Redefining the marine carbon cycle

There is a fundamental unsolved problem in our understanding of the oceanic carbon cycle: what is the contribution and role of heterotrophic anaplerotic fixation in the production of new organic matter? Resolving this enigma could redefine the marine carbon cycle, by placing heterotrophs as an integral part of the global ocean, not only as consumers but also as key producers of new particulate organic carbon available for the marine food web. Thus, this project will test the hypothesis that anaplerotic pathways are key contributors to the vast flux of oceanic dark CO2 fixation; and that this, thus far, neglected C flux is relevant in the marine C cycle at present and will become more important in response to ocean warming. To test our hypothesis we will perform field and lab studies combining basic biochemical rate measurements and applying state-of-the art molecular tools. With the field experiments we will provide the first specific single-cell level dark CO2 fixation rates for anaplerotic bacteria in the sea, and see how they compare to the heterotrophic growth based on organic C (and therefore their relevance in marine C cycling). With the lab culture studies, we will be able to calculate how much the anaplerotic DIC fixation contributes to the bacterial C production, and how relevant the anaplerotic dark DIC fixation of key microorganisms is as compared to other important C fluxes such as respiration. Finally, to test the effect of warming on anaplerotic DIC fixation, we will perform both of the above-mentioned field and lab culture investigations under different seawater temperatures. With this information we will determine the effect of warming on anaplerotic DIC fixation, and compare it to other C fluxes (i.e., respiration, growth) of key marine microbes. Our project is original because the magnitude/contribution of the anaplerotic metabolism of heterotrophic microbes has never been determined in natural environments, and thus it is not considered in the oceanic carbon cycle. If this project is successful its findings would redefine the marine C cycle as we know it by incorporating anaplerotic pathways as key elements in the C cycle. Our results would also imply that not only autotrophs are responsible for organic C production, but that also heterotrophs/mixotrophs are main actors in organic C production, potentially changing textbooks on the functioning of the C cycle. Bearing in mind the similarities of microbial communities among different environments, it is very likely that the key role of anaplerotic DIC fixation is not only relevant in the ocean but also in other environments, further upscaling the transformative potential of our project. The results obtained in this project will set the base for a larger project to model/estimate the global contribution of anaplerotic DIC fixation to global C budgets.

Our project addressed a critical aspect of marine ecology and biogeochemistry: dissolved inorganic carbon (DIC) fixation. We focused on studying the contribution and role of heterotrophic microbes, particularly through anaplerotic fixation, in producing new organic matter. Early on, we identified a previously unknown heterotrophic microbe, a gammaproteobacteria called UBA868, which dominates DIC fixation globally despite its low abundance. UBA868 significantly contributes to both heterotrophic and autotrophic activities within the microbial community, especially in the mesopelagic ocean. It dominates the total expression of RuBisCO (rbcL type II) genes and key sulfur oxidation (soxB and rdsrA) genes in this region. UBA868's ability to utilize alternative energy sources, such as sulfur compound oxidation, makes it a major contributor to the autotrophic and heterotrophic metabolic activities in the mesopelagic ocean. This microbe fills a crucial gap in oceanic elemental cycles. Traditionally, the oceanic water column's ecology and biogeochemistry have been associated with obligate autotrophs (phototrophs or chemoautotrophs) and heterotrophs. The prevailing paradigm in deep ocean biogeochemistry posits that organic matter sources sustaining deep ocean metabolism originate from the euphotic layer (via the biological carbon pump) or are generated in situ by chemoautotrophic ammonia-oxidizing Crenarchaeota and nitrite-oxidizing Nitrospina and Nitrospira. Our study reveals that the mixotrophic bacteria UBA868 plays a remarkable role in biogeochemical transformations of organic matter in the deep open ocean. This highlights the importance of mixotrophs, alongside obligate autotrophs and heterotrophs, in oceanic biogeochemical cycles. Our findings significantly advance and potentially transform the field by showcasing a previously unknown and globally relevant microbe that dominates DIC transformation into organic carbon in the deep ocean. Furthermore, our research demonstrates that significant DIC fixation in the ocean is not only carried out by autotrophs (such as surface obligate photoautotrophs and deep-water chemoautotrophs) but also by mixotrophic microbes. This project redefines the marine carbon cycle by incorporating microbial mixotrophs as key elements in carbon cycling and anaplerotic pathways. Our results imply that both autotrophs and heterotrophs/mixotrophs are central to organic carbon production, potentially altering existing textbooks on the carbon cycle. Given the similarities of microbial communities across different environments, it is likely that the role of mixotrophs and anaplerotic DIC fixation extends beyond the ocean, amplifying the transformative potential of our project. The results lay the foundation for a larger project to model and estimate the global contribution of anaplerotic DIC fixation to global carbon budgets. Although our research is fundamentally ecological and does not anticipate direct effects beyond the scientific community, it indirectly impacts areas such as fisheries and global warming, which in turn affect the economy and society. Given the global scale of our discoveries, we anticipate that our research will have worldwide relevance and implications.