Nitrogen (N) is essential to the growth and metabolism of life, and it often limits primary productivity in the ocean. Biological N₂ fixation is an important source of new N in the euphotic zone of oceanic water, which is mainly conducted by cyanobacterial diazotrophs. Crocosphaera watsonii is a major unicellular cyanobacterial diazotroph that is highly abundant in the warm and oligotrophic waters of tropical and subtropical oceans. In this thesis, we explored the top-down and bottom-up controls on C. watsonii, with specific focus on the effects of microzooplankton grazing and ocean warming. By examining the diel grazing pattern of protistal grazers on C. watsonii and non-diazotrophic unicellular microalgae, we found that all the tested grazers increased grazing selectivity on C...[ Read more ]

Nitrogen (N) is essential to the growth and metabolism of life, and it often limits primary productivity in the ocean. Biological N₂ fixation is an important source of new N in the euphotic zone of oceanic water, which is mainly conducted by cyanobacterial diazotrophs. Crocosphaera watsonii is a major unicellular cyanobacterial diazotroph that is highly abundant in the warm and oligotrophic waters of tropical and subtropical oceans. In this thesis, we explored the top-down and bottom-up controls on C. watsonii, with specific focus on the effects of microzooplankton grazing and ocean warming. By examining the diel grazing pattern of protistal grazers on C. watsonii and non-diazotrophic unicellular microalgae, we found that all the tested grazers increased grazing selectivity on C. watsonii when C. watsonii was fixing N at night. It suggested that diazotrophs are more nutritious (inferred by C:N ratio) when they are fixing N and may act as an important N source to the microzooplankton in the ocean. In the context of ocean warming, diazotrophs are facing temperature elevation and phosphorus (P) limitation in oceanic waters (as a result of intensifying water stratification). By comparing the physiology of C. watsonii under P-limited and P-replete conditions with temperature gradient, we found that elevated temperature relieved the P limitation of its growth, N₂ and CO₂ fixation. P use efficiency of C. watsonii increased with temperature under P limitation. As revealed by the expression levels of the genes involved in P metabolism, P recycling and acquisition of C. watsonii were upregulated by both P limitation and temperature elevation. Our results provided new insights to the ecological function and environmental adaptation of an important marine diazotroph, which contributed to the mechanistic understanding of the biogeochemical cycling of the warming ocean.