Ulla Li Zweifel

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

ABSTRACT 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.

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

ABSTRACT 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.

A comparison of methods for the measurement of dissolved organic carbon in natural waters

A small suite of natural samples spanning a wide range of DOC concentrations and salinities were analyzed by three high-temperature combustion (HTC) techniques and persulfate oxidation. One of the HTC techniques, sealed-tube combustion (STC), served as a referee method. Using this method, it was possible to obtain absolute DOC concentrations for the natural samples free of any analytical blank offset. Prior to the comparison of samples, an exchange of carbon-free water and calibration standards showed that all methods were equally well calibrated. Linear correlation analysis was used to differentiate whether the differences observed between methods was due to variable oxidation yields or to an artifact of the instrument blank. Agreement among the various methods was quite good, but yields for all methods decreased compared to the STC technique at concentrations of > 400 μM C. Persulfate results were found to be very similar to HTC results. Finally, carbon-free distilled water prepared by UVH2O2 oxidation or Milli-Q systems had near-zero DOC concentrations and was adequate for blank correction of all the various techniques yielding DOC concentrations in excellent agreement with the referee method.

Pelagic Plankton Growth and Resource Limitations in the Baltic Sea

The Baltic Sea is a large transition area between fresh and marine conditions with a drainage area that includes vast forested lands as well as some of the most densely populated regions in Europe. Substantial quantities of allochthonous material enter the Baltic via rivers and streams and the net outflow of water through the Danish straits is very large, being comparable to the Mississippi River (Bergstrom and Carlsson 1994). In view of such large terrestrial influences extensive monitoring programs exist whose goal is to gain predictive knowledge of ecosystem behavior and ecosystem response to perturbation.

Culturability and Coexistence of Colony-Forming and Single-Cell Marine Bacterioplankton

ABSTRACT 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.