The Ecology of Microbial Communities Associated with Macrocystis pyrifera.

Kelp forests are characterized by high biodiversity and productivity, and the cycling of kelp-produced carbon is a vital process in this ecosystem. Although bacteria are assumed to play a major role in kelp forest carbon cycling, knowledge of the composition and diversity of these bacterial communities is lacking. Bacterial communities on the surface of Macrocystis pyrifera and adjacent seawater were sampled at the Hopkins Marine Station in Monterey Bay, CA, and further studied using 454-tag pyrosequencing of 16S RNA genes. Our results suggest that M. pyrifera-dominated kelp forests harbor distinct microbial communities that vary temporally. The distribution of sequence tags assigned to Gammaproteobacteria, Alphaproteobacteria and Bacteriodetes differed between the surface of the kelp and the surrounding water. Several abundant Rhodobacteraceae, uncultivated Gammaproteobacteria and Bacteriodetes-associated tags displayed considerable temporal variation, often with similar trends in the seawater and the surface of the kelp. Bacterial community structure and membership correlated with the kelp surface serving as host, and varied over time. Several kelp-specific taxa were highly similar to other bacteria known to either prevent the colonization of eukaryotic larvae or exhibit antibacterial activities. Some of these kelp-specific bacterial associations might play an important role for M. pyrifera. This study provides the first assessment of the diversity and phylogenetic profile of the bacterial communities associated with M. pyrifera.

Diversity and abundance of glycosyl hydrolase family 5 in the North Atlantic Ocean.

The diversity and abundance of glycosyl hydrolase family 5 (GH5) were studied in the North Atlantic Ocean. This family was chosen because of the large number of available sequences from cultured bacteria, the variety of substrates it targets, and the high number of similar sequences in the Sargasso Sea environmental genome database. Three clone libraries of a GH5 subcluster were constructed from the Mid-Atlantic Bight and the eastern and western North Atlantic Ocean. The two North Atlantic Ocean libraries did not differ from each other but both were significantly less diverse than the Mid-Atlantic Bight library. The abundance of GH5 genes estimated by quantitative PCR was positively correlated with chlorophyll concentrations in the eastern part of a transect from Fort Pierce, Florida, to the Azores and in a depth profile, suggesting that the supply of labile organic material selects for GH5-bearing bacteria in these waters. However, the data suggest that only <1% of all bacteria harbor the GH5 subcluster. These and other data suggest that the hydrolysis of polysaccharides requires complicated multi-enzyme systems.

Light-stimulated bacterial production and amino acid assimilation by cyanobacteria and other microbes in the North Atlantic ocean.

We examined the contribution of photoheterotrophic microbes--those capable of light-mediated assimilation of organic compounds--to bacterial production and amino acid assimilation along a transect from Florida to Iceland from 28 May to 9 July 2005. Bacterial production (leucine incorporation at a 20 nM final concentration) was on average 30% higher in light than in dark-incubated samples, but the effect varied greatly (3% to 60%). To further characterize this light effect, we examined the abundance of potential photoheterotrophs and measured their contribution to bacterial production and amino acid assimilation (0.5 nM addition) using flow cytometry. Prochlorococcus and Synechococcus were abundant in surface waters where light-dependent leucine incorporation was observed, whereas aerobic anoxygenic phototrophic bacteria were abundant but did not correlate with the light effect. The per-cell assimilation rates of Prochlorococcus and Synechococcus were comparable to or higher than those of other prokaryotes, especially in the light. Picoeukaryotes also took up leucine (20 nM) and other amino acids (0.5 nM), but rates normalized to biovolume were much lower than those of prokaryotes. Prochlorococcus was responsible for 80% of light-stimulated bacterial production and amino acid assimilation in surface waters south of the Azores, while Synechococcus accounted for on average 12% of total assimilation. However, nearly 40% of the light-stimulated leucine assimilation was not accounted for by these groups, suggesting that assimilation by other microbes is also affected by light. Our results clarify the contribution of cyanobacteria to photoheterotrophy and highlight the potential role of other photoheterotrophs in biomass production and dissolved-organic-matter assimilation.