Ecology of Photosynthetic Picoeukaryotes in the Oligotrophic Ocean: Diversity, Activity, and Dynamics

Abstract

Picophytoplankton (≤3 µm), including cyanobacteria and photosynthetic picoeukaryotes (PPE), account for ~60% of total phytoplankton biomass and are responsible for greater than 40% of total net primary production in large regions of the world’s oceans. Eukaryotic picophytoplankton are globally significant contributors to primary productivity; however, to date, relatively little is known about the temporal dynamics underlying their production and diversity. In this dissertation, picophytoplankton contributions to 14C primary productivity and group-specific contributions by Prochlorococcus, Synechococcus, and PPE were determined along a transect spanning a large trophic gradient in the South East Pacific Ocean (SEP) and during monthly scale sampling over a one-year period at Station ALOHA in the North Pacific Subtropical Gyre (NPSG). Results revealed that cell-specific rates of PPE carbon fixation were elevated in the well-lit regions of the euphotic zone (<75 m). In addition, depth-dependent variability in PPE taxa composition was determined in biogeochemically distinct regions of the SEP, and depth- and time-dependent variability was examined throughout the euphotic zone (0-175 m) at Station ALOHA in the NPSG over a two-year period. Diverse and distinct PPE taxa were observed in the SEP and in the NPSG between the well-lit and the dimly lit regions of the euphotic zone. Moreover, PPE taxa composition in the NPSG varied seasonally, with taxa typically residing near the deep chlorophyll maximum entrained into the upper euphotic zone during periods of winter and spring convective mixing. Such variations in PPE communities appeared to influence the types of organisms best poised to respond to nutrient perturbations in the surface waters at Station ALOHA at different times of the year. Overall, results revealed greater diversity of PPE taxa in oligotrophic environments with potentially mixotrophic taxa more prevalent in these low-nutrient regions. This dissertation contributes knowledge to understanding the ecology of picoeukaryotes in the euphotic zone of the open ocean, and lends insight into the contributions of these microorganisms to carbon cycling in these oligotrophic habitats.