Marine O2 and the ecology of microbial eukaryotes in past and present oceans

Oxygen availability has controlled eukaryotic evolution and ecology for over a billion years. While ongoing marine deoxygenation redirects energy to lower trophic levels, deep-time increases in marine oxygen levels likely made these higher trophic levels possible. Early eukaryotic diversification and proliferation, including the onset of appreciable eukaryotic marine primary productivity and the origin of animal and plant life, have been attributed to marine oxygenation and its influence on eukaryotic grazing and trophic specialization – although these proposed relationships remain to be tested. For over ten years, Saanich Inlet – a seasonally anoxic fjord on the coast of Vancouver Island, British Columbia, Canada – has served as a natural laboratory for exploring relationships between marine oxygen availability and the dynamics of microbial communities, including marine protists. Building off previous efforts in Saanich Inlet, we present here time-series observations of ITS, 16S, and 18S rDNA gene diversity over a 12-month period, covering six depths between 10 and 200 m and O₂ concentrations between 250 and <1 µM. Consistent with previous reports, we identified a relatively high abundance of Alveolata OTUs (Dinoflagellata and Ciliophora), in addition to high relative abundances of Opalozoa and Ochrophyta from the Stramenopiles, both primarily between 120-200m, with the Opalozoa most abundant during the winter months between 8 and <1 µM O₂, and the Ochrophyta most abundant during the summer and fall months between 40 and <1 µM O₂. The addition of fungal-specific ITS sequences suggests relatively high abundances of chytrids and basidiomycetes, with no obvious structure with depth and O₂ concentration. Understanding the physiological and ecological mechanisms underpinning these distributions will elucidate both the deep-time and near-future controls of dissolved O₂ concentration on eukaryotic community structure.