Dynamics and interactions of protists in marine ecosystems

Abstract

Protists play a crucial role in marine ecosystems due to the diversity of their functional traits. This thesis focuses on their taxonomic and functional diversity in two oceanic regions: the Arctic and the Tropics. The thesis begins by investigating the community dynamics of photosynthetic protists in the ice and underlying water column during the Arctic spring under-ice bloom (Chapter 2). The temporal dynamics of these primary producers were characterised using DNA metabarcoding of the 18S rRNA gene. It demonstrates the significant contributions from diatoms and from overlooked classes: Pelagophyceae, Bolidophyceae and Cryptophyceae. Biogeography analyses showed the transition as the bloom progresses from polar to polar-temperate taxa, especially in pico-sized phytoflagellates. The importance of analysing the bloom at the microdiversity (amplicon sequence variants) level was also emphasized. Chapter 3 then utilizes a RNA metabarcoding approach to unveil the succession of ice and water protist functional groups. Network analyses uncovered ecological interactions between groups. The community shifted from having high functional diversity (phototrophs, mixotrophs and varying heterotrophs) to phototroph dominance accompanied by increased predation by eukaryvores. Chapter 4 describes a novel parasitic species Labyrinthula merlionensis isolated from Singapore coastal waters located in the Tropics. It reports cell-to-cell interactions with its diatom host by integrating various microscopy techniques (light, SEM, TEM and confocal). This interaction appears to be a combination of intra- and extra-cellular parasitism of the diatom hosts. This thesis contributes to a deeper understanding of protist diversity and interactions, particularly in the context of changing environmental conditions in the Arctic and the Tropics.