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More organic carbon gives less organic carbon

The investigation of the stoichiometric coupling between carbon and growth-limiting nutrients within the biomass of marine microorganisms is central to understand biogeochemical dynamics such as the marine carbon cycle.

A recent paper by T.F. Thingstad and 14 coauthors (Nature 455, 18 September 2008) reports surprising results about the carbon to nutrient coupling in an Arctic pelagic ecosystem. The accumulation rate of added organic carbon to mesocosm systems depended critically on the state of the microbial food web. Mineral nutrient limitation of bacterial growth resulted in higher carbon accumulation than organic carbon limitation.

The mechanism behind this surprising effect was that addition of dissolved carbon stimulated bacterial competition for nutrients and thereby reduced phytoplankton biomass and carbon production. This effect was even more pronounced in Silicate enriched, and therefore diatom dominated, mesocosms where carbon/mineral nutrient ratios of phytoplankton were high. As both, carbon/nutrient ratios of inorganic consumption and the release of dissolved organic carbon may increase with increasing CO2 levels (Riebesel et al. 2007) these results show that it is not possible to predict future states of the marine organic carbon cycle without understanding stoichiometric dynamics in autotrophic and heterotrophic food web processes.

Additionally, the study demonstrates the importance of experimental analyses to identify and investigate key processes regulating marine pelagic food web dynamics and thereby biogeochemical cycles. Mesocosm experiments have recently become an essential tool for such investigations demonstrated for example by the analyses of carbon consumption in a high CO2 ocean (Riebesell et al. 2007) or the control of marine pelagic trophic cascades by top predators and zooplankton (Stibor et al. 2004).

Review by Herwig Stibor. LMU Munich

References:
Riebesell, U. et al. Enhanced biological consumption in a high CO2 ocean. Nature 450, 545-548 (2007)
Stibor et al. Copepods act as a switch between alternative trophic cascades in marine pelagic food webs. Ecology Letters 7: 321-328 (2004)
T. F. Thingstad et al., 2008. Counterintuitive carbon-to-nutrient coupling in an Arctic pelagic ecosystem, Nature 455 http://dx.doi.org/10.1038/nature07235



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