The main aim of my PhD work was to evaluate the impact of photochemical transformations of dissolved organic matter (DOM) on the bacterioplankton community in a coastal lagoon with high dissolved organic carbon (DOC) concentrations (Laguna de Rocha, Uruguay). In order to accomplish that, I evaluated the short-term effects of photochemical transformations of DOM on bacterial abundance, activity and community composition through several experiments. I found a clear DOM photobleaching signal in experiments where DOM with high chromophoric content dominated. In those cases, bacterial abundance and activity were stimulated in the treatments where DOM was previously exposed to UV radiation, suggesting increased bioavailability after photoalteration. This stimulation was accompanied by the enrichment of Betaproteobacteria, particularly, by a population that hybridized with a probe for ammonia-oxidizers. Thus, the results suggest that DOM photoalteration not only stimulate bacterioplankton growth but also induce rapid changes in bacterioplankton composition triggered by photoproducts such as ammonium.
In addition, I have performed a seasonal study in order to evaluate the dynamics of bacterial community in the lagoon, using the same bacterial parameters described above. I found that Alphaproteobacteria formed the dominant microbial group in both zones throughout the sampling period (one year). After an intrusion of marine water, members of the SAR11 lineage became abundant in the brackish-water zone. These bacteria were apparently distributed over the lagoon during the following months until they constituted almost 30% of all prokaryotic cells at both sampling sites. At the first sampling date (March 2003) a single alphaproteobacterial species unrelated to SAR11, Sphingomonas echinoides, dominated the microbial assemblages in both zones of the lagoon concomitantly with a bloom of filamentous cyanobacteria. Pronounced maxima of leucine incorporation were observed once in each zone of the lagoon. In the freshwater zone, this highly active microbial assemblage was a mix of the typical bacteria lineages expected in aquatic systems. By contrast, a single bacterial genotype with >99% similarity to the facultative pathogen gammaproteobacterial species Stenotrophomonas maltophilia formed >90% of the bacterial assemblage (>107 cell ml-1) in the brackish-water zone at the time point of highest bacterial leucine incorporation. Moreover, these bacteria were equally dominant, albeit less active, in the freshwater zone. Thus, the pelagic zone of the studied lagoon harbored repeated short-term blooms of single bacterial species. As Laguna de Rocha is a protected area and an important feeding and breeding place for many animal species, this finding may have consequences for environmental protection.
Lastly, I performed experiments to test the effect of photodegradation of potential DOM sources (plants, sediment and soil from the littoral zone of the lagoon) on the bacterioplankton community. The results showed that plant extracts rapidly stimulated two different bacterial populations that were scarcely present in situ: a filamentous bacterium belonging to Exiguobacterium genus and a population from the Acinetobacter genus. On the other hand, irradiated sediment and soil extracts did not provoke significant changes on bacterial community.
The results obtained through this study suggest that variation in some physicochemical characteristics of water, which are induced by the complex hydrology of the lagoon (i.e. nutrients concentration, DOM origin and time of exposure to solar radiation), provokes drastic changes on bacterioplankton community. Changes in land management around Laguna de Rocha are currently being done (such as increased use of commercial fertilizers commonly used in agriculture), which in combination with increased influx of organic carbon by terrestrial runoff might induce the development of large populations of potentially pathogenic bacteria.