Biofilm formation at warming temperature: acceleration of microbial colonization and microbial interactive effects

Abstract

River biofilms grown on any wet benthic surface are mainly composed of bacteria, algae, cyanobacteria and protozoa embedded in a polysaccharide matrix. We investigated the effects of warming river water temperatures on biofilm formation. A mesocosm laboratory experiment was designed considering two temperatures (low temperature: 7-11°C, night-day; high temperature: 11-15°C, night-day) and two nutrient levels (low nutrient: 0.054 mgP/L, 0.75 mgN/L; high nutrient: 0.54 mgP/L, 7.5 mgN/L). Biofilm samples were collected from each mesocosm periodically for 58 days. Biofilm formation at increased water temperatures was faster, while the biomass of the mature biofilm was mainly determined by nutrient availability. The specific response of the three microbial groups (algae, bacteria and ciliates) was modulated by interactions between them. The greater bacterial growth rate and earlier bacterial colonization at higher temperatures and higher nutrient conditions was not translated into higher bacterial biomass accrual. This might result from a significant effect of ciliates grazing on bacteria, as shown by the earlier increase in peritrichia at higher temperatures, both at high and low nutrient conditions. Ciliate grazing and temperature might determine the growth of a distinctive bacterial community under warming conditions, which was detected by DGGE analysis during the colonization process. Temperature accelerated algal colonization, but the effect of temperature on algal biomass was only significant under high nutrient availability. Warming conditions also produced a thicker biofilm (measured by confocal laser scanning microscopy), while functional responses were much less evident (increases in the heterotrophic use of polysaccharides and peptides, but no significant increase in primary production and respiration). In aquatic ecosystems, biofilm changes due to increasing river water temperatures might lead to faster biofilm recolonization after disturbances, with a distinct biofilm community structure that might affect the trophic web. Warming effects would be expected to be highly relevant under eutrophic conditions.