On the role of DOM under varying climate change conditions in Crocosphaera

Diazotrophs provide the main source of reactive nitrogen to the ocean, sustaining primary productivity and CO2 uptake. Climate change is raising temperatures, decreasing pH and reducing nutrient availability. How microbes respond to these changes is largely unexplained. Similarly, the role of DOM in the growth and survival of certain diazotrophic organisms is poorly understood. Moreover, growing evidence indicates some diazotrophs are capable of utilizing distinct DOM compounds via osmotrophy providing them with additional metabolic plasticity and ecological advantages compared to other non-diazotrophic microbes. We aimed to understand how osmotrophy could modify carbon uptake and alleviate energy stress in diazotrophs under ongoing climate change perturbations. We hypothesized that Crocosphaera preferentially uses DOM when labile as a carbon source in present pH conditions, as compared to future more acidic scenarios with higher access to inorganic carbon. Alternatively, the lower pH may cause Crocosphaera to be energy limited when trying to maintain intracellular homeostasis which would favour DOM uptake as an extra source of energy. Overall design: We exposed cultures of the unicellular cyanobacterium Crocosphaera watsonii WH8501 to current and future climate change scenarios (RCP 4.5 and 6.0 from IPCC 2014) combining temperature and pH levels (26ºC and pH 8.1, 28ºC and pH 8.0 and 30ºC and pH 7.9). We followed their growth, nitrogen and dissolved inroganic carbon fixation rates, DOM specific uptake rates and gene expression responses along 8 days comparing DOM amended samples with not amended controls. We discuss how the different treatments alter Crocosphaera metabolism, allowing or not to cope with warmer and more acidic ocean conditions, while providing an improved mechanistic understanding of the role of osmotrophy in diazotrophs and allowing for a better comprehension of the global nitrogen cycle in the future ocean.