Counterintuitive carbon-to-nutrient coupling in an Arctic pelagic ecosystem

Abstract

Predicting the ocean's role in the global carbon cycle requires an understanding of the stoichiometric coupling between carbon and growth-limiting elements in biogeochemical processes. A recent addition to such knowledge is that the carbon/nitrogen ratio of inorganic consumption and release of dissolved organic matter may increase in a high-CO(2) world. This will, however, yield a negative feedback on atmospheric CO(2) only if the extra organic material escapes mineralization within the photic zone. Here we show, in the context of an Arctic pelagic ecosystem, how the fate and effects of added degradable organic carbon depend critically on the state of the microbial food web. When bacterial growth rate was limited by mineral nutrients, extra organic carbon accumulated in the system. When bacteria were limited by organic carbon, however, addition of labile dissolved organic carbon reduced phytoplankton biomass and activity and also the rate at which total organic carbon accumulated, explained as the result of stimulated bacterial competition for mineral nutrients. This counterintuitive 'more organic carbon gives less organic carbon' effect was particularly pronounced in diatom-dominated systems where the carbon/mineral nutrient ratio in phytoplankton production was high. Our results highlight how descriptions of present and future states of the oceanic carbon cycle require detailed understanding of the stoichiometric coupling between carbon and growth-limiting mineral nutrients in both autotrophic and heterotrophic processes.

Pelagic community production and carbon-nutrient stoichiometry under variable ocean acidification in an Arctic fjord

“…Tanaka et al (2013) also described no pCO 2 effect on community respiration. These findings imply that the role of bacterioplankton as competitor for mineral nutrients could be strengthened in the Arctic Ocean (Thingstad et al, 2008), while their role in recycling organic matter into inorganic carbon and nutrients could remain unchanged.…”

“…Concentrations of DOC, however, did not change significantly after nutrient addition, indicating higher DOC consumption by bacteria with increasing pCO 2 . Like phytoplankton, bacteria require inorganic nutrients to grow and to increase their biomass (Thingstad et al, 2008), thus higher abundance of both phytoplankton and bacteria in mesocosms with high pCO 2 results in an increased demand for mineral nutrients. Phytoplankton growth in phase II was again terminated by viral infection .…”

CO<sub>2</sub> increases <sup>14</sup>C primary production in an Arctic plankton community

“…Such a counterintuitive cycling of carbon (i.e., higher autotrophic carbon fixation leads to less net production of the whole community) has been hypothesized for Arctic systems previously (Thingstad et al, 2008).…”

“…An increased input of labile DOC (glucose) was rapidly consumed by bacteria and other osmotrophs during an earlier mesocosm study at Svalbard, resulting in enhanced competition for inorganic nutrients between phyto-and bacterioplankton, and in an overall reduction of autotrophic productivity of the system (Thingstad et al, 2008). A hypothesis that came out of the study of Thingstad et al (2008) was that stimulation of the microbial loop in Arctic waters by increased DOC release under high pCO 2 may result in a counterintuitive carbon cycling (i.e., "more organic carbon gives less organic carbon") and not necessarily enhance carbon export to the deep sea.…”

Gammaproteobacterial diazotrophs and nifH gene expression in surface waters of the South Pacific Ocean

“…While some heterotrophic bacteria can fix N 2 in anaerobic or aerobic conditions, the organisms need to consume large amounts of carbon to obtain energy. Carbon is known to be an important growth-limiting factor for oceanic heterotrophic bacteria (Kirchman et al, 2000;Thingstad et al, 2008). We speculate that the most reasonable explanation for low N 2 fixation rates and low growth rates of heterotrophic prokaryotes in the open ocean is a lack of sufficient energy in the form of bioavailable carbon.…”