Response of bacterial and viral communities to nutrient manipulations in seawater mesocosms

Abstract

Changes in natural bacterial and viral assemblages were studied in seawater meso- cosms manipulated with inorganic (nitrate + phosphate) and inorganic + organic (glucose) nutrient additions. As inferred from the gel band patterns obtained by DGGE, only moderate changes within the bacterial community took place when mineral nutrients were added alone. Supplementing the mineral nutrients with glucose in excess of what the bacteria could consume led, however, to major changes in band patterns. Based on fluorescence in situ hybridisation (FISH), the major bacterial response was identified as an increase in the population of γ-Proteobacteria with a smaller response in α -Proteobacteria. Sequencing of bands from the DGGE gels indicated that glucose + mineral nutri- ents led to a Vibrio-dominated bacterial community. A specific FISH probe was designed from a band sequence affiliated to Vibrio splendidus, and linked a large-celled bacterial morphotype to the DGGE-gel bands dominating in glucose-amended mesocosms. A similar difference in the response of the viral populations among treatments was demonstrated using pulsed field gel electrophoresis (PFGE). The number of bands on DGGE gels and PFGE gels were similar (mean ratio 0.98). We suggest an interpretation of these results where coexistence of nutrient-competing bacterial hosts is controlled by viral lysis. We also suggest that the success of large bacteria in glucose-replete treat- ments was not based on superior glucose-utilisation abilities, but rather on an advantage in competi- tion for limiting mineral nutrients derived from the combination of a large cell surface with a low cellular content of the limiting element, possible for cells with large C-rich inclusion bodies.