GSE208287
Marine phytoplankton downregulate core photosynthesis and carbon storage genes upon rapid mixed layer shallowing and increased daily irradiance
Marine phytoplankton are a diverse group of photoautotrophic organisms and key mediators in the global carbon cycle.  Phytoplankton physiology and biomass accumulation are closely tied to mixed layer depth, but the intracellular metabolic pathways activated in response to changing mixed layer depths remain unexplored.  Here, metatranscriptomics was used to characterize the phytoplankton community response to a mixed layer shallowing from 233 meters to 5 meters over the course of two days during the late spring in the Northwest Atlantic.  Most phytoplankton genera downregulated core photosynthesis, carbon storage, and carbon fixation genes as the system transitioned from a deep to a shallow mixed layer and shifted towards catabolism of stored carbon ic pathways supportive of rapid cell growth.  In contrast, phytoplankton genera exhibited divergent transcriptional strategies for photosystem light harvesting complex genes during this transition. Active infection, taken as the ratio of virus to host transcripts, increased in the Bacillariophyta (diatom) phylum and decreased in the Chlorophyta (green algae) phylum upon mixed layer shallowing.  A conceptual model is proposed to provide ecophysiological context for our findings, in which light limitation during deep mixing induces populations into a transcriptional state which maximizes interrupts the oscillating levels of transcripts related to photosynthesis, carbon storage, and carbon fixation found in shallow mixed layers with relatively higher growth rates.  We propose that upon sensing high light levels during mixed layer shallowing, phytoplankton resume diel oscillation of core sets of genes enabling photoprotection, biosynthesis and cell replication. Our findings highlight the shared and unique transcriptional response strategies within phytoplankton communities acclimating to the dynamic light environment associated with transient deep mixing and shallowing events during the annual North Atlantic bloom. Overall design: Comparative gene expression of phytoplankton comunities between different water depths corresponding to 40%,20% and 1% of surface PAR, and between the same relative irradiances in different mixed layer depths.  Incubations were performed on deck of the ship to simulate in situ light and temperature conditions of the North Atlantic.  The first incubation ("I") lasted for 3 days, while the next incubation lasted for 5 days. Biological replicate numbers are not related to each other in terms of incubated and in situ samples - ie. 1_5_04_I_1 is not the incubated treatment of 1_5_04_B_1