Marine Biogeochemistry

Environmental impacts on the cycling of organic matter by heterotrophic bacteria

Heterotrophic bacteria are the main producers of CO2 in the ocean, thereby counteracting the biological drawdown of CO2 by primary production. The transfer of organic carbon to the deep ocean and the subsequent long term removal of CO2 from the atmosphere are strongly attenuated by heterotrophic recycling processes that regenerate inorganic nutrients and CO2 in the microbial loop. Bacterial activity in the ocean is controlled by multiple abiotic and biotic environmental factors. Major constraints on bacterial growth and activity are temperature and the availability of organic matter and inorganic nutrients. Effects of climate change on heterotrophic processes driven by bacterial activity in the ocean are still largely unknown. Studies conducted in the last decade have shown a high potential of warming and acidification to enhance heterotrophic bacterial activity. Tipping the balance of autotrophic carbon fixation and heterotrophic recycling would have a high potential to change biogenic carbon fluxes in the ocean. Our studies investigate the impact of warming, acidification and deoxygenation on the bacterial turnover of organic matter and aim to identify processes that are potentially sensitive to upcoming changes in the marine environment.

While single bacterial species and processes may be affected strongly by environmental stressors, natural bacterial communities can handle changes effectively due to their functional diversity and redundancy.

    • Ocean acidification
      We studied the effects of elevated seawater pCO2 on a natural plankton community during a large-scale mesocosms (KOSMOS) studies in the North Sea, Baltic Sea and the Arctic Ocean. During the 2011 KOSMOS study, bacterial growth as well as total and cell-specific aminopeptidase activities were elevated under low pH conditions (Endres et al. 2014). We conclude that ocean acidification has the potential to stimulate the bacterial community and facilitate the microbial recycling of freshly produced organic matter, thus strengthening the role of the microbial loop in the surface ocean.
    • Ocean warming 
      The period of 1995-2005 was the warmest decade in the Arctic since at least the 17th century, with air temperatures 2 °C above the 1951-1990 average. We test individual and combined effects of warming and organic matter amendment on growth, biomass production and extracellular enzyme activities of Arctic bacterioplankton. Results show that the supply of carbohydrates strongly enhanced temperature effects on bacterial growth, suggesting synergistic combined effects of temperature and organic matter availability (Piontek et al. 2015). Hence, the complexity of combined effects must be considered to better assess the potential of climate change to alter biogenic carbon and energy fluxes in marine systems.
    • Oxygen limitation

      Little is known about the oxygen sensitivity of heterotrophic bacterial metabolism in the Baltic Sea and potential consequences for elemental cycles. We study microbial cycling in coastal areas and the deep basins of the Baltic Sea during cruises and incubation studies. Additionally, we study Shewanella baltica, an important denitrifying bacterium of the Gotland Deep under oxic and anoxic conditions in chemostats (Maßmig et al., in prep.)

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