Phytoplankton carbon fixation gene (RuBisCO) transcripts and air-sea CO2 flux in the Mississippi River plume

Abstract

River plumes deliver large quantities of nutrients to oligotrophic oceans, often resulting in significant CO 2 drawdown. To determine the relationship between expression of the major gene in carbon fixation (large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase, RuBisCO) and CO 2 dynamics, we evaluated rbcL mRNA abundance using novel quantitative PCR assays, phytoplankton cell analyses, photophysiological parameters, and pCO 2 in and around the Mississippi River plume (MRP) in the Gulf of Mexico. Lower salinity (30-32) stations were dominated by rbcL mRNA concentrations from heterokonts, such as diatoms and pelagophytes, which were at least an order of magnitude greater than haptophytes, a-Synechococcus or high-light Prochlorococcus. However, rbcL transcript abundances were similar among these groups at oligotrophic stations (salinity 34-36). Diatom cell counts and heterokont rbcL RNA showed a strong negative correlation to seawater pCO 2 . While Prochlorococcus cells did not exhibit a large difference between low and high pCO 2 water, Prochlorococcus rbcL RNA concentrations had a strong positive correlation to pCO 2 , suggesting a very low level of RuBisCO RNA transcription among Prochlorococcus in the plume waters, possibly due to their relatively poor carbon concentrating mechanisms (CCMs). These results provide molecular evidence that diatom/pelagophyte productivity is largely responsible for the large CO 2 drawdown occurring in the MRP, based on the cooccurrence of elevated RuBisCO gene transcript concentrations from this group and reduced seawater pCO 2 levels. This may partly be due to efficient CCMs that enable heterokont eukaryotes such as diatoms to continue fixing CO 2 in the face of strong CO 2 drawdown. Our work represents the first attempt to relate in situ microbial gene expression to contemporaneous CO 2 flux measurements in the ocean.

Effects of increased atmospheric CO<sub>2</sub> on small and intermediate sized osmotrophs during a nutrient induced phytoplankton bloom

“…A number of CCM variants, differing in manner of operation and efficiency, are found among different phytoplankton, and nutrient availability is also known to play a significant role in modulating CCMs (reviewed by Giordano et al, 2005). From our results alone it is therefore difficult to judge whether increase in atmospheric CO 2 might have a greater effect when production is based on regenerated nutrients, or whether our observations possibly reflect that small and intermediate sized osmotrophs are not equipped with carbon concentration mechanisms as efficient as the diatoms and therefore benefit more from increased CO 2 levels (John et al, 2007). An observed a shift from diatoms to nanophytoplankton when Hare et al, 2007 incubated phytoplankton communities at elevated pCO 2 support the latter explanation.…”

Comparison of lysogeny (prophage induction) in heterotrophic bacterial and Synechococcus populations in the Gulf of Mexico and Mississippi river plume

“…Although overall primary productivity is highest in plume samples, whole and 0.2 mm size-fractionated primary productivity demonstrated that a higher proportion of the total primary productivity in the plume was driven by larger phytoplankton, such as diatoms, rather than Synechococcus (John et al, 2007).…”

Diversity and distribution of autotrophic microbial community along environmental gradients in grassland soils on the Tibetan Plateau

“…The large subunit of form I RubisCO is encoded by the cbbL genes, which further divide form I RubisCO into four forms (IA, IB, IC, and ID) (Tabita et al 2008). cbbL genes encoding forms IA, IB, IC, and ID have been widely used as phylogenetic biomarkers to characterize biodiversity of autotrophic microorganisms in diverse habitats, e.g., oceans, lakes, and soils (John et al 2007;Kong et al 2012a, b;Yuan et al 2012a).…”