Little is known about the abundance, distribution, and ecology of aerobic anoxygenic phototrophic (AAP) bacteria, particularly in oligotrophic environments, which represent 60% of the ocean. We investigated the abundance of AAP bacteria across the South Pacific Ocean, including the center of the gyre, the most oligotrophic water body of the world ocean. AAP bacteria, Prochlorococcus, and total prokaryotic abundances, as well as bacteriochlorophyll a (BChl a) and divinyl-chlorophyll a concentrations, were measured at several depths in the photic zone along a gradient of oligotrophic conditions. The abundances of AAP bacteria and Prochlorococcus were high, together accounting for up to 58% of the total prokaryotic community. The abundance of AAP bacteria alone was up to 1.94 ؋ 10 5 cells ml ؊1 and as high as 24% of the overall community. These measurements were consistent with the high BChl a concentrations (up to 3.32 ؋ 10 ؊3 g liter ؊1 ) found at all stations. However, the BChl a content per AAP bacterial cell was low, suggesting that AAP bacteria are mostly heterotrophic organisms. Interestingly, the biovolume and therefore biomass of AAP bacteria was on average twofold higher than that of other prokaryotic cells. This study demonstrates that AAP bacteria can be abundant in various oligotrophic conditions, including the most oligotrophic regime of the world ocean, and can account for a large part of the bacterioplanktonic carbon stock.

Proteorhodopsin (PR)-containing bacteria are hypothesized to use both light and organic compounds as energy sources. Recent studies have found that PR is common in marine microorganisms, but the impact of light on the growth of PR-containing organisms and PR transcription in the environment remains unclear. We examined the diversity of PR genes and transcripts by PCR amplification and sequencing in Delaware coastal waters. Clone libraries of PR DNA and cDNA (from mRNA) revealed large differences between bacterial groups in expression of PR genes. We then evaluated by quantitative PCR the impact of light on growth and PR expression in PR-containing SAR11 bacteria (SAR11-PR) and a population of Flavobacteria (Flavobacteria-PR). This experiment was conducted in 30 l microcosms exposed to continuous light, continuous dark, and 12 h-12 h dark-light cycles for 5 days. We found a strong upregulation of PR expression by light in Flavobacteria-PR and SAR11-PR. The abundance of PR transcripts per PR cell was enhanced up to 120-fold under continuous light and up to 20-fold under dark-light cycles while continuous darkness led to very low levels of PR mRNA. This upregulation of PR expression was correlated with the abundance of PR genes, indicating net growth of SAR11-PR cells and Flavobacteria-PR under dark-light cycles. SAR11-PR and Flavobacteria-PR abundance decreased under continuous light despite upregulation of PR expression, and continuous darkness led to low abundances of both populations. Collectively, these data suggest that light affects growth of PR-containing bacteria and regulation of PR mRNA synthesis in natural communities.

Abstract. The chemical and biological characteristics of the surface microlayer were determined during a transect across the South Pacific Ocean in October-December 2004. Concentrations of particulate organic carbon (1.3 to 7.6-fold) and nitrogen (1.4 to 7-fold), and POC:PON ratios were consistently higher in the surface microlayer as compared to surface waters (5 m). The large variability in particulate organic matter enrichment was negatively correlated to wind speed. No enhanced concentrations of dissolved organic carbon were detectable in the surface microlayer as compared to 5 m, but chromophoric dissolved organic matter was markedly enriched (by 2 to 4-fold) at all sites. Based on pigment analysis and cell counts, no consistent enrichment of any of the major components of the autotrophic and heterotrophic microbial community was detectable. CE-SSCP fingerprints and CARD FISH revealed that the bacterial communities present in the surface microlayer had close similarity (>76%) to those in surface waters. By contrast, bacterial heterotrophic production ( 3 H-leucine incorporation) was consistently lower in the surface microlayer than in surface waters. By applying CARD-FISH and microautoradiography, we observed that Bacteroidetes and Gammaproteobacteria dominated leucine uptake in the surface microlayer, while in surface waters Bacteroidetes and Alphaproteobacteria were the major groups accounting for leucine incorporation. Our results demonstrate that the microbial community in the surface microlayer closely resembles that of the surface Correspondence to: I. Obernosterer (ingrid.obernosterer@obs-banyuls.fr)waters of the open ocean. Even a short residence in the surface microlayer influences leucine incorporation by different bacterial groups, probably as a response to the differences in the physical and chemical nature of the two layers.