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The fatty acid and sterol compositions of zooxanthellae and animal fractions of the scleractinian coral Turbinaria reniformis were investigated under different light and feeding conditions, to study the symbiont-host exchanges. Nubbins were maintained during 6 weeks under two light levels (100 mmol photons m 22 s 21 and 300 mmol photons m 22 s 21 ) and two feeding levels (starved and fed with zooplankton) in a factorial experiment. There were greater proportions of some polyunsaturated fatty acids (PUFA; e.g., C18:4 n-3, C20:5 n-3, C22:6 n-3) in the zooxanthellae than in the host, suggesting that these PUFA were synthesized by the algae and transferred to the animal. Conversely, C20:4 n-6 exhibited a greater proportion in the host and might have been synthesized by the animal. Light affected the chlorophyll content, the rates of photosynthesis, and the lipid production of all coral samples. Corals maintained in high-light conditions had lower relative phytol content but higher concentrations of fatty acids (FA) and sterols than the shaded corals. Feeding also affected coral metabolism, but differently according to the light level and despite the fact that the host did not directly incorporate the zooplankton lipids (PUFA and cholesterol). In low light, feeding resulted in an increase of growth rates and storage lipid concentrations, mainly saturated fatty acids (SAFA) and membrane constituents (PUFA and sterols). In high light, the lipid energy from the food was directed toward an increase in calcification, as well as in chlorophyll content and protein content. This study highlights the importance of feeding in sustaining coral metabolism, especially when light, or stress events, is limiting photosynthesis.

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The effects of grazing pressure and inorganic nutrient availability on the direct carbon transfer from freshly produced phytoplankton exudates to heterotrophic bacteria biomass production were studied in Mediterranean coastal waters. The short-term incorporation of ¹³C (H¹³CO₃) in phytoplankton and bacterial lipid biomarkers was measured as well as the total bacterial carbon production (BP), viral lysis and the microbial community structure under three experimental conditions: (1) High inorganic Nutrient and High Grazing (HN + HG), (2) High inorganic Nutrient and Low Grazing (HN + LG) and (3) under natural in situ conditions with Low inorganic Nutrient and High Grazing (LN + HG) during spring. Under phytoplankton bloom conditions (HN + LG), the bacterial use of freshly produced phytoplankton exudates as a source of carbon, estimated from ¹³C enrichment of bacterial lipids, contributed more than half of the total bacterial production. However, under conditions of high grazing pressure on phytoplankton with or without the addition of inorganic nutrients (HN + HG and LN + HG), the ¹³C enrichment of bacterial lipids was low compared with the high total bacterial production. BP therefore seems to depend mainly on freshly produced phytoplankton exudates during the early phase of phytoplankton bloom period. However, BP seems mainly relying on recycled carbon from viral lysis and predators under high grazing pressure.

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Molecular lipid biomarkers (hydrocarbons, alcohols, sterols and fatty acids) and compound-specific isotope analysis of suspended particulate organic matter (SPM) and surface sediments of the Mackenzie Shelf and slope (southeast Beaufort Sea, Arctic Ocean) were studied in summer 2009. The concentrations of the molecular lipid markers, characteristic of known organic matter sources, were grouped and used as proxies to evaluate the relative importance of fresh algal, detrital algal, fossil, C₃ terrestrial plants, bacterial and zooplankton material in the organic matter (OM) of this area. Fossil and detrital algal contributions were the major fractions of the freshwater SPM from the Mackenzie River with ~34% each of the total molecular biomarkers. Fresh algal, C₃ terrestrial, bacterial and zooplanktonic components represented much lower percentages, 17, 10, 4 and <1%, respectively. In marine SPM from the Mackenzie slope, the major contributions were fresh and detrital algal components (>80%), with a minor contribution of fossil and C₃ terrestrial biomarkers. Characterization of the sediments revealed a major sink of refractory algal material mixed with some fresh algal material, fossil hydrocarbons and a small input of C₃ terrestrial sources. In particular, the sediments from the shelf and at the mouth of the Amundsen Gulf presented the highest contribution of detrital algal material (60–75%), whereas those from the slope contained the highest proportion of fossil (40%) and C₃ terrestrial plant material (10%). Overall, considering that the detrital algal material is marine derived, autochthonous sources contributed more than allochthonous sources to the OM lipid pool. Using the ratio of an allochthonous biomarker (normalized to total organic carbon, TOC) found in the sediments to those measured at the river mouth water, we estimated that the fraction of terrestrial material preserved in the sediments accounted for 30–40% of the total carbon in the inner shelf sediments, 17% in the outer shelf and Amundsen Gulf and up to 25% in the slope sediments. These estimates are low compared to other studies conducted 5–20 yr earlier, and they support the increase in primary production during the last decade mainly because of the increase in the number of ice-free days and due to the strength and persistence of winds favouring upwelling

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