Particulate (POM) and dissolved organic matter (DOM) released by the cold water corals Lophelia pertusa (L.) and Madrepora oculata (L.) was collected, analysed and quantitatively compared to that released by warm water reef-building corals. Particulate nitrogen (PN) and particulate organic carbon (POC) release rates of L. pertusa were 0.14 ± 0.07 mg N m -2 h -1 and 1.43 ± 1.22 mg C m -2 h -1, respectively, which is in the lower range of POM release rates measured for warm water corals, while dissolved organic carbon (DOC) release was 47 ± 19 mg C m -2 h -1. The resulting high DOC:POC ratio indicates that most cold water coral-derived organic matter immediately dissolved in the water column. Cold water corals, similar to their warm water counterparts, produced large amounts of nitrogen-rich coral mucus with C:N ratios of 5 to 7 for Lophelia-and 7 to 9 for Madrepora-derived mucus. A 7-fold increase in the oxygen consumption rates in cold water coral mucus-amended seawater containing the natural microbial assemblage indicates that this organic matter provided an attractive food source for pelagic microbes. In situ investigations at Røst Reef, Norway, showed that microbial activity in the seawater closest to the reef was 10 times higher than in the overlying water column. This suggests that cold water corals can stimulate microbial activity in the direct reef vicinity by the release of easily degradable and nutrient-rich organic matter, which may thereby function as a vector for carbon and nutrient cycling via the microbial loop in cold water coral reef systems. KEY WORDS: Coral reefs · Cold water corals · Lophelia pertusa · Madrepora oculata · Organic matter release · Microbial ecology · Fauna-microbe interaction Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 372: [67][68][69][70][71][72][73][74][75] 2008 products and mucus (e.g. Harrison et al. 1984, Crossland 1987, in dissolved (Ferrier-Pages et al. 1998) or particulate form into their surroundings. The release of organic matter by corals makes an important contribution to the ecological functioning of tropical coral reefs by controlling key processes such as the transport of organic matter. This, in turn, may influence planktonic and benthic metabolism as well as the associated recycling of essential elements. Mass release of eggs and sperm during the annual coral spawning event can have extensive biogeochemical consequences (Wild et al. 2004c, Eyre et al. 2008, Glud et al. 2008. Furthermore, mucus continuously released by tropical corals can act as an energy carrier and particle trap (Wild et al. 2004a) and consequently initiates element cycling and interaction between fauna and microorganisms in tropical reef ecosystems. Within this context, coral-derived mucus can strongly influence planktonic microbial metabolism (Ferrier-Pages et al. 2000), microbial abundance (Wild et al. 2004b) and microbial community composition (Allers et al. 2008).Cold water coral reefs exist in different environmental sett...
Glacial retreat is changing biogeochemical cycling in the Arctic, where glacial runoff contributes iron for oceanic shelf primary production. We hypothesize that in Svalbard fjords, microbes catalyze intense iron and sulfur cycling in loworganic-matter sediments. This is because low organic matter limits sulfide generation, allowing iron mobility to the water column instead of precipitation as iron monosulfides. In this study, we tested this with high-depth-resolution 16S rRNA gene libraries in the upper 20 cm at two sites in Van Keulenfjorden, Svalbard. At the site closer to the glaciers, iron-reducing Desulfuromonadales, iron-oxidizing Gallionella and Mariprofundus, and sulfur-oxidizing Thiotrichales and Epsilonproteobacteria were abundant above a 12-cm depth. Below this depth, the relative abundances of sequences for sulfate-reducing Desulfobacteraceae and Desulfobulbaceae increased. At the outer station, the switch from iron-cycling clades to sulfate reducers occurred at shallower depths (ϳ5 cm), corresponding to higher sulfate reduction rates. Relatively labile organic matter (shown by ␦ 13 C and C/N ratios) was more abundant at this outer site, and ordination analysis suggested that this affected microbial community structure in surface sediments. Network analysis revealed more correlations between predicted iron-and sulfur-cycling taxa and with uncultured clades proximal to the glacier. Together, these results suggest that complex microbial communities catalyze redox cycling of iron and sulfur, especially closer to the glacier, where sulfate reduction is limited due to low availability of organic matter. Diminished sulfate reduction in upper sediments enables iron to flux into the overlying water, where it may be transported to the shelf. IMPORTANCE Glacial runoff is a key source of iron for primary production in the Arctic. In the fjords of the Svalbard archipelago, glacial retreat is predicted to stimulate phytoplankton blooms that were previously restricted to outer margins. Decreased sediment delivery and enhanced primary production have been hypothesized to alter sediment biogeochemistry, wherein any free reduced iron that could potentially be delivered to the shelf will instead become buried with sulfide generated through microbial sulfate reduction. We support this hypothesis with sequencing data that showed increases in the relative abundance of sulfate reducing taxa and sulfate reduction rates with increasing distance from the glaciers in Van Keulenfjorden, Svalbard. Community structure was driven by organic geochemistry, suggesting that enhanced input of organic material will stimulate sulfate reduction in interior fjord sediments as glaciers continue to recede.
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