Marine ecosystems are significant sources of the powerful greenhouse gas nitrous oxide (N 2 O). A by-product of nitrification and an intermediate in the denitrification pathway, N 2 O is formed primarily in oxygen-deficient waters and sediments. We describe the isolation of a group of alphaproteobacteria from the suboxic waters of the Arabian Sea that are phylogenetically affiliated with Labrenzia spp. and other denitrifiers. Quantitative PCR assays revealed that these organisms were very broadly distributed in this semienclosed ocean basin. Their biogeographical range extended from the productive, upwelling region off the Omani shelf to the clear, oligotrophic waters that are found much further south and also included the mesotrophic waters overlying the oxygen minimum zone (OMZ) in the northeastern sector of the Arabian Sea. These organisms actively expressed NosZ (N 2 O reductase, the terminal step in the denitrification pathway) within the OMZ, an established region of pelagic denitrification. They were found in greatest numbers outside the OMZ, however, and nosZ mRNAs were also readily detected near the base of the upper mixed layer in nutrient-poor, oxic regions. Our findings provide firm molecular evidence of a potential sink for N 2 O within well-ventilated, oceanic surface waters in this biogeochemically important region. We show that the Labrenzia-like denitrifiers and their close relatives are habitual colonizers of the pseudobenthic environment provided by Trichodesmium spp. We develop the conjecture that the O 2 -depleted microzones that occur within the colonies of these filamentous, diazotrophic cyanobacteria might provide unexpected niches for the reduction of nitrogen oxides in tropical and subtropical surface waters. E missions of the greenhouse gas nitrous oxide (N 2 O) have increased steadily since the early part of the 19th century. Atmospheric N 2 O concentrations are higher today than at any time during the past 650,000 years (1, 2). Apart from its significant warming potential (ϳ300-fold that of CO 2 over a 100-year period), rising N 2 O is of further environmental concern because it is presently the single most destructive source of emissions contributing to stratospheric ozone depletion (3). The growing inventory of atmospheric N 2 O has occurred primarily as a result of an increase in emissions from the terrestrial environment owing to changes in agricultural practices, the combustion of fossil fuels, and other anthropogenically driven perturbations of the nitrogen cycle (2). The marine environment is also an important net source of N 2 O to the atmosphere, however, and the unperturbed (nonanthropogenic) rates of emissions from coastal margins, shelf, and open waters are of the same order as those from land (1, 4, 5).Significant feedbacks on marine N 2 O emissions are anticipated over the coming decades as a result of increasing ocean acidification (5) and the expansion of hypoxic waters owing to surface warming and an acceleration in the prevailing rates of eutrophication from anthrop...

We report a pronounced diel rhythm in ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO) gene expression in a natural population of the coccolithophorid Coccolithus pelagicus sampled during a Lagrangian experiment in the Northeast Atlantic. Our observations show that there is greater heterogeneity in the temporal regulation of RubisCO expression among planktonic chromophytes than has been reported hitherto.Despite the importance of the oceans in the global carbon cycle, comparatively little is known of the regulation of photosynthetic carbon fixation in marine phytoplankton. As in higher plants, the bulk of CO 2 is fixed via the Calvin cycle enzyme ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO) (10) in these organisms. Therefore, it is of considerable interest to understand how the environment structures the expression of this key enzyme. In the present study, we investigated the diel variability in rbcL mRNA abundance in a natural population of the coccolithophorid Coccolithus pelagicus.Observations were made in subpolar waters to the south of Iceland within an anticyclonic eddy with a cold surface temperature anomaly (6,12). A patch of the tracer SF 6 was deployed at the eddy center (5), and the research vessel operated in Lagrangian mode by analyzing surface SF 6 concentrations continuously (4). Seawater samples were obtained during conductivity-temperature-depth hydrocasts, and the abundance of C. pelagicus cells was determined by flow cytometry (13). For RNA, near-surface (ϳ2.5-m depth) seawater samples (10 to 20 liters) were rapidly filtered (Ͻ10 min) onto 90-mm-diameter Whatman GF/C filters, and the filters were stored at Ϫ70°C in extraction buffer (15) following snap-freezing in a propan-2-ol cooling bath. RNA was isolated and prepared for Northern analysis (16) under the hybridization conditions described below.An internal region of rbcL was amplified from DNA isolated from C. pelagicus cells collected at the study site (16) and from laboratory cultures of the haptophytes Emiliania huxleyi and Pavlova salina. The identity of the products was confirmed by comparison to previously published sequences (2, 3), and sense strand in vitro transcription products were synthesized as previously described (16). Northern slot blots were hybridized at 55°C in Easy-Hyb solution (Roche) amended with 25 ng of C. pelagicus rbcL probe DNA (labeled with digoxigenin-dUTP) ml Ϫ1 . Stringency washes were performed on the following day with 0.05ϫ SSPE (1ϫ SSPE is 150 M NaCl, 10 M Na 2 HPO 4 , 1 M EDTA)-0.1% sodium dodecyl sulfate at 68°C, and hybrids were detected with alkaline phosphatase-conjugated antidigoxigenin in conjunction with the chemiluminescent substrate CDP-Star (Roche). This hybridization protocol enables the specific detection of C. pelagicus rbcL mRNA and was optimized empirically by testing in vitro transcription products under conditions of increasing stringency (Fig. 1). Like the rbcL genes of the other coccolithophorids that have been characterized to date, the nucleotide sequence of the ...