[Department of Ocean Science] MPhil Thesis Defense Seminar: Ecological Differentiation of Picocyanobacteria Communities in the Subtropical and Subarctic Pacific
 

Abstract:

Marine picocyanobacteria of the genera Prochlorococcus and Synechococcus are the most abundant photosynthetic organisms in the ocean and major contributors to global primary production. Their ecological success rests on extensive genetic diversity, which partitions each genus into clades and finer ecotypes with distinct light, temperature, and nutrient preferences. Yet most field studies rely on chlorophyll, cell counts, or genus-level abundance, measures that average over physiologically distinct lineages and can obscure, or even invert, the underlying community response. Two questions remain open: whether ecologically meaningful structure exists below the clade level, particularly along trace-metal gradients, and how physical transport by ocean currents reshapes community composition. This thesis addresses both queations by resolving picocyanobacteria to the ecotype and subclade level along two contrasting gradients, pairing high-resolution amplicon sequencing with concurrent hydrographic and biogeochemical measurements. At Xianbei Seamount in the oligotrophic South China Sea, topographic upwelling produced approximately 10m of isopycnal doming and a 4.5-fold enrichment in dissolved inorganic nitrogen at the summit deep chlorophyll maximum, sustaining warm-water, high-light ecotypes within their canonical habitat, while downstream, lee-wave mixing lifted deep-water, low-light ecotypes into the upper euphotic zone, even as total picocyanobacterial abundance collapsed nearly tenfold. This decoupling shows that bulk biomass misrepresents the community response to seamount forcing, with the synchronous shoaling of taxa with divergent physiologies recording vertical advection rather than shared adaptation. In the western subarctic Pacific and Bering Sea, where biogeochemical forcing dominates, the Bering Slope Current spatially dissolved iron from macronutrients. Cold-water clade I prevailed throughout (>90% of reads), but this apparent uniformity masked a reciprocal split between iron-replete shelf and iron-depleted basin subclades, revealing iron nitrogen coupling as the axis structuring sub-ecotype differentiation. Together, these findings demonstrate that fine-scale lineage turnover, not bulk abundance, is the primary biological signature of environmental forcing in the ocean.