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Photosynthesis in the surface ocean produces approximately 100 gigatonnes of organic carbon per year, of which 5 to 15 per cent is exported to the deep ocean (1,2). The rate at which the sinking carbon is converted into carbon dioxide by heterotrophic organisms at depth is important in controlling oceanic carbon storage (3). It remains uncertain, however, to what extent surface ocean carbon supply meets the demand of water-column biota; the discrepancy between known carbon sources and sinks is as much as two orders of magnitude (4-8). Here we present field measurements, respiration rate estimates and a steady-state model that allow us to balance carbon sources and sinks to within observational uncertainties at the Porcupine Abyssal Plain site in the eastern North Atlantic Ocean. We find that prokaryotes are responsible for 70 to 92 per cent of the estimated remineralization in the twilight zone (depths of 50 to 1,000 metres) despite the fact that much of the organic carbon is exported in the form of large, fast-sinking particles accessible to larger zooplankton. We suggest that this occurs because zooplankton fragment and ingest half of the fast-sinking particles, of which more than 30 per cent may be released as suspended and slowly sinking matter, stimulating the deep-ocean microbial loop. The synergy between microbes and zooplankton in the twilight zone is important to our understanding of the processes controlling the oceanic carbon sink., The global carbon cycle is affected by biological processes in the oceans, which export carbon from surface waters in the form of organic matter and store it at depth, in [...]

Planktonic algae < 5 Inn in size are major fixers of inorganic carbon in the ocean (1). They dominate phytoplankton biomass in postbloom, stratified oceanic temperate waters (2). Traditionally, large [...]

Studies of the picoautotroph-dominated oligotrophic areas of the Eastern Atlantic Ocean show that plankton community respiration in these regions exceeds gross production. These areas also exhibit greater phytoplankton turnover rates than bacteria turnover rates, and high ratios between heterotrophic and autotrophic biomass.

A clade of alpha-proteobacteria related to Roseobacter spp., a Cytophaga-Flavobacterium cluster, and a cluster of gamma-proteobacteria produce most of the total bacterioplankton metabolic activity in the Celtic Sea. The smallest cells of the gamma-proteobacteria had the highest growth-specific rates, accounting for their dominance in nutrient-limited waters.

An assay was designed to measure protein biomass of marine planktonic bacteria by flow cytometry. The procedure was calibrated by employing five species of Bacteria isolated from seawater samples and grown in culture at various temperatures. The intensity of SYPRO-protein fluorescence of these bacteria strongly correlated with their total protein content, measured by the bicinchoninic acid method to be in the range of 60 to 330 fg of protein/cell.

Four categories of picoplankton were discriminated from plankton samples collected from 10 depths at 25 stations along a transect from the British Isles to the Falkland Islands. Each category was counted by flow cytometry and the mean size of bacterial cells was determined. Prochlorococcus occurred at all stations between 47 degrees N and 38 degrees S. There were contrasting compositions of picoplankton communities in the North and South Atlantic between mesotrophic waters.

A series of fluorescent oligonucleotide probes that can hybridize with unique sequences of the small subunit (SSU) ribosomal RNA (rRNA) of Paraphysomonas vestita were prepared for bacterial identification. The fluorescent oligonucleotide probes specifically bonded to Paraphysomonas vestita and imparted a strong fluorescence to the nanoflagellate. Furthermore, SSU rRNA gene of the nanoflagellate exhibited numerous sequence differences compared to other Paraphysomonas species.

Fluorescence microscopy was utilized to identify the basic characteristics of specific fluorescent oligonucleotide probes to the small-subunit (SSU) ribosomal ribonucleic acid (rRNA) of Paraphysomonas vestita. The batch cultures of flagellate cells exhibited a decrease in fluorescence during the stationary phase due to a decrease in biomass. The oligonucleotide probes gave higher fluorescence with exponentially growing cells of both flagellate and ciliate cultures compared to cells from the late stationary phase.

Research shows that Prochlorococcus cyanobacteria contribute about 33% of organic nitrogen in the form of amino acids to the total bacterioplankton production in the oligotrophic regions of the Arabian Sea, suggesting its competitive dominance over algae and other bacteria.