Previous work in our lab has revealed the utility of molecular (genetic) assays for evaluating the role of various phytoplankton groups in marine carbon fixation. To determine the relationship between expression of the major gene in carbon fixation (large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase, or rbcL) and CO2 dynamics, we evaluated rbcL mRNA abundance using novel quantitative PCR assays which enhance the quantitative and phylogenetic specificity of measurements for determining transcription activity of certain phytoplankton groups. These RuBisCO RNA measurements were compared with phytoplankton cell analyses, photophysiological parameters, and pCO2 in and around the Mississippi River plume (MRP) in the Gulf of Mexico.
The surface water chemical characteristics measured allow us to categorize the sampling stations into “plume” stations (those with salinities between 30 and 32 and pCO2 under 310 µatm) and “non-plume” stations (those with salinities greater than 34 and pCO2 greater than 400 µatm). CO2 influx to the surface ocean was estimated in the plume region, while a smaller degree of CO2 efflux was estimated outside the plume.
Plume stations were dominated by rbcL mRNA concentrations from heterokonts, such as diatoms and pelagophytes, that were at least an order of magnitude greater than haptophytes, α-Synechococcus or high-light Prochlorococcus. However, rbcL transcript abundances were similar among these groups at oligotrophic (non-plume) stations. Diatom cell counts and heterokont rbcL RNA showed a strong negative correlation to seawater pCO2. This evidence indicates that (>2 µm) eukaryotes, particularly diatoms, were responsible for the carbon fixation and CO2 drawdown we and others (Cai, 2003; Lohrenz & Cai, 2006) have observed in the MRP, and not picoplankton such as Synechococcus or picoeukaryotes which exist in much greater numbers throughout the plume area.
While Prochlorococcus cells did not exhibit a large difference between low and high pCO2 water, Prochlorococcus rbcL RNA concentrations had a strong positive correlation to pCO2, suggesting a very low level of RuBisCO RNA transcription among Prochlorococcus in the plume waters. If the amount of rbcL RNA per Prochlorococcus cell is estimated, those outside the plume contained an average of over 14X more rbcL RNA per cell, all else being equal.
One hypothesis that merits further investigation is whether the success of some phytoplankton in highly-productive nutrient plumes is enabled by efficient carbon concentrating mechanisms (CCM). Diatoms have been shown to possess multiple CCMs and an ability to regulate active inorganic carbon uptake under conditions of reduced CO2 (Giordano et al, 2005).
These CCMS include external (periplasmic) carbonic anhydrases, which convert HCO3-, generally the dominant inorganic carbon species in marine water, into CO2 which is more readily transported across the cell membrane, and inducible active CO2 and HCO3- transport mechanisms (Burkhardt et al, 2001; Matsuda et al, 2001). Prochlorococcus, which are adapted to oligotrophic environments with DIC concentrations of at least 2 mM, as in our non-plume stations, apparently have limited CCM capabilities, particularly the high-light Prochlorococcus with their highly reduced genomes (Badger & Price, 2003); (Badger et al, 2006).
Measurement of rbcL mRNA from the environment appears to be good for characterizing large-scale differences found among differing oceanic environments such as river plumes and other coastal areas with the open ocean, and for determining the active carbon fixing organisms based on gene sequence specificities. Continued development and application of gene expression techniques will hopefully enable enhanced understanding of the underlying physiologic mechanisms regulating community dynamics and important ecological phenomena such as carbon flux and marine productivity.
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