Strong correlations between bacterial communities and algal seasonal succession have been previously observed. In aquatic systems, dissolved organic carbon produced by algae is an important resource for bacteria. Quality and quantity of algal exudates can have a strong influence on bacterial community composition. Light and nutrient availability are two factors that influence biomass and abundance of algae in aquatic systems. These environmental factors also regulate the quantity and quality of algal exudates available to support bacterial communities. Glycolate is an algal-specific exudate produced under excess light conditions. Glycolate uptake by bacteria has been shown to correlate with algal primary productivity. In this study, development of algal biofilms in an experimental stream system was monitored across different light and phosphorus levels, and samples were collected over time. Bacterial communities were monitored using DNA fingerprinting techniques based on rRNA genes.  Bacterial populations that utilize glycolate possess the glcD gene (glycolate oxidase subunit D). This gene was used as a marker to identify changes in specific bacterial populations that respond to algal exudates.

Results/Conclusions

Bacterial community composition changed significantly over the course of biofilm development (ANOSIM R =0.56, P = 0.001). Distinct bacterial communities have been observed across varying P and light treatments (P and light effect account for 69.9% and 73.9% of explained variance, respectively). Differences in bacterial community composition were correlated with light: P ratio and algal biomass. This implies that those differences in bacterial community composition may result from quality of algal exudates via light: P ratios. Differences in abundance and diversity of glcD gene were also examined in response to algal biofilms grown under various light and phosphorus levels. Glycolate-utilizing bacterial communities were shown to differ across light and P treatments. Light has a stronger effect on bacterial community composition than P. This indicates that bacterial community is influenced by glycolate production through algal response to light levels. Our results demonstrate that algal community composition and activity can significantly impact aquatic bacterial communities. These results increase our understanding of the ecological drivers that impact benthic biofilm communities that carry transformation of nutrients in streams.