Bacterial communities associated to outdoor cultures of the marine microalga Tetraselmis suecica

Microalgae in culture collections are usually not axenic, i.e. they are associated with bacteria that were co-enriched and isolated together with the algal cells and then selected during algal maintenance and culture. Moreover outdoor cultures of microalgae are carried out in non-sterile conditions and are thus susceptible to contaminations. Several studies have reported that bacteria may both stimulate or inhibit algal growth (1); they are able to produce algicidal compounds, regulate algal production of toxins (2) and alter algal physiology (3). Therefore they can play an important role on the stability and productivity of microalgal cultures. Although many microalgal cultivation technologies have been developed and culture conditions are optimized to improve biomass productivity, little is known about bacterial communities associated to microalgal mass cultures. The objective of the present study was to analyze the bacterial community associated with an outdoor culture of Tetraselmis suecica strain OR and to determine the influence of this community on the algal growth. Tetraselmis suecica is a marine flagellate green alga of the class Prasinophyceae widely used in aquaculture, which has shown a high probiotic potential and an inhibitory activity against Vibrio species (4). T-RFLP was applied to follow the dynamics of the bacterial community associated to the culture in different periods of the year. Culturable bacteria isolated from the laboratory culture (used as inoculum for the outdoor culture) and from the outdoor culture collected eight months after inoculation were clustered into operational taxonomic units by ARDRA analysis and then identified through 16S rDNA sequencing. Batch cultures inoculated with outdoor, laboratory and axenic cultures were carried out in 0.5-L bubbled tubes under laboratory conditions to evaluate the influence of bacterial communities on algal growth. The outdoor algal culture showed a lower diversity in the associated bacterial communities, compared to the laboratory one, and a seasonal variation of the bacterial population, with only few ribotypes constantly present in the algal culture. Strains belonging to the Roseobacter clade and to the Flavobacteriales and Rhizobiales groups appeared to be closely associated to the culture. The presence of an associated bacterial community demonstrated a positive influence on the algal growth under laboratory conditions (higher productivity than the axenic culture). From these results it appears that the bacterial flora in Tetraselmis suecica cultures is only in part resilient and that it has a positive effects on culture productivity. Further studies will be carried out to better understand the role of the closely associated bacteria on algal growth and bioactivity under laboratory conditions.