Effect of primary producers on the heterotrophic metabolism of a stream biofilm

1. The influence of benthic algae on heterotrophic metabolism in a forested Mediterranean stream was investigated. Bacterial density and ectoenzymatic activities, as well as algal biomass (chlorophyll-a) and metabolism (the rate of C-14 incorporation), were measured during colonization over 60 days of artificial substrata (clay tiles) under light and dark conditions. 2. Chlorophyll-a and the rate of C-14 incorporation were significantly higher in light-grown than in dark-grown biofilms. Bacterial density and ectoenzymatic activity (especially beta-glucosidase) were also significantly higher in light-grown biofilms. 3. Regressions of chlorophyll-a and C-14 incorporation values on the ectoenzymatic activities were significant. The slopes of regression lines obtained for dark-grown biofilms were significantly higher than those obtained for light -grown biofilms. 4. The differences in the slope (of the regression lines) between dark and light-grown biofilms suggest that the response of the heterotrophs is faster in biofilms with low algal biomass accrual and slows down when algal biomass is increased. 5. It is concluded that algal accumulation in the epilithic biofilm influences the use of organic matter by the heterotrophic community by increasing the amount of organic substrate available for bacteria.