Structure and Activity of Rock and Sand Biofilms in a Mediterranean Stream

The microbial benthic community living in streams has a key role in organic-matter processing. The respective role of epilithic biofilms (growing on rocks) and epipsammic biofilms (growing on surficial sandy substrata) in the degradation of organic matter was evaluated in a Mediterranean stream (Riera Major, northeastern Spain). Along with structural variables (algal chlorophyll a, bacterial density), we measured extracellular enzymatic activities (β-glucosidase, β-xylosidase, and phosphatase), photosynthetic activity (H14CO3− incorporation) and microbial community respiration (electron-transport-system activity) monthly for one year on different biofilms. Variations of temperature, dissolved oxygen and ammonium influenced the metabolism of sand-colonizing biofilm. However, none of these factors influenced metabolism of the epilithon, which did not follow a clear seasonal pattern. The capacity of microorganisms to degrade polysaccharides differed between the two types of biofilm. Epipsammon had higher β-glucosidase and β-xylosidase activities (16.4 and 7.4 nmol·cm−2·h−1 in annual average, respectively) than epilithon (6.1 and 3.3 nmol·cm−2·h−1 in annual average, respectively), indicating that degradation of cellulosic and hemicellulosic molecules was greater on sand than on tiles. The higher extracellular enzymatic activity in the epipsammon was related, in part, to the accumulation of organic matter in the sediment—suggesting dependence on substrate concentration. Bacterial biomass and cell size were lower on sand, but its biofilm was much more efficient in the utilization of polysaccharidic compounds: the extracellular enzyme activity per cell was up to 60–240 × 10−10 nmol methylumbelliferyl (MUF) per cell per hour in sand, but only reached maximum values of 4–7 × 10−10 nmol MUF per cell per hour on rock surrogates. These results emphasize the importance that sand habitats have in processing of organic matter in low-order, forest streams, where biofilms are potentially able to degrade a higher volume of macromolecules (especially polysaccharides) than those in the rocky streambed.