[A-OS13] Marine ecosystems and biogeochemical cycles: theory, observation and modeling

[AOS13-05] Diversity in physiology and genomic function of Prochlorococcus across the Eastern Indian Ocean

*Siyu Jiang¹, Fuminori Hashihama², Hongbin Liu³, Kazutoshi Yoshitake⁴, Hideto Takami^1,5, Koji Hamasaki^1,6, Idha Yulia Ikhsani⁷, Hajime Obata¹, Hiroaki Saito¹ (1.Atmosphere and Ocean Research Institute, The University of Tokyo, 2.Department of Ocean Sciences, Tokyo University of Marine Science and Technology, 3.Department of Ocean Science, Hong Kong University of Science and Technology, 4.Graduate School of Agricultural and Life Sciences, The University of Tokyo, 5.Center for Mathematical Science and Advanced Technology, JAMSTEC, 6.Collaborative Research Institute for Innovative Microbiology, The University of Tokyo, 7.Research Center for Oceanography - National Research and Innovation Agency, Indonesia)

Keywords:Eastern Indian Ocean, oligotrophic, Prochlorococcus, physiology, growth rate, functional metagenomics

The widespread distribution (40°N to 40°S) and domination of Prochlorococcus in phytoplankton communities in global oligotrophic oceans could be attributed to the extensive genetic diversity that allows them to adapt to various nutrient environments. However, it is unknown whether Prochlorococcus could also show diverse physiologies at the population-level. To fill the gap of knowledge, we conducted a meridional cruise in the surface Eastern Indian Ocean (EIO, 16.5°N to 20°S). The growth rate of Prochlorococcus population (division rate and pigment production rate), and its response to macronutrient enrichments (NH₄Cl and KH₂PO₄) were examined by dilution experiments. Additionally, the Prochlorococcus ecotype composition and abundance of functional modules that related to nutrient utilization were determined by the metagenomic analysis. Although the dissolved inorganic nitrogen was depleted (~58 nM) and the Prochlorococcus population was dominated by the highlight-adapted II ecotype throughout, incubation experiments clearly showed Prochlorococcus populations had distinct physiologies especially the response to macronutrient enrichments, indicating their adaptation to local nutrient environments. Moreover, the distinct physiologies of Prochlorococcus population could be explained by the spatially varied abundance of functional modules to some extent. At the northernmost station in Bay of Bengal, the significant increase in growth rate with macronutrient enrichments and the highest abundance of phosphate starvation response two-component regulatory system collectively indicated the phosphorus limitation. In contrast in the southern EIO, the low cell division rate, insignificant increase in growth rate with macronutrient enrichments, and the higher abundance of iron complex transport system suggested the Prochlorococcus growth was possibly limited by the iron availability. Our results showed the current understanding of Prochlorococcus needs to be revised to also consider its physiological diversity at the population-level, since it is the population that gets involved in marine ecological processes and biogeochemical cycle.