The influence of nitrifiers on the oceanic carbon cycle

About one quarter of the carbon dioxide (CO2) released to the atmosphere dissolves in the surface ocean and equilibrates with the inorganic carbon compounds present in seawater. Phytoplankton in the sunlit surface ocean are responsible for half of the Earths photosynthesis, converting some of this dissolved inorganic carbon to organic matter. When phytoplankton dies and consequently sinks through the water column, organic matter is transported to deeper depths. This sinking organic matter supports most of the marine food web. While the majority of the organic carbon is eventually respired back to CO2 by marine micro- and macroorganisms, a small fraction sinks to the seafloor, resulting in the sequestration of carbon for several thousand years. Understanding how this carbon is cycled on the way to the ocean floor has important implications for the way we model the carbon cycle, and predict the ocean`s role in mitigating climate change. In addition to microorganisms that consume organic carbon, the deep ocean is also home to an abundant community of microorganisms that can use chemical energy to convert inorganic carbon into biomass. These organisms are able to oxidize, for example, reduced nitrogen compounds such as ammonia (NH3) and nitrite (NO2-) to generate the energy they need for the fixation of inorganic carbon. Hence, similar to phytoplankton in the surface ocean, ammonia- and nitrite-oxidizing microorganisms make organic carbon available for microorganisms that consume these compounds in the deep ocean. In addition to the organic material that is released upon cell death and lysis, living microbial cells can also leak organic compounds into their ambient environment. These compounds could represent an important additional source of organic carbon in the deep ocean, which is not yet accounted for in models of the oceans carbon cycle. This scientific project aims to quantify and characterize the organic carbon released by ammonia- and nitrite-oxidizing microorganisms using laboratory cultures and field measurements. The overall goal is to integrate estimates of previously unaccounted sources of organic carbon into global carbon cycle models to better understand future climate change scenarios.

Understanding how carbon is cycled on the way to the ocean floor has important implications for the way we model the carbon cycle, and predict the ocean's role in mitigating climate change. In addition to microorganisms that consume organic carbon, the deep ocean is also home to an abundant community of microorganisms that can use chemical energy to convert inorganic carbon into biomass (chemoautotrophy). These organisms are able to oxidize, for example, reduced nitrogen compounds such as ammonia and nitrite to generate the energy they need for the fixation of inorganic carbon. This newly fixed carbon represents an important nutritional foundation for heterotrophic food webs in the deep ocean. In addition to the organic material that is released upon cell death and lysis, living microbial cells can also leak organic compounds into their ambient environment. Quantifying how much inorganic carbon these microorganisms fix and release again into the ambient seawater is critical to a complete understanding of the oceanic carbon cycle. To address this knowledge gap, we measured the inorganic carbon fixation yields and dissolved organic carbon release rates of laboratory microbial cultures. The results obtained within the framework of this research project suggest that ammonia- and nitrite-oxidizing microorganisms release 5 to 15% of their recently fixed inorganic carbon as dissolved organic carbon. This would equate to yearly global ocean fluxes of 6000 to 20,000kilotons of carbon. The released compounds might represent an important additional source of organic carbon in the deep ocean, which is not yet accounted for in models of the ocean's carbon cycle. Elucidating the lability and fate of released organic carbon will be the crucial next step to understand its implications for marine food-web functioning and the biological sequestration of carbon in the ocean. Another important result originating from this research proposal was the discovery that some marine nitrite oxidizers can use formate as alternative energy source. While the activity of ammonia and nitrite oxidizers are typically tightly coupled in the ocean through their combined involvement in the nitrification process, the metabolic versatility of nitrite oxidizers challenges this traditional view. Future research will focus on the implications of alternative metabolisms of nitrite oxidizers in the marine environment. In summary, the results of this scientific research project elucidate the diverse roles of nitrifying microorganisms in the oceanic carbon cycle, including the release of dissolved organic carbon into the environment and the use of organic carbon as energy source. Furthermore, these findings provide values for biogeochemical models of the global carbon cycle, and help to further constrain the relationship between carbon and nitrogen fluxes in the nitrification process.