A day in the life in the dynamic marine environment: how nutrients shape diel patterns of phytoplankton photosynthesis and carbon fixation gene expression in the Mississippi and Orinoco River plumes

Abstract

This research addresses the hypothesis that environmental conditions affect temporal connectivity between daily transcription of ribulose-1,5 bisphosphate carboxylase/oxygenase (Rubisco) genes and photosynthetic capacity among phytoplankton. From surface samples collected in the Mississippi River plume (MRP) and Orinoco River plume (ORP), we made size-fractionated measurements of Rubisco (rbcL) mRNA from four phytoplankton groups (heterokonts, haptophytes, Synechococcus, and Prochlorococcus) and chlorophyll-normalized photosynthesis–irradiance parameters (including light-saturated photosynthetic rate—PmaxB), plus nutrient uptake and inorganic carbon. Chlorophyll, photosynthesis, nutrient uptake, and total rbcL mRNA levels were substantially greater in the MRP. Rubisco mRNA and PmaxB exhibited characteristic diel patterns. Regressions with temporally offset data revealed photosynthesis cycles correlated to rbcL mRNA, but with a time lag. This delay was greater in the MRP, and greater among cells >2 μm. At both sites, PmaxB of <2 μm cells exhibited an earlier increase and closer temporal coupling to rbcL mRNA, suggesting carbon fixation is under tighter transcriptional control in picoplankton. The data support our hypothesis such that in the MRP, high productivity and apparent nutrient stress (perhaps including pCO2) resulted in earlier transcription and longer delay to increasing PmaxB among larger phytoplankton. Under stress, larger eukaryotic cells may need longer time to translate adequate Rubisco enzyme because of cellular resource limitation or diversion to nutrient acquisition. In the relatively oligotrophic ORP, phytoplankton may conserve cellular resources by restricting the time for Rubisco translation. These findings provide an interesting glimpse into ways marine algae can modulate the physiology of carbon fixation in response to environmental challenges.