[Department of Ocean Science] PhD Qualifying Examination Defense Seminar: Top-down control on picoplankton in Hong Kong coastal waters
 

Abstract:

Marine picoplankton contribute substantially to primary production, microbial biomass and carbon cycling in coastal waters. Their populations are regulated by viral lysis, which recycles host-derived organic matter within the microbial loop, and protistan grazing, which transfers picoplankton production to higher trophic levels. Yet the identities and seasonal dynamics of the viruses and grazers involved, and their relative contributions to mortality, remain poorly resolved. This study integrates seasonal viral metagenomics, RNA stable-isotope probing (RNA-SIP) and modified dilution experiments to investigate top-down control of picoplankton in subtropical Hong Kong coastal waters. Analysis of 24 viral metagenomes recovered 354,029 viral operational taxonomic units, 96.4% of which were dsDNA phages. Although viral richness and Shannon diversity were comparable between Deep Bay and Tai Tan, community composition was significantly associated with both season and habitat. NCLDVs were enriched in Deep Bay during spring, whereas ssDNA viruses were more abundant in Tai Tan during autumn and winter. Persistent dominance of Uroviricota also concealed substantial family-level turnover. RNA-SIP with labelled cyanobacterial, picoeukaryotic and heterotrophic bacterial prey is being used to identify prey-specific grazers and determine how grazing relationships vary seasonally. Modified dilution experiments further partition mortality between grazing and viral lysis. Preliminary results indicate that grazing accounted for 86-99% of cyanobacterial and 90-100% of picoeukaryotic mortality, whereas viral lysis was comparatively limited. Nevertheless, total mortality (the sum of viral lysis and grazing) matched gross growth in both groups during the preliminary sampled period. A year-long monthly time series will determine how these contributions vary among seasons. Together, these approaches will integrate virus-host associations, grazer-prey interactions and process rates into a seasonally resolved picoplankton carbon-flow network, clarifying how natural seasonality and other environmental drivers regulate the balance between viral recycling and trophic transfer in Hong Kong coastal microbial food webs.