The trophic significance of bacterial carbon in a marine intertidal sediment: Results of an in situ stable isotope labeling study

نویسندگان

  • Dick van Oevelen
  • Leon Moodley
  • Karline Soetaert
  • Jack J. Middelburg
چکیده

We report the results of an in situ tracer experiment in an intertidal sediment, where bacterial carbon was tagged with stable carbon–isotope label, after the injection of 13C-glucose. The appearance of label in bacteria (based on label incorporation in bacteria-specific, phospholipid-derived fatty acids) and subsequent transfer to meiobenthos (group level) and macrobenthos (species level) was followed for 36 days. The label dynamics of benthic taxa were either fitted with a simple-isotope model or evaluated against enrichment in bacteria, to derive the importance of bacterially derived carbon for the meiobenthos and macrobenthos. Although selective uptake of bacteria was evident, as 2.4 times more bacterial carbon was grazed as expected from indiscriminate feeding, bacterial carbon accounted on average for only 0.08 and 0.11 of the carbon requirements of meiobenthic and macrobenthic taxa, respectively. Additionally, the contribution of bacterial carbon to total carbon requirements did not depend on the living/feeding depth in the sediment or organism size (evaluated over a size range of four orders of magnitude). The observed overall low contribution of bacterial carbon implies that most intertidal benthic fauna depend primarily on other carbon resources that may assert a stronger control on the structure of intertidal-sediment communities. Deposit-feeding organisms face the formidable task of gathering digestible resources that are diluted with minerals and refractory organic matter (Lopez and Levinton 1987). Bacteria are ubiquitous in marine sediments, and because of their high abundance, production, and nutritional value, they are considered an important resource for sedimentdwelling fauna (Zobell and Feltham 1937; Gerlach 1978; Tsuchiya and Kurihara 1979). Transfer of bacterial carbon to benthic fauna is often discussed from the bacterial side: Is bacterial-carbon production a link or sink in the benthic food web (Kemp 1990)? Most studies show that grazing losses are generally restricted to less than 20% of bacterial-carbon production (Kemp 1987; Epstein and Shiaris 1992; Sundback et al. 1996). Although grazing may represent a minor fate of bacterial production, a crucial complementary ecological question is how much the bacterially derived carbon contributes to the total carbon requirements of benthic fauna. However, observations on the trophic significance of bacterial carbon for different benthic taxa are limited. For some meiobenthos (e.g., nematodes), estimates on the relative importance of bacterial carbon are based on qualitative gut-contents analysis (Moens and Vincx 1997). Quantitative data are available for some macrobenthic deposit feeders that are based on laboratory measurements of sediment-ingestion rate, bacterial abundance, and bacterial-digestion efficiency on the one hand and physiologic data on carbon requirements on the other hand (Cammen 1980; Kemp 1987; Andresen and Kristensen 2002). Most of these studies show that bacterial carbon contributes less than 10% of the total carbon requirements. However, deposit feeders are known for their selective uptake of organic matter (Lopez and Levinton 1987; Neira and Höpner 1994), which may result in ingestion of sediment with higher bacterial abundances, as compared with bulk sediment (Plante and Jumars 1993; Andresen and Kristensen 2002). Therefore, the bacterial contribution as a carbon source may have been underestimated in previous studies that used bulk sediment–bacterial abundance. Moreover, deep-living meiobenthic and subsurface macrobenthic deposit feeders have restricted access to recently produced or deposited labile organic matter, as compared with meiobenthos and macrobenthos living close to the sediment surface. This circumstance may have repercussions on the extent to which they exploit bacteria as a carbon resource, but quantitative data are lacking so far. Quantitative data on the link between bacteria and benthic fauna are essential for understanding the extent to which this trophic link structures sediment communities. In this paper, we use an in situ stable isotope–labeling method to quantify the importance of bacteria as a carbon source for the meiobenthic and macrobenthic community. 13C-glucose was injected into an intertidal marine sediment to isotopically enrich the bacterial community. Incorporation of 13C-glucose into bacterial carbon was traced through 13C enrichment of bacteria-specific, phospholip1 Corresponding author ([email protected]).

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تاریخ انتشار 2006