Diversity and abundance of Gammaproteobacteria during the winter-spring transition at station Kabeltonne - Helgoland

Abstract

Recent evidence has suggested that Gammaproteobacteria are strongly associated with phytoplankton blooms and may play a role in the processing of the algal derived organic substrate. Therefore, the succession of specific groups within the Gammaproteobacteria may be strongly influenced by the availability of this organic matter. Other factors such as the increase of temperature during the winter-spring transition time may also be important for such a succession as it promotes the bacterial growth rate. The aim of this thesis was to study the population dynamics of the gammaproteobacterial groups caused by a massive spring algal bloom in the North Sea. Accordingly, surface seawater was investigated, that had been taken twice weekly in years 2009 and 2010 from the long-term ecological research station Kabeltonne located at the Helgoland Roads. Bacterial diversity was examined in samples of 11.02.2009 (winter) and 14.04.2010 (spring) by comparative sequence analysis of two full-length 16S rRNA gene clone libraries. Around 800 sequences were obtained from each library and revealed that Bacteroidetes and Gammaproteobacteria comprised the major fraction of the bacterial community in the coastal North Sea in April in contrast to February when Alphaproteobacteria was retrieved in high frequency. Within the Gammproteobacteria the SAR86 clade dominated the community in the winter library while the SAR92 clade was the dominating group in the spring one. Reinekea, Balneatrix-related clade and Galciecola were genera only occurring in the spring library. For the newly occurring gammaproteobacterial groups oligonucleotide probes were designed. The full sequences of both winter and spring clone libraries were used for the in silico validation of the probe specificities. Subsequently, the abundances of the gammaproteobacterial subgroups were monitored for the years 2009 and 2010 using catalyzed reporter deposition-fluorescence in situ hybridization (CARD-FISH) and a probe set consisting of the newly designed probes and previously designed ones. The sum of all counts as detected by this set of probe covered almost 100% of cells detected by the general probe GAM42a. In the year 2009, a tight succession of members of the genera Reinekea, the SAR92, OM182 and NOR5/OM60 clades, Balneatrix-related clade, Vibrio, Glaciecola and Pseudoalteromonas was observed about one month after a discrete spring algal bloom in late March which mainly consisted of diatoms. These groups seem to respond to the substrate generated by the bloom (bottom-up effect). A succession of Reinekea spp. was observed reaching up to 140000 cells/ml. This was followed by a particular succession of SAR92 clade showing the strongest increase within the Gammaproteobacteria, up to total abundances of 300000 cells/ml. OM182 and NOR5/OM60 clades subsequently reached total abundances of 120000 cells/ml and 90000 cells/ml, respectively. These larger groups likely play a major role in the degradation of the organic matter produced by the diatoms spring bloom. Although the abundance of other gammaproteobacterial groups such as Balneatrix-related clade, Glaciecola, Vibrio and Pseudoalteromans remained lower, they also participated into the upshift with maximum abundances of 36000 cells/ml, 10000 cells/ml, 30000 cells/ml and 9000 cells/ml respectively. Members of the SAR86 clade were no major players in the succession directly after the bloom, yet, they dominated later in May reaching up to 200000 cells/ml. In year 2010, a similar succession pattern was observed for the so far investigated groups Reinekea, SAR92, SAR86 and OM182 clades, Balneatrix-related clade and Glaciecola also within one month after the phytoplankton blooming peak. This indicates that this is an annual phenomenon occurring in the North Sea during the winter-spring transition period and further suggests substrate specialization for those groups.