Research Progress in Area 1 方向 ( 一 ) 課題進展 69 Abstract The aims of this collaborative study are to test and refine an experimental approach for quantifying and comparing growth and grazing of marine microbes on a taxon-specific basis (species or clades) and to use it to investigate microbial food web dynamics in estuarine and open-ocean field studies. The approach is based on the seawater dilution technique, which resolves growth and grazing rates using natural and diluted seawater treatments to manipulate the encounter rates of protistan consumers with their prey. Research Activities and Progress Due to continuing COVID-19 travel restrictions, P.I. Landry could not travel to Hong Kong to conduct Y2 field work. However, Y2 dilution experiments were conducted in Hong Kong coastal/South China Sea waters by post-docs Shunyan Cheung and Kailin Liu. We also completed analyses of experiments from the Bering Sea and eastern Indian Ocean for early insights on process rates and relationships for the 2022 field study. • Conducted dilution experiments on transect from Pearl River to the South China Sea; • Analyzed grazing mortality rates and instantaneous growth rates of UCYN-A sublineages in the western Bering Sea and western Subarctic Gyre using a combination of dilution technique and qPCR; • Assessed temperature sensitivities of picophytoplankton growth at 17 stations along a natural 10°C (18-28°C) temperature gradient in the eastern Indian Ocean characterized by deep mixing and consistently low dissolved nitrogen. Population biomass (B), cell carbon and chlorophyll were measured by flow cytometry. Instantaneous growth (µ) and production (P) were calculated from dilution incubations at four light levels. Key Findings • Contrary to most empirical and Metabolic Theory of Ecology predictions, Prochlorococcus showed insignificant temperature sensitivity of growth rate in oligotrophic waters along a natural temperature gradient; • Synechococcus were also below prediction, and picoeukaryotes showed high variability, including negative rates suggesting viral lytic cycles, at high temperature. Research Output Publication 2021 1 Trained personnel 1 Population-specific Dynamics of Microbial Growth and Grazing Prof. Michael Landry University of California San Diego Fig 1. Production:Biomass (P:B) ratios and growth rates for Prochloroccus (PRO), Synechococcus (SYN) and photosynthetic eukaryotes (EUK) along a 10°C gradient in environmental temperature in the oligotrophic eastern Indian Ocean.
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