Marine Microbiology Group - College of Marine Science - University of South Florida
Marine Microbiology Group - College of Marine Science - University of South Florida Marine Microbiology Group - College of Marine Science - University of South Florida Marine Microbiology Group - College of Marine Science - University of South FloridaMarine Microbiology Group - College of Marine Science - University of South Florida
Marine Microbiology Group - College of Marine Science - University of South Florida
Marine Microbiology Group - College of Marine Science - University of South Florida Marine Microbiology Group - College of Marine Science - University of South Florida Marine Microbiology Group - College of Marine Science - University of South Florida Marine Microbiology Group - College of Marine Science - University of South Florida
Marine Microbiology Group - College of Marine Science - University of South Florida

RESEARCH - REGULATION OF CARBON FIXATION

Coastal seas, estuaries, and river plumes are a fundamental part of the global carbon cycle because they link terrestrial, oceanic, and atmospheric carbon reservoirs. River-dominated ocean margins are the most important class of margins in terms of their impact on carbon sequestration (Green et al, 2006). River plumes deliver large quantities of nutrients to oligotrophic oceans, often resulting in significant CO2 drawdown. This CO2 consumption is driven by phytoplankton primary productivity in surface waters of marine environments.

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.


Click here to see a larger version of the above images. (Top) Composite SeaWiFS image of estimated surface chlorophyll a concentrations during cruise dates in July 2005. Sampling stations are shown, with 1 oligotrophic station out of view (Station 1). Station 7X was not sampled for data reported here. (Bottom) Estimated CO2 flux over the Mississippi River plume area, showing sampling sites and cruise track within or near the plume. The negative sign of numbers in the lower panel indicates the direction of pCO2 flux is from atmosphere to surface ocean. Two sampling sites far outside the MRP are not shown (Stations 1 and 8).

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.


Click here to see a larger version of the above image. Average rbcL RNA concentrations L-1 of the four groups measured by qPCR, in the whole (unfiltered) and <2 µm size fractions. Means are grouped by Mississippi River plume (salinity 30-32) and open Gulf of Mexico (salinity >34) stations. The heterokont group detects rbcL RNA from diatoms, pelagophytes, dictyochophytes (silicoflagellates) and pinguiophytes, which have closely related rbcL genes. Only high-light Prochlorococcus are detected by the qPCR assay. Error bars show standard deviation. Only samples taken from 6:00 -13:00 are included.

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).


Correlation of rbcL mRNA concentrations to seawater pCO2 for samples from 06:00-13:00 EDT, showing heterokont (gold diamonds) and highlight Prochlorococcus (blue circles) rbcL RNA concentrations, with log-linear regressions placed right and left respectively. The divergence of data from plume and open Gulf of Mexico stations is apparent, in that outside the plume in higher pCO2 water, the heterokont group and Prochlorococcus rbcL RNA concentrations are similar. In the plume, at lower pCO2 ≤300 µatm, heterokont rbcL RNA abundance is dramatically greater than that of Prochlorococcus. The values for Prochlorococcus are extremely low, at the limits of detection for the qPCR assay, and are not resolved as accurately, thus giving rise to the observed spread along the low end of this logarithmic scale.

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.



References:

Badger MR, Price GD (2003) "CO2 concentrating mechanisms in cyanobacteria: molecular components, their diversity and evolution." J Exp Bot 54: 609-622
Badger MR, Price GD, Long BM, Woodger FJ (2006) "The environmental plasticity and ecological genomics of the cyanobacterial CO2 concentrating mechanism." J Exp Bot 57: 249-265
Burkhardt S, Amoroso G, Riebesell U, Sultemeyer D (2001) "CO2 and HCO3- uptake in marine diatoms acclimated to different CO2 concentrations." Limnol Oceanogr 46: 1378-1391
Cai WJ (2003) "Riverine inorganic carbon flux and rate of biological uptake in the Mississippi River plume." Geophys Res Lett 30
Giordano M, Beardall J, Raven JA (2005) "CO2 concentrating mechanisms in algae: Mechanisms, environmental modulation, and evolution." Annu Rev Plant Biol 56: 99-131
Green RE, Bianchi TS, Dagg MJ, Walker ND, Breed GA (2006) "An organic carbon budget for the Mississippi River turbidity plume and plume contributions to air-sea CO2 fluxes and bottom water hypoxia." Estuaries Coasts 29: 579-597
Lohrenz SE, Cai WJ (2006) "Satellite ocean color assessment of air-sea fluxes of CO2 in a river-dominated coastal margin." Geophys Res Lett 33
Matsuda Y, Hara T, Colman B (2001) "Regulation of the induction of bicarbonate uptake by dissolved CO2 in the marine diatom, Phaeodactylum tricornutum." Plant Cell Environ 24: 611-620

  • To find out more about Rubisco genes in the environment, click here.

  • To learn more about carbon and nitrogen dynamics in the Mississippi, click here.

  • To learn more about the metagenomic analysis of oceanic picoplankton communities click here.

  • To learn more about the relationship between CO2 drawdown and phytoplankton gene expression click here.



Marine Microbiology Group - College of Marine Science - University of South Florida Marine Microbiology Group - College of Marine Science - University of South Florida Marine Microbiology Group - College of Marine Science - University of South Florida Marine Microbiology Group - College of Marine Science - University of South Florida Marine Microbiology Group - College of Marine Science - University of South Florida Marine Microbiology Group - College of Marine Science - University of South Florida Marine Microbiology Group - College of Marine Science - University of South Florida
Marine Microbiology Group - College of Marine Science - University of South Florida Marine Microbiology Group - College of Marine Science - University of South Florida Marine Microbiology Group - College of Marine Science - University of South Florida Marine Microbiology Group - College of Marine Science - University of South Florida
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