Taking the metabolic pulse of the world's coral reefs.

Worldwide, coral reef ecosystems are experiencing increasing pressure from a variety of anthropogenic perturbations including ocean warming and acidification, increased sedimentation, eutrophication, and overfishing, which could shift reefs to a condition of net calcium carbonate (CaCO3) dissolution and erosion. Herein, we determine the net calcification potential and the relative balance of net organic carbon metabolism (net community production; NCP) and net inorganic carbon metabolism (net community calcification; NCC) within 23 coral reef locations across the globe. In light of these results, we consider the suitability of using these two metrics developed from total alkalinity (TA) and dissolved inorganic carbon (DIC) measurements collected on different spatiotemporal scales to monitor coral reef biogeochemistry under anthropogenic change. All reefs in this study were net calcifying for the majority of observations as inferred from alkalinity depletion relative to offshore, although occasional observations of net dissolution occurred at most locations. However, reefs with lower net calcification potential (i.e., lower TA depletion) could shift towards net dissolution sooner than reefs with a higher potential. The percent influence of organic carbon fluxes on total changes in dissolved inorganic carbon (DIC) (i.e., NCP compared to the sum of NCP and NCC) ranged from 32% to 88% and reflected inherent biogeochemical differences between reefs. Reefs with the largest relative percentage of NCP experienced the largest variability in seawater pH for a given change in DIC, which is directly related to the reefs ability to elevate or suppress local pH relative to the open ocean. This work highlights the value of measuring coral reef carbonate chemistry when evaluating their susceptibility to ongoing global environmental change and offers a baseline from which to guide future conservation efforts aimed at preserving these valuable ecosystems.

Phytoplankton carbon fixation gene (RuBisCO) transcripts and air-sea CO(2) flux in the Mississippi River plume.

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, alpha-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 co-occurrence 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.

Physiological responses of Acropora cervicornis to increased solar irradiance.

The effects of increased UV radiation (UV-B [280-320 nm] + UV-A [320-400 nm]; hereafter UVR) on the growth, production of photosynthetic pigments and photoprotective mycosporine-like amino acids (MAAs) were studied in the threatened Caribbean coral Acropora cervicornis transplanted from 20 to 1 m depth in La Parguera, Puerto Rico. The UVR exposure by the transplanted colonies was significantly higher than that at 20 m, while photosynthetically active radiation (PAR) only increased by 9%. Photosynthetic pigments, quantified with HPLC, as well as linear extension rates and skeletal densities, were significantly reduced 1 month after transplantation to 1 m depth, while MAAs increased significantly despite immediate paling experienced by transplanted colonies. While these colonies showed a significant reduction in photosynthetic pigments, there were no significant reductions in zooxanthellae densities suggesting photoacclimation of the coral's symbionts to the new radiation conditions. The results suggest that while corals might be able to survive sudden increases in UVR and PAR, their skeletal structure can be greatly debilitated due to a reduction in the photosynthetic capacity of their symbionts and a possible relocation of resources.

Geochemical rate-RNA integration study: ribulose-1,5-bisphosphate carboxylase/oxygenase gene transcription and photosynthetic capacity of planktonic photoautotrophs.

A pilot field experiment to assess the relationship between traditional biogeochemical rate measurements and transcriptional activity of microbial populations was carried out at the LEO 15 site off Tuckerton, N.J. Here, we report the relationship between photosynthetic capacity of autotrophic plankton and transcriptional activity of the large subunit gene (rbcL) for ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO), the enzyme responsible for primary carbon fixation during photosynthesis. Similar diel patterns of carbon fixation and rbcL gene expression were observed in three of four time series, with maxima for photosynthetic capacity (P(max)) and rbcL mRNA occurring between 10 a.m. and 1 p.m. The lowest P(max) and rbcL levels were detected between 6 p.m. and 10:30 p.m. A significant correlation was found between P(max) and form ID rbcL mRNA (R(2) = 0.56) and forms IA and IB (R(2) = 0.41 and 0.47, respectively). The correlation between the abundance of "diatom" rbcL and P(max) mRNA was modest (R(2) = 0.49; n = 12) but improved dramatically (R(2) = 0.97; n = 10) upon removal of two outliers which represented afternoon samples with high P(max) but lower mRNA levels. Clone libraries from reverse transcription-PCR-amplified rbcL mRNA indicated the presence of several chromophytic algae (diatoms, prymnesiophytes, and chrysophytes) and some eukaryotic green flagellates. Analogous results were obtained from amplified small rRNA sequences and secondary pigment analysis. These results suggest that diatoms were a major contributor to carbon fixation at LEO 15 at the time of sampling and that photosynthetic carbon fixation was partially controlled by transcriptional regulation of the RubisCO gene.

Increase of nitrous oxide flux to the atmosphere upon nitrogen addition to red mangroves sediments.

Response of nitrous oxide N20 sediment/air flux to nitrogen addition was assessed in mangrove (Rhizophora mangle) sediments. Fluxes were enhanced with both ammonium and nitrate loading. Greatest fluxes (52 micromol m(-2) h(-1)) were obtained with ammonium addition and saturation was achieved with additions of 0.9 mol m(-2). Maximum flux following ammonium addition was 2785 times greater than control plots and 4.5 times greater during low tide than with equivalent ammonium addition at high tide. Nitrate enrichment resulted in exponential growth, with maximal mean flux of 36.7 micromolm(-2) h(-1) at 1.9 molm(-2); saturation was not achieved. Differential response to ammonium and nitrate, and to tide and elevation, indicate that microbial nitrification is responsible for most of the observed gas flux. Mangrove sediments constitute an important source of global atmospheric N20 and increases in nitrogen loading will lead to significant increases in the flux of this atmospherically active gas.