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.     

Depth-dependent photoadaption by zooxanthellae of the reef coral Montastrea annularis

Zooxanthellae living in colonies of the Caribbean reef coral Montastrea annularis photoadapt to depth-dependent attenuation of submarine light. Studies carried out at Discovery Bay, Jamaica, show that in shallow-living coral colonies, the zooxanthellae appear photoadapted to function at high light intensities, and do poorly if transplanted to low light intensities; in contrast, zooxanthellae in deeper-living coral colonies can be damaged by high light intensities. The adaptation to decreasing light intensity and changing spectral quality appears to be accomplished by increasing the size of the photosynthetic unit (PSU), as opposed to increasing the number of PSU's per cell. Whole cell absorption increases with depth, partially offsetting the loss of light energy due to depth-dependent attenuation. Calculations of photosynthetically usable radiation, the light an alga is capable of absorbing in its own submarine habitat, suggest that the algae at different depths are optimizing rather than maximizing their ability to harvest submarine light energy.     

Tolerance of estuarine benthic diatoms to high concentrations of ammonia,nitrite ion,nitrate ion and orthophosphate

A carbon-14 assimilation method was used to determine action spectra and photosynthesis versus irradiance (P versus I) curves of natural populations of phytoplankton and zooxanthellae from a coral reef fringing Lizard Island in the Australian Barrier Reef. The action spectra were related to the phytoplankton species composition. The curves showed shade adaptation in phytoplankton from deeper waters and in the zooxanthellae. Rates of photosynthesis of zooxanthellae were shown to be highly but variably dependent on their host organisms. Photosynthetic production by zooxanthellae was about 0.9 gC m^-2 day^-1, which is about three times higher than phytoplankton production in the waters close to the reef.     

The effects of antifoulant-paint-contaminated sediments on coral recruits and branchlets

The 184-m cargo ship "Bunga Teratai Satu" ran aground on Sudbury Reef, within the Great Barrier Reef Marine Park, on 2 November 2000. Although no cargo or fuel was lost, the ship remained aground for 12 days and a large quantity of antifoulant paint containing tributyltin (TBT), zinc, and copper was scraped from the hull during the grounding and subsequent refloating operation. This resulted in extensive contamination of the reef sediments for up to 250 m surrounding the grounding site. Two laboratory-based experiments assessed the impact of contaminated sediments on the survival of both newly settled corals of Acropora microphthalma and branchlets of A. formosa. Newly settled corals exposed to sediments containing 8.0 mg kg^–1 TBT, 72 mg kg^–1 Cu, and 92 mg kg^–1 Zn or greater suffered significantly higher mortality after 72 h, compared to control or low-concentration treatments. Coral recruits exposed to 40 mg kg^–1 TBT (Sn), 306 mg kg^–1 Cu, and 403 mg kg^–1 Zn were all killed within 38 h. Branchlets from adult corals exposed to sediments with a high concentration of contaminants (TBT 160 mg kg^–1, Cu 1,180 mg kg^–1, and Zn 1,570 mg kg^–1) suffered significant mortality (38%), whereas branchlets placed in treatments with lower levels of contaminants suffered no mortality. Visual bleaching of the branchlets was observed at high contaminant levels, but an overall reduction in the symbiotic zooxanthellae populations was not observed in surviving corals. The photosynthetic yields of light-adapted zooxanthellae remained constant in live branchlets, indicating that the TBT-contaminated sediment may be more toxic to the host than the symbiont. Our results show that antifoulant contamination at ship-grounding sites has the potential to cause major mortality of resident coral communities and can have a negative impact on the recovery of adult populations.Communicated by P.W. Sammarco, Chauvin     

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.     

Photosynthesis,photorespiration, and dark respiration in eight species of algae

The rates of photosynthesis and dark respiration for 7 marine algae and 1 fresh-water alga were measured and compared. The dinoflagellates Glenodinium sp. and zooxanthellae have high dark respiration rates relative to photosynthetic rates, which may decrease their net growth rates. Photorespiration in the 8 algal species was studied by examining the effects of the concentration of oxygen on the rates of photosynthesis, on the incorporation of ¹⁴CO₂ into the photorespiratory pathway intermediates glycine and serine, and on the postillumination burst of carbon dioxide production and oxygen consumption. A combination of these results indicates that all the algae tested can photorespire, but that Glenodinium sp., Thalassiosira pseudonana, and zooxanthellae either have a photorespiratory pathway different from that proposed for freshwater algae (Tolbert, 1974), or an additional pathway for glycolate metabolism.     

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.     

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.     

Alternate coral–bryozoan competitive superiority during coral bleaching

Bleaching of corals results from the loss of their symbiotic algae (zooxanthellae) and/or pigments. The supply of photoassimilates provided by the zooxanthellae to the coral declines during bleaching and reduces the ability to activate energy-costly processes such as maintenance, growth and reproduction. In the present study we compared the competitive outcomes, expressed as overgrowth and changes in colony sizes of Oculina patagonica (an encrusting Mediterranean stony coral) and the bryozoan Watersipora sp., growing in contact with each other, during and between bleaching events. Year-round observations of tagged colonies showed alternating competitive outcomes: O. patagonica wins over Watersipora sp. between bleaching events, but loses during bleaching events. Using the ¹⁴C-point-labeling technique on coral tissue, we examined intra-colonial translocation of photosynthetic products from the point-tissue labeling towards interaction zones. In non-bleached O. patagonica, competition resulted in preferentially oriented translocation of ¹⁴C products to the interaction zone located up to 8 cm away from the tissue-labeling site. Sites opposite the interaction zone received significantly less labeled photoassimilates compared to the interaction zone. In bleached colonies (40-85% bleached surface area), such translocation did not occur, probably explaining the failure to compete with the encrusting neighbor Watersipora sp. Our findings demonstrate the importance of colonial integration and resource orientation for the competitive superiority of O. patagonica.     

High rates of nitrogen fixation on coral skeletons after predation by the crown of thorns starfish Acanthaster planci

At One Tree Reef, Great Barrier Reef, Australia, between 1983 and 1985, corals killed by the crown of thorns seastar Acanthaster planci L. gave rise to skeletons which were colonised rapidly by blue-green and other algae. For the next 3 to 9 mo these coral skeletons showed over three times more nitrogen fixation (acetylene reduction) than control substratum rates (9 to 32 nmol vs 3 to 10 nmol C₂H₂ cm^-2 h^-1, over all seasons). These values convert to relatively high annual fixation rates of 37 to 127 kg N ha^-1 yr^-1 but, at the low densities of A. planci on One Tree Reef (ca. 0.65 ha^-1), this has little impact on the total nitrogen fixation rate and, as a result, on the level of organic nitrogen in the system. However, it is suggested that on reefs subjected to high aggregations of a. planci such an effect would enhance the level of organic nitrogen and lead to greater primary and secondary production throughout the reef system.     

Nitrogen excretion by some demersal macrozooplankton in Heron and One Tree Reefs,Great Barrier Reef,Australia

Nitrogen excretion rates of demersal macrozooplankton were measured together with nitrogen concentrations in the water column and sediments in lagoons of Heron Reef and One Tree Reef, Great Barrier Reef, Australia, during August and November 1991. Excretion rates increased with body weight, and weight-specific excretion rates of the demersal macrozooplankton were comparable to those of pelagic zooplankton and meiofauna in the Great Barrier Reef. Values of demersal macrozooplankton abundance from previous studies and excretion rates from this study were combined to estimate fluxes of ammonium from demersal macrozooplankton in coral reef lagoons. The estimated fluxes in the water column and sediments were 12 M NH₄ m^-2 d^-1 and 34 M NH₄ m^-2d^-1, respectively. These fluxes were compared with reported fluxes of ammonium in coral reef lagoons in the Great Barrier Reef, Australia. The estimated flux from the demersal macrozooplankton in the water column was 29 and 9% of those reported for microheterotroph regeneration and phytoplankton utilization, respectively. It was 10% of the reported advective flux during periods of low advection and 13% of the maximum efflux from sediments computed from diffusion models. The estimated flux from the demersal macrozooplankton in the sediments exceeded those reported for meiofauna, and was 5 to 32% and 2 to 13% of those reported for ammonification and utilization in sediments, respectively. The potential importance of demersal macrozooplankton in mediating sediment-water column exchanges in the absence of diffusive effluxes and when they swarm is discussed.     

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

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

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.     

Daily photosynthesis,respiration, and carbon budgets in a tropical marine jellyfish (Mastigias sp.)

From measured diel photosynthesis and respiration rates, using oxygen electrodes, estimates of carbon flux between symbiotic algae (zooxanthellae) and host animal are presented for the marine scyphomedusan Mastigias sp. from a marine lake in Palau, Western Caroline Islands, during February and March 1982. The carbon budgets calculated for these lake medusae indicate that carbon fixed photosynthetically by zooxanthellae and made available to the host may satisfy up to 100% of the host's daily metabolic carbon demand (CZAR). The stable carbon isotope (¹³C) signature of the mesogleal carbon of lake Mastigias sp. was close to that of the zooxanthellae, supporting the interpretation that while these medusae may feed holozoically, some of their carbon comes from their symbionts. The diel photosynthesis, respiration, and preliminary estimates of carbon budgets of three individuals of another ecotype of Mastigias sp. collected from nearby oceanic lagoons are also given. Photosynthesis of lagoon medusae was generally greater than that for lake medusae of similar size, and lagoon medusae were phototrophic with respect to carbon, with commensurately greater CZAR values. Carbon translocated from the symbiotic algae also may contribute to the growth requirements of both lake and lagoon medusae. From carbon flux data, the lake jellyfish were estimated to contribute about 16% to the total primary productivity of their marine lake habitat.     

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.     

Feeding behavior and acquisition of zooxanthellae by planula larvae of the sea anemone Anthopleura elegantissima

Symbiotic associations between cnidarians and photosynthetic dinoflagellates (i.e., zooxanthellae) are common in the marine environment. Many symbiotic cnidarians produce offspring that are initially nonsymbiotic. These new hosts must acquire symbiotic algae from environmental sources. We examined zooxanthella acquisition by laboratory-reared planula larvae of the temperate sea anemone Anthopleura elegantissima. Larvae ingested zooxanthellae while they were feeding. However, the signal that prompted larval feeding behavior did not originate from the symbiotic algae; the addition of algal cells to larval cultures never elicited a feeding response. In contrast, the addition of macerated animal tissue from several sources invariably generated a strong feeding response, which resulted in the larvae indiscriminately ingesting any particulate matter that was present, including zooxanthellae or other unicellular algae. Ingested zooxanthellae were incorporated into endodermal cells, where they remained undigested, while all other ingested material was digested or expelled within 24 h. Our results provide evidence that one source of zooxanthellae likely to serve as a route of infection in the natural environment is zooxanthella-laden mucus egested by anemones. This egested material fulfilled both of the criteria necessary for successful infection: it prompted larvae to begin feeding and provided an abundant supply of zooxanthellae that were ingested and taken up into endodermal cells of the new host.     

In situ thermal dynamics of shallow water corals is affected by tidal patterns and irradiance

We studied the diel variation of in situ coral temperature, irradiance and photosynthetic performance of hemispherical colonies of Porites lobata and branching colonies of Porites cylindrica during different bulk water temperature and tidal scenarios on the shallow reef flat of Heron Island, Great Barrier Reef, Australia. Our study presents in situ evidence that coral tissue surface temperatures can exceed that of the surrounding water under environmental conditions typically occurring during low tide in shallow reef or lagoon environments. Such heating may be a regular occurrence on shallow reef flats, triggered by the combined effects of high irradiance and low water flow characteristic of low Spring tides. At these times, solar heating of corals coincides with times of maximum water temperature and high irradiance, where the slow flow and consequent thick boundary layers impede heat exchange between corals and the surrounding water. Despite similar light-absorbing properties, the heating effect was more pronounced for the hemispherical P. lobata than for the branching P. cylindrica. This is consistent with previous laboratory experiments showing the evidence of interspecific variation in coral thermal environment and may result from morphologically influenced variation in convective heat transfer and/or thermal properties of the skeleton. Maximum coral surface warming did not coincide with maximum irradiance, but with maximum water temperature, well into the low-tide period with extremely low water flow in the partially drained reef flat, just prior to flushing by the rising tide. The timing of low tide thus influences the thermal exposure and photophysiological performance of corals, and the timing of tidally driven coral surface warming could potentially have different physiological impacts in the morning or in the afternoon.     

Reproduction in Anthelia glauca (Octocorallia: Xeniidae). II. Transmission of algal symbionts during planular brooding

The soft coral Anthelia glauca Lamarck, 1816, of the family Xeniidae, is found on the reefs of KwaZulu-Natal, South Africa. Its gastrodermal cells contain numerous endosymbiotic unicellular algae (zooxanthellae). A. glauca is a gonochoric species that simultaneously broods its planulae within the pharyngeal cavity of the polyps. Symbiotic algae appear with zygote formation within the pharynx, embedded in amorphous material. The algal cells adhere to the ciliated ectodermal surface of immature planulae and are most probably endocytosed by them. Zooxanthellae are translocated towards the basal part of the ectoderm. Gaps are subsequently opened in the mesoglea into which symbionts surrounded by ectodermally derived material, including plasma membrane, pass. The basal membrane of endodermal cells disintegrates, and the algae bulge into spaces formed in the underlying endoderm. Throughout the process, each zooxanthella resides within a vacuolar membrane in the detached ectodermal cytoplasm. The acquisition process is essentially one in which zooxanthellae are translocated from the pharyngeal cavity into the ectoderm and then through the mesoglea into the endoderm, culminating in the final symbiotic state. The direct transmission of symbiotic algae to the eggs or larvae probably provides the most efficient means whereby zooxanthellae are acquired by the host progeny. Received: 15 July 1997 / Accepted: 25 February 1998     

Mineralization of particulate organic matter derived from coral-reef organisms in reef sediments of the Gulf of Aqaba

In situ and laboratory incubation experiments in a fringing reef in the Gulf of Aqaba were performed to study degradation rates of particulate organic matter in reef sediments. Coral mucus, clam eggs, and zooxanthellae were used as model particulate organic compounds for these experiments. Aerobic and anaerobic mineralization rates were calculated by dissolved inorganic carbon (DIC) and O₂ fluxes from the sediments under different particulate organic matter additions. Fast enhancement (approximately twofold) of O₂ and DIC fluxes were found with the addition of coral mucus and clam eggs compared with control incubations without addition. Most of the degradation is believed to have occurred anaerobically rather than aerobically (DIC:O₂ ratios were 4.3–28.1). Higher degradation rates of coral mucus and clam eggs were estimated in carbonate sediment than in silicate sediment (1.2–1.6-fold), which was attributed to the different physical and chemical properties of both sediments. Our study shows the significance of the reef sediment as a suitable site for microbial degradation of particulate organic material excreted from different reef community organisms. This may increase the regeneration of nutrients in the reef environment necessary to sustain high biological productivity.