Science Serving Maryland's Coasts

Luca Cherubini, Loyola University Chicago

Class Year: 

Project Title: 

An Analysis of Protist Diversity in Oxygen Deficient Zones: Understanding the Relationship Between Prochlorococcus 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, with 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 (1500 basepairs) to identify the protist community and create a phylogenetic tree. Metagenomic reads were placed on the phylogenetic tree. Results 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 lower concentrations of heterotrophic Dinoflagellate orders: Prorocentrales and Gymnodiniales. However, nearly all protist population concentrations decreased dramatically near the lower bounds of the oxycline where LLIV Prochlorococcus is present, potentially signifying reduced grazing pressures.