Composition and Seasonality of Membrane Transporters in Marine Picoplankton.

In this study, we examined transporter genes in metagenomic and metatranscriptomic data from a time-series survey in the temperate marine environment of the Baltic Sea. We analyzed the abundance and taxonomic distribution of transporters in the 3µm-0.2µm size fraction comprising prokaryotes and some picoeukaryotes. The presence of specific transporter traits was shown to be guiding the succession of these microorganisms. A limited number of taxa were associated with the dominant transporter proteins that were identified for the nine key substrate categories for microbial growth. Throughout the year, the microbial taxa at the level of order showed highly similar patterns in terms of transporter traits. The distribution of transporters stayed the same, irrespective of the abundance of each taxon. This would suggest that the distribution pattern of transporters depends on the bacterial groups being dominant at a given time of the year. Also, we find notable numbers of secretion proteins that may allow marine bacteria to infect and kill prey organisms thus releasing nutrients. Finally, we demonstrate that transporter proteins may provide clues to the relative importance of biogeochemical processes, and we suggest that virtual transporter functionalities may become important components in future population dynamics models.

Annual variations in the diversity, viability, and origin of airborne bacteria.

The presence of bacteria in aerosols has been known for centuries, but information on their identity and role in dispersing microbial traits is still limited. This study monitored the airborne bacterial community during an annual survey using samples collected from a 25-m tower near the Baltic Sea coast. The number of CFU was estimated using agar plates while the most probable number (MPN) of viable bacteria was estimated using dilution-to-extinction culturing assays (DCAs). The MPN and CFU values produced quantitatively similar results, displaying a pronounced seasonal pattern, with the highest numbers in winter. The most dominant bacteria growing in the DCAs all formed colonies on agar plates, were mostly pigmented (80%), and closely resembled (>97%) previously cultured bacteria based on their 16S rRNA gene sequences. 16S rRNA gene clone libraries were constructed on eight occasions during the survey; these revealed a highly diverse community with a few abundant operational taxonomic units (OTUs) and a long tail of rare OTUs. A majority of the cloned sequences (60%) were also most closely related to previously "cultured" bacteria. Thus, both culture-dependent and culture-independent techniques indicated that bacteria able to form colonies on agar plates predominate in the atmosphere. Both the DCAs and clone libraries indicated the dominance of bacteria belonging to the genera Sphingomonas sp. and Pseudomonas sp. on several sampling occasions. Potentially pathogenic strains as well as sequences closely resembling bacteria known to act as ice nuclei were found using both approaches. The origin of the sampled air mass was estimated using backward trajectories, indicating a predominant marine source.

Culturability and coexistence of colony-forming and single-cell marine bacterioplankton.

Culturability and coexistence of bacterioplankton exhibiting different life strategies were investigated in the Baltic Sea and Skagerrak Sea. Bacterial numbers were estimated using a dilution-to-extinction culturing assay (DCA) and calculated as the most probable number, based on six different methods to detect bacterial growth in the DCA. Irrespective of the method used to detect growth, the fraction of multiplying cells never exceeded 10%, using the total count of 4',6'-diamidino-2-phenylindole (DAPI)-stainable cells as a reference. Furthermore, the data also showed that non-colony-forming bacteria made up the majority of the viable cells, confirming molecular results showing dominance of non-colony-forming bacteria in clone libraries. The results obtained are in agreement with previous observations, indicating that bacterial assemblages in seawater are dominated by small, active subpopulations coexisting with a large group of inactive cells. The ratio of colony-forming to non-colony-forming bacteria was approximately 10 to 20 times higher in the brackish Baltic Sea than in the Skagerrak Sea. These two sea areas differ in (for example) their levels of bacterial production, dissolved organic carbon, and salinity. We suggest that the relative importance of colony-forming versus non-colony-forming bacterioplankton may be linked to environmental characteristics.

Use of 16S ribosomal DNA for delineation of marine bacterioplankton species.

All of the marine bacterioplankton-derived 16S ribosomal DNA sequences previously deposited in GenBank were reanalyzed to determine the number of bacterial species in the oceanic surface waters. These sequences have been entered into the database since 1990. The rate of new additions reached a peak in 1999 and subsequently leveled off, suggesting that much of the marine microbial species richness has been sampled. When the GenBank sequences were dereplicated by using 97% similarity as a cutoff, 1,117 unique ribotypes were found. Of the unique sequences, 609 came from uncultured environmental clones and 508 came from cultured bacteria. We conclude that the apparent bacterioplankton species richness is relatively low.