Positive autotrophic-heterotrophic ecological interactions may be good reasons to live in a biofilm

ROMANÍ, ANNA; DÍAZ VILLANUEVA, VERÓNICA; FONT, JORDI; YLLA, IRENE

Simposio; 12th Symposium on Aquatic Microbial Ecology (SAME 12); 2011

Biofilms develop on any solid wet surface. The biofilm community under light conditions is mainly composed by microbial autotrophs and heterotrophs which make use of their respective capabilities. The biofilm gives physical refuge and available organic molecules from within the polymeric matrix. At the same time, biofilm microorganisms may compete for the resources from the flowing water and/or the main biofilm. Although it is not always clear when microorganisms became benthic, results suggest that when possible they prefer to be attached in biofilms. The autotrophic-heterotrophic interactions in biofilms is analysed based on empirical results and under the view of the known ecological interactions usually described for higher organisms (facilitation, mutualism, competition). Biofilm autotrophic-heterotrophic interactions have been mainly defined in one direction, describing the positive effect of algae on bacteria by providing algal exudates. However, algae can also benefit from bacteria. Algae require special vitamins for their growth which are basically synthesized by heterotrophic bacteria. Bacteria may also release extracellular enzymes that degrade complex organic molecules not transportable across cell membranes, making organic and inorganic compounds available. This algal-bacterial relationship has been generally defined as mutualism or co-operation within the biofilm. However, at the initial biofilm formation phase, the great difficulties observed for lonely algae (with no bacteria) to form a biofilm suggest that a facilitation interaction occurs at this first step. Bacterial colonization may facilitate algal colonization providing a sticky active surface similarly than the facilitation mechanism described for higher plants where the first colonizers provide shadow. During biofilm formation autotrophic-heterotrophic ecological interactions may change from facilitation to mutualism, while competition may also occur specially in mature biofilms