The Proteomic Response of the Green Alga Micromonas to Phosphorus Deficiency and Recovery

The impact of nutrient limitation on phytoplankton growth and community composition is a major question in oceanography and global change science. Phosphorus is an important nutrient for marine ecosystem, it is essential for many fundamental processes that sustain life, including nucleic acid synthesis, membrane synthesis, energy metabolism, signaling, redox reactions, and modification of protein activities. Thus, phosphate supplementation influences microbial diversity and global oceanic primary production. The unicellular green alga Micromonas was first reported as a dominant phytoplankton in the 1950s and noted since for its global distribution. However, little is known about the environmental and physiological factors that regulate their growth and contributions to primary production, or forces driving their divergence. The availability of complete genome sequences for two Micromonas species make Micromonas an excellent candidate for in-depth systems analyses. Here, we focus on the response of the Micromonas under phosphate limited growth. Micromonas was grown in a technologically advanced bioreactor wherein continuous culturing experiments could be performed that transitioned cells from phosphate replete to phosphate limited conditions and, finally, phosphate re-fed conditions. Concurrent with a reduction in growth rate under phosphate-limited conditions we observed a decline in F_v/F_m, which provides a measure of impact of a stress on the organism’s photosystem. Global changes in the proteome were assessed using high throughput LC/MS methods. 420 differentially expressed proteins were identified at a significance level of FDR-adjusted p-value < 5% and fold change in protein abundance ≥2. Among those, 197 proteins were more abundant in P-replete condition and 130 proteins were more abundant in the P-deficient condition. Under phosphate-limited conditions, proteins involved in starch metabolism, glycolysis and major CHO metabolism, as well as several other pathways were more abundant, while proteins involved in nitrate assimilation, protein synthesis, modification and degradation declined in abundance. This indicates a sophisticated response to phosphate-limited growth strongly affecting cellular carbon metabolism and mirroring responses observed in multicellular plants.