THESIS
2020
xii leaves, 80 pages : color illustrations ; 30 cm
Abstract
Nitrogen (N) is essential to the growth and metabolism of life, and it often limits primary
productivity in the ocean. Biological N
2 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...[
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Nitrogen (N) is essential to the growth and metabolism of life, and it often limits primary
productivity in the ocean. Biological N
2 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, N
2 and CO
2 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.
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