Abstract
The surface waters of oligotrophic oceans have chronically low phosphate (Pi) concentrations, which renders dissolved organic phosphorus (DOP) an important nutrient source. In the subtropical North Atlantic, cyanobacteria are often numerically dominant, but picoeukaryotes can dominate autotrophic biomass and productivity making them important contributors to the ocean carbon cycle. Despite their importance, little is known regarding the metabolic response of picoeukaryotes to changes in phosphorus (P) source and availability. To understand the molecular mechanisms that regulate P utilization in oligotrophic environments, we evaluated transcriptomes of the picoeukaryote Micromonas pusilla grown under Pi-replete and -deficient conditions, with an additional investigation of growth on DOP in replete conditions. Genes that function in sulfolipid substitution and Pi uptake increased in expression with Pi-deficiency, suggesting cells were reallocating cellular P and increasing P acquisition capabilities. Pi-deficient M. pusilla cells also increased alkaline phosphatase activity and reduced their cellular P content. Cells grown with DOP were able to maintain relatively high growth rates, however the transcriptomic response was more similar to the Pi-deficient response than that seen in cells grown under Pi-replete conditions. The results demonstrate that not all P sources are the same for growth; while M. pusilla, a model picoeukaryote, may grow well on DOP, the metabolic demand is greater than growth on Pi. These findings provide insight into the cellular strategies which may be used to support growth in a stratified future ocean predicted to favor picoeukaryotes.
Highlights
Picophytoplankton (< 3 μm), composed of both prokaryotic and eukaryotic organisms, dominate autotrophic biomass in oligotrophic oceans
Given the importance of dissolved organic phosphorus (DOP) to picophytoplankton in oligotrophic oceans, we investigated the response in M. pusilla cultures grown under replete conditions with ATP as the only P source
M. pusilla is considered a model organism [43] yet it has been the target of only two other studies which have interrogated changes in gene expression [44, 45]
Summary
Picophytoplankton (< 3 μm), composed of both prokaryotic and eukaryotic organisms, dominate autotrophic biomass in oligotrophic oceans. While single-celled cyanobacteria are the most abundant autotrophs, picoeukaryotes can dominate biomass and productivity in the subtropics [1,2,3], making them important contributors to ocean carbon production [4] and export [5]. In the central North Atlantic Ocean, picoeukaryotes accounted for approximately 87% of the carbon biomass and 68% of the picophytoplankton primary production [1]. Eukaryotes in the subtropical North Atlantic Ocean were found to be biochemically different. Picoeukaryote Response to Phosphorus Availability supported by the Office of Science of the U.S Department of Energy under Contract No DE-AC0205CH11231
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