An Analysis of Protist Diversity in Oxygen Deficient Zones: Understanding the Relationship between Prochloroccocus and Grazing

The cyanobacteria Prochlorococcus is an integral part of oxygen deficient zone (ODZ) microbial ecology. In the Eastern Tropical North Pacific, Prochlorococcus displays two maxima, one within the epipelagic zone at 50-70 meters (Low-Light I ecotype) and one in the ODZ at ~100 meters (Low-Light IV ecotype (LLIV)). LLIV Prochlorococcus reproduces slowly, making populations vulnerable to grazing. Previous non-quantitative studies indicate that heterotrophic protists are in the ODZ. We investigated if ODZ LLIV Prochlorococcus are subject to heterotrophy from microbial eukaryotes. The two prevailing hypotheses predict that: (A) only symbiont-bearing mixotrophic protists, which consume fewer prey due to autotrophy, would be present at the second Prochlorococcus maximum, or (B) heterotrophic and mixotrophic protist populations would decrease along the oxycline, reducing grazing pressure on LLIV Prochlorococcus. We used 18S rDNA long amplicon sequences (62847 1500 basepair amplicons) to identify the protist community and create a phylogenetic tree. Metagenomic reads were placed on the phylogenetic tree, creating depth profiles of protist frequency. Results from both amplicons and metagenomics identified that both heterotrophic and mixotrophic protists were present at the second Prochlorococcus maximum, specifically high concentrations of the mixotrophic Radiolarian order, Spumellaria, along with heterotrophic Dinoflagellate orders: Prorocentrales and Gymnodiniales. However, metagenomic data indicated that most protist populations decreased at the top of the ODZ where LLIV Procholorococcus is present, potentially signifying reduced grazing pressures.