Phytoplankton phenology through gene expression during the North Atlantic spring bloom decline

While phytoplankton dynamics in the annual North Atlantic spring bloom have been well characterized, the physiological underpinnings driving these changes and their net impact on the biogeochemistry of the region are less understood. Phytoplankton metabolism is both affected by, and influences the regions nutrient cycling, primary production, and ultimately, the fate of carbon export. Thus, developing an understanding of these processes is critical. Phytoplankton biomass, biological rates, and gene expression data along with associated environmental parameters were measured as part of the NASA EXport Processes in the Ocean from RemoTe Sensing programs campaign to the North Atlantic to evaluate the relationships amongst these processes within the four most dominant phytoplankton groups (diatoms, dinoflagellates, haptophytes, and chlorophytes) during the spring bloom. We observe a transition from a period dominated by active diatom growth (defined as Phase I) to a period dominated by non-diatom phytoplankton groups (Phase II). Silicic acid depletion appears to limit overall production and reduce competition from diatoms, likely leading to enhanced contributions of haptophytes in Phase II. Expression of key protein-encoding genes involved in cell maintenance, photosynthesis, and nitrogen and vitamin metabolisms varied amongst the taxa throughout the observation period. Expression patterns of diatom genes involved in silicon transport suggest an apparent uncoupling between genes involved in nitrate uptake and photosynthesis, resulting in an increase in silicification independent growth. Our analysis demonstrates the utility in combining gene expression with biological rate processes to provide a more holistic view of phytoplankton bloom dynamics and phenology.