Reef-forming corals cycle carbon (C) between the coral host, their endosymbiotic algae, and their skeleton. At elevated sea-surface temperatures this relationship breaks down and the corals bleach by expelling their endosymbiotic algae or these algae lose their photosynthetic pigments. The effect of thermally induced bleaching on the C cycling of 2 ecologically important coral species was investigated. The acquisition and assimilation of photoautotrophically and heterotrophically acquired C was examined via pulse-chase labeling experiments in thermally bleached and nonbleached Montipora capitata and Porites compressa corals. In non-bleached corals photoautotrophic and heterotrophic C were acquired and assimilated very differently. Namely, photoautotrophically acquired C was used to meet short-term metabolic demands and calcification, whereas heterotrophically acquired C was retained in both the coral host and endosymbiotic algae. In bleached corals there was a dramatic reduction in the assimilation of photoautotrophically acquired C by the endosymbiotic algae, in the translocation of C from the algae to the coral host, and in the C assimilated in the skeleton. The level of heterotrophically acquired C assimilated into bleached corals was similar to that in non-bleached corals, and was a direct source of organic C to the endosymbiotic algae. This host-to-endosymbiotic algal supply of heterotrophic C may stimulate endosymbiotic algal recovery. These findings show the importance of both photoautotrophic and heterotrophic C to coral function and demonstrate that both play a crucial role in the recovery from bleaching.

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The degradation of phytoplankton and seagrass organic carbon (OC) in sulfate-reducing (SR) and methaneproducing (MP) sediments was tracked by measuring concentrations of particulate OC (POC), hydrolysis products (HP), fermentation products (FP), and inorganic end products (EP). This experiment used the novel approach of amending sediment with organic substrates having ␦ 13 C values unique from that of the OC pool originally present in the sediment. As a result, we could monitor changes in the dynamic dissolved OC pool and gain insight into processes that control the fate of OC. Rates of hydrolysis, fermentation, and terminal metabolism were greater in the phytoplankton-amended treatments than in the seagrass-amended treatments during the period of active decomposition. At the end of the incubation, concentrations of HP and FP in amended treatments were not significantly different from those in the controls. An analysis of the ␦ 13 C values of HP from the amended treatments indicated that the addition of fresh organic matter stimulated the decomposition of carbon present in the sediment at the time of collection. Hydrolysis of this carbon accounted for Ն50% of the total carbon hydrolyzed in the sediment amended with seagrass.

The natural abundance of radiocarbon ( 14 C ) provides unique insight into the source and cycling of sedimentary organic matter. Radiocarbon analysis of bacterial phospholipid lipid fatty acids (PLFAs) in salt-marsh sediments of southeast Georgia (USA) -one heavily contaminated by petroleum residues -was used to assess the fate of petroleum-derived carbon in sediments and incorporation of fossil carbon into microbial biomass. PLFAs that are common components of eubacterial cell membranes (e.g., branched C 15 and C 17 , 10-methyl-C 16 ) were depleted in 14 C in the contaminated sediment (mean ∆ 14 C value of +25 ± 19 ‰ for bacterial PLFAs) relative to PLFAs in uncontaminated "control" sediment (∆ 14 C = +101 ± 12 ‰). We suggest that the 14 Cdepletion in bacterial PLFAs at the contaminated site results from microbial metabolism of petroleum and subsequent incorporation of petroleum-derived carbon into bacterial membrane lipids. A mass balance calculation indicates that 6-10% of the carbon in bacterial PLFAs at the oiled site could derive from petroleum residues. These results demonstrate that even weathered petroleum may contain components of sufficient lability to be a carbon source for biomass production by marsh sediment microorganisms.Furthermore, a small but significant fraction of fossil carbon is assimilated even in the presence of a much larger pool of presumably more-labile and faster-cycling carbon substrates.3

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