were measured during periods of high-density-band (HDB) and low-density-band (LDB) formation. To test their value as indirect measures of competition between zooxanthellae and host, as well as coral health and performance in both species, these biological parameters were also measured, during the HDB-formation period, in corals collected at La Blanquilla. This reef is located in the Gulf of Mexico, in an area of suboptimal environmental conditions for coral growth. M. faveolata had a significantly higher skeletal extension rate than M. annularis. Corals growing in Mahahual had significantly higher skeletal extension rate than those living in Chinchorro Bank. This is consistent with inshore–offshore gradients in growth rates observed by other authors in the same and other coral species. This is probably due to less favorable environmental conditions for coral growth in near shore Mahahual, where there is high hydraulic energy and high sedimentation rate. Contrary to observations of other authors, skeletal extension rate did not differ significantly between HDB- and LDB-formation periods for both species of Montastraea. Both species produced their HDB between July and September, when the seawater temperatures are seasonally higher in the Mexican Caribbean. Tissue thickness indicated that environmental conditions are more favorable for coral health and performance during the HDB-formation period. Mitotic index data support the idea that zooxanthellae have competitive advantages for carbon over the host during the LDB-formation period. So, corals, during the LDB-formation period, with less favorable environmental conditions for coral performance and at a disadvantage for carbon with zooxanthellae, add new skeleton with little or no opportunity for thickening the existing one. This results in an equally extended skeleton with lower density, and the stretching response of skeletal growth, proposed for M. annularis growing under harsher environmental conditions, also occurs during the LDB-formation period.Communicated by P.W. Sammarco, Chauvin     

Lipogenesis in the intact coral Pocillopora capitata and its isolated zooxanthellae: Evidence for a light-driven carbon cycle between symbiont and host

Surface tissue of the reef coral Pocillopora capitata contained approximately 34% lipid on a dry weight basis. Of this, 75% was storage lipid (wax ester and triglyceride) and 25% structural (phospholipid, galactolipid, etc.). Based on chlorophyll a: lipid ratios of intact coral and isolated zooxanthellae, it was determined that over 90% of the storage lipid resided in the host tissue. One half of the structural lipids was found in the host and the other in the symbiotic algae. Gentle fractionation of coral tissue indicated that zooxanthellae possessed less than 14% of the total coral protein. Coral tips and isolated zooxanthellae were incubated with sodium acetate-1-¹⁴C in light and dark to obtain lipogenic rates and proportions of fatty acids and lipid classes synthesized. The rate of lipid synthesis from acetate-1-¹⁴C by intact coral was stimulated three-fold in the light (1200 lux), which indicated that the majority of coral lipogenesis occurred in the zooxanthellae. Intact coral triglycerides contained ca. 68% of the ¹⁴C-activity and wax esters ca. 21%. Zooxanthellae isolated by the Water Pik technique synthesized negligible amounts of wax ester, which implied that wax ester synthesis was a property of the animal tissue. Isolated zooxanthellae and intact coral synthesized identical triglyceride fatty acids from acetate-1-¹⁴C. This study provides evidence for a carbon cycle between host and symbiont whereby the zooxanthellae take up host-derived carbon (probably in the form of acetate), synthesize fatty acids using their photosynthetically derived energy, and return the lipid to the host where it appears as wax ester and triglyceride.     

Experimental ecology of the temperate scleractinian coral Astrangia danae

The combined effects of temperature, light and symbiont density on the metabolic rate and calcification of the temperate coral Astrangia danae were studied experimentally using colonies containing different concentrations of zooxanthellae. After acclimation to five temperatures between 6.5° and 27°C, and incubation at three light levels and in darkness, respiration and photosynthesis were measured and corrected for rates due to commensals alone. Calcification rates were regressed on zooxanthellae concentration and production in order to define “symbiotic” and “non-symbiotic” averages, and the enhancement of calcification by symbiotic interactions in the polyps. Respiration by the polyparium varied less with temperature between 11.5° and 23°C than that of the commensals, suggesting physiological acclimation by the coral tissue. In-vivo zooxanthellae photosynthesis increased linearly with temperature and was near its maximum at 400 μEin m^?2 s^?1, but the photosynthesis of the endolithic algae of the corallum varied little between 11.5° and 27°C. Calcification at any given temperature was near its maximum at 40 μEin m^?2 s^?1 in both symbiotic and non-symbiotic corals. CaCO₃ deposition increased linearly with temperature in non-symbiotic colonies and in symbiotic colonies incubated in the dark. In symbiotic colonies, calcification in the light increased above these basic rates as temperature rose above 15°C. Below 15°C, symbiotic interactions failed to stimulate calcification, apparently due both to a lowering of zooxanthellae photosynthesis and to a decrease in the enhancing effect of any given level of primary production.     

Release of zooxanthellae with intact photosynthetic activity by the coral<Emphasis Type="Italic"> Galaxea fascicularis</Emphasis> in response to high temperature stress

Damage to zooxanthellae photosynthetic apparatus has been proposed to be the underlying mechanism of coral bleaching, but how the expulsion of zooxanthellae is triggered is still not understood. The present study compared the photosystem II (PSII) functioning and overall photosynthesis of retained and released zooxanthellae from the reef-building coral Galaxea fascicularis exposed to high temperature stress. The use of pulse-amplitude-modulated (PAM) fluorometry for isolated zooxanthellae was validated and used to measure photosynthesis. There was no significant difference in PSII functioning and the overall photosynthesis between retained zooxanthellae, which were isolated immediately after stress treatment, and those released from the coral exposed to either 30 or 32°C, while the zooxanthellae population released at 28°C showed significantly lower PSII functioning than those retained in the polyps. The release of healthy-looking zooxanthellae by polyps exposed to elevated temperatures was significantly higher than those in the control (28°C). Higher release of undischarged cnidae, indicative of host cell necrosis or detachment, was observed in 32°C treatments. These findings indicate that the zooxanthellae released in 30 or 32°C treatments exhibited normal morphology and intact photosynthetic activity. The present results strongly suggest that the release of zooxanthellae from G. fascicularis at 30 or 32°C is a non-selective process with respect to the zooxanthellar PSII functioning and thus the host seems to be the first partner to be physiologically affected in temperature-induced bleaching.Communicated by T. Ikeda, Hakodate     

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     

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     

Why does the white tip of stony coral grow so fast without zooxanthellae?

The photosynthesis of zooxanthellae in a coral polyp greatly enchances the calcification rate of a coral. However, the white tip of a coral branch is free of zooxanthellae yet still has a very high calcification rate. Furthermore, the reason for the difference is not clear. In this study, the amount of photopigment, total protein (TP), total organic carbon (TOC), ATP, and lipid in polyps from the white tip and brown stalk of a branch of stony coral were measured. Samples of Acropora hyacinthus and A. formosa were collected from southern Taiwan between 1985 and 1987. The results showed that the ATP concentration in polyps of the white tip was much higher than that in polyps of the brown stalk. Conversely, the amount of TP, TOC and measured lipids in polyps of the brown stalk were all higher than those of the white tip. It was the high concentration of ATP in cells that gave these polyp tips the vitality to sustain the energy requirements of such a rapid calification rate. Facilitated diffusion, due to the high metabolite gradient created by cell activity, could be the major driving force for the transport of photosynthetic product from stalk to tip.     

Pink-line syndrome, a physiological crisis in the scleractinian coral Porites lutea

Coral diseases are one of the major factors that alter coral cover and their diversity. We have earlier reported the “Pink-line syndrome” (PLS) in the scleractinian coral Porites lutea wherein a colored band appears between the dead and healthy tissue of a colony. About 20% of the P. lutea colonies were affected in Kavaratti of the Lakshadweep Islands in the Arabian Sea during April 1996 and the incidence increased fourfold within the next 4 years. Fungi were associated in both PLS-affected and healthy specimens, whereas the cyanobacterium Phormidium valderianum occurred exclusively in the PLS-affected specimens. There was an increased expression of a 29 kDa protein without any significant increase in total protein content in the PLS-affected colonies. A reduced number of zooxanthellae and an increase in zooxanthellae size, mitotic index, and chl a concentrations were some of the characteristics of the PLS-affected colonies. PLS induction experiments conducted using selected fungi and the cyanobacterium P. valderianum isolated from the affected colonies and abiotic factors, such as CO₂ enrichment and the effect of cyanobacterial photosynthesis inhibition, indicated that the CO₂ build-up around the host tissue caused the pink coloration. We hypothesize that these physiological changes disturb the mutualism between the zooxanthellae and the host. When the symbiosis is disturbed by the external CO₂, the host loses control over the zooxanthellae, causing their uncontrolled division. This process may lead to a break in photosynthate transfer to the host, thereby resulting in starvation and finally leading to partial mortality. We further hypothesize that these degenerative processes are triggered by the CO₂ produced by P. valderianum through its carbon concentration mechanism. In this context, any opportunistic cyanobacteria or other agents having potential to interfere with the physiology of the host or the symbiont can cause such a physiological disorder. The mechanism of PLS formation is an early warning to protect corals as the increasing atmospheric CO₂ could induce PLS-like physiological disorder in corals.     

Effects of light of altered spectral composition on coral zooxanthellae associations and on zooxanthellae in vitro

Pocillopora damicornis (Linnaeus) and Montipora verrucosa (Lamarck) were collected from Hawaiian reefs. In two experiments (September 1979-January 1980: ca. 4 mo; August-October 1980; ca. 2 mo), these reef corals were grown under sunlight passed through filters producing light fields of similar quantum flux but different spectral composition. In vitro cultures of symbiotic zooxanthellae (Symbiodinium microadriaticum Freudenthal) from M. verrucosa were cultured under similar conditions for 15 d. Blue or white light promoted more coral skeletal growth than green or red light. In both coral species, blue light increased the total amount of chlorophyll a of the coral-zooxanthellae association. In the perforate species, M. verrucosa, the pigment concentration was elevated by an increase in the density of zooxanthellae, but the pigment concentrations per algal cell remained unchanged; in the non-perforate species, P. damicornis, it appears that pigment concentration was elevated by an increase in pigment per algal cell, and not by an increase in density of zooxanthellae. The sunloving reef-flat coral P. damicornis did not grow as rapidly as the shade-species M. verrucosa at the low quantum flux (about 10% sunlight) provided by the experimental treatments. The in vitro cultures of zooxanthellae from M. verrucosa exhibited growth rates in light of altered spectral quality that correlated with the responses of the host coral species: blue and white light supported significantly greater growth than green light, and red light resulted in the lowest growth rate.Contribution No. 678 of the Hawaii Institute of Marine Biology     

Effects of feeding frequency and symbiosis with zooxanthellae on nitrogen metabolism and respiration of the coral Astrangia danae

Colonies of the temperate coral Astrangia danae occur naturally with and without zooxanthellae. Basal nitrogen excretion rates of nonsymbiotic colonies increased with increasing feeding frequency [average excretion rate was 635 ng-at N (mg-at tissue-N)^-1 h^-1]. Reduced excretion rates of symbiotic colonies were attributed to N uptake by the zooxanthellae. Nitrogen uptake rates of the zooxanthellae averaged 8 ng-at N (10⁶ cells)^-1 h^-1 in the dark and 21 ng-at N (10⁶ cells)^-1 h^-1 at 200 Ein m^-2 s^-1. At these rates the zooxanthellae could provide 54% of the daily basal N requirement of the coral if all of the recycled N was translocated. Basal respiration rates were 172 nmol O₂ cm^-2 h^-1 for starved colonies and 447 nmol O₂ cm^-2 h^-1 for colonies fed three times per week. There were no significant differences between respiration rates of symbiotic and nonsymbiotic colonies. N excretion and respiration rates of fed (symbiotic and nonsymbiotic) colonies increased greatly soon after feeding. N absorption efficiencies decreased with increasing feeding frequency. A N mass balance, constructed for hypothetical situations of nonsymbiotic and symbiotic (3×10⁶ zooxanthellae cm^-2) colonies, starved and fed 15 g-at N cm^-2wk^-1, showed that the presence of symbionts could double the N growth rate of feeding colonies, and reduce the turnover-time of starved ones, but could not provide all of the N requirements of starved colonies. Rates of secondary production, estimated from rates of photosynthesis and respiration were similar to those estimated for reef corals.     

Diel cycles of expansion and contraction in coral reef anthozoans

Diel patterns of expansion and contraction are widespread in coral reef anthozoans, yet no theory adequately explains this behavior. We have observed a wide variety of behavior patterns in 14 sea anemone species at 9 sites along the Caribbean coast. The distribution of zooxanthellae in anemone tissues was quantified by sectioning preserved specimens and calculating zooxanthella density in the endoderm. We show that polyp structures containing dense populations of zooxanthellae respond positively to light (expansion, positive orientation) and those with few or no zooxanthellae respond negatively (contraction, negative orientation). Structures capable of prey capture, feeding tentacles, are expanded at night when prey is available. Structures adapted for photosynthesis, auxiliary structures of the column and tentacles with dense zooxanthellae, are expanded during the day. Such independent reactions of structures acting as functional units for photosynthesis and/or prey capture combine to give the observed variety of behavior patterns. We hypothesize that the need to conserve limiting nutrients and energy could be the ultimate cause of expansion and contraction rhythms in coral reef anthozoams.     

A floating mid-water coral nursery as larval dispersion hub: testing an idea

The global decline in reef health has prompted the need for effective management methodologies, including the development of active restoration measures. One such approach is the ‘gardening concept’ that involves use of underwater nurseries where coral fragments are farmed before their transplantation into denuded reefs. Here we document enhanced sexual reproduction in colonies of the coral Stylophora pistillata cultured in mid-water floating nursery situated in nutrient enriched water, near the fish farms in Eilat, Red Sea. We found that after 2 years of nursery, the average number of oocytes per polyp in farmed colonies was ca. 35% higher than in corresponding naturally growing colonies. Small branches in the nursery developed gravid colonies that released equal (or more) numbers of planula larvae as compared to similar size, 5-year old naturally growing colonies. These nursery-borne planulae possessed more zooxanthellae and contained more chlorophyll per larva. While settled and metamorphosed in equal rates compared to planulae originated from reef-grown colonies, the nursery borne planulae developed faster growing young colonies. We estimate that a coral nursery could generate, during the reproductive season, tens of millions of planula larvae and therefore should be regarded as a ‘larval dispersion hub’ that can be used as a management tool for natural recruitment enhancement.     

Some factors influencing selective release of soluble organic material by zooxanthellae from reef corals

Studies were carried out to determine optimum conditions for the investigation of symbiotic zooxanthellae in vitro and to gain insight into factors influencing release of photosynthate by the symbionts. Zooxanthellae isolated from the reef coral Agaricia agaricites and incubated with an homogenate of host tissue release twice as much photosynthate as controls in seawater. The animal homogenate retained its stimulatory activity for 3 h at room temperature (ca. 26°C). Release of photosynthate was markedly influenced by time after isolation of algae from the host, variation in homogenate concentration, and prolonged exposure to homogenate. Release was not influenced by cell concentration, light intensity, or glycerol in the incubation medium. If zooxanthellae are labelled in vitro with glucose ¹⁴C, the principle product released is alanine ¹⁴C. The mechanism of action of homogenate on zooxanthellae in vitro is discussed in terms of its effect on algal cell membrane permeability. A preliminary fractionation of host homogenate is described.     

A “CO2 supply” mechanism in zooxanthellate cnidarians: role of carbonic anhydrase

Carbonic anhydrase (CA, EC 4.2.1.1) activity was detected in 22 species of tropical cnidarians which contain endosymbiotic dinoflagellates (=zooxanthellae). CA activity was 2 to 3 times higher in animal tissue than in algae and ca. 29 times higher in zooxanthellate than azooxanthellate species. It was also higher in the zooxanthellate tentacle tissue than in the azooxanthellate column tissue of the anemone Condylactis gigantea. CA was therefore significantly related to the presence of endosymbiotic algae. Further results indicated that CA functions in the photosynthetic carbon metabolism of zooxanthellate cnidarians as evidenced by (1) low CA activity in shade-adapted and deep water colonies compared to the more productive shallow water, light-adapted colonies of the coral Stylophora pistillata, and (2) the 56 to 85% reduction in photosynthetic carbon assimilation by zooxanthellae in situ in the presence of Diamox, an inhibitor of CA. Although CA has been proposed to function in calcification, its association with zooxanthellae and photosynthetic activity in both calcifying and non-calcifying associations suggests a role in photosynthetic metabolism of algal/cnidarian symbioses. It is proposed that CA acts as a CO₂ supply mechanism by releasing CO₂ from bicarbonate, and enabling zooxanthellae to maintain high rates of photosynthesis in their intracellular environment.     

Light responses of a scleractinian coral (Plerogyra sinuosa)

During daytime Plerogyra sinuosa Dana displays globular expandable tentacles (bubbles) which foster the photosynthetic ability of the coral. Adaptational responses of this coral to different depths (5–25 m) and light conditions were investigated by photosynthetic pigment analysis, insitu measurements of oxygen production, transplantation and shading experiments. Pigment concentrations per unit tissue dry weight were variable, but unrelated to depth. Pigment concentrations per zooxanthellae cell remained constant and bubble size increased with depth. Light intensity at 25 m was 20 to 25% of the 5-m value, but daily integrated rates of photosynthesis were 65% of the 5-m rates, indicating a higher light utilization efficiency in deeper corals. Coral heads transplanted from 25 to 5 m died within 20 d if not protected against UV-radiation, but corals transplanted from 5 to 25 m acclimatized to the new light condition. Photosynthetic oxygen production and bubble size increased in shaded, sun-adapted corals within 60 min and decreased in sun-exposed, shade-adapted corals. The variable bubble size is interpreted as an adaptational mechanism to optimize light exposure of zooxanthellae.     

Effects of particulate peat on the behavior and physiology of the Jamaican reef-building coral Montastrea annularis

Corals in an in situ respirometer exposed to suspended peat during the day greatly decreased net oxygen production, probably due to a reduction of intensity and spectral quality of light reaching the symbiotic zooxanthellae. Net production returned to pre-exposure levels after the chambers were cleared; the corals showed no behavioral effects. In contrast, after exposure during the night, corals displayed clearing behavior (such as extreme distension of the coenosarc and trapping of peat particles in thick clumps of mucus) and an increase in respiration rate comparable to the decrease in net production observed during the daytime exposure. The following morning, net production values were significantly lower than pre-exposure production values although ambient light intensity was slightly higher. This decrease in production as well as a 22% reduction of chlorophyll content in the coral tissue indicated loss of zooxanthellae from the stressed corals. Long-term exposure to such a stress could reduce coral growth rates and substantially alter coral reef communities.     

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.