AME 16:131-141 (1998)  -  doi:10.3354/ame016131

ABSTRACT: The bedrock of La Solana (NE Spain) stream is a thick calcareous cyanobacterial crust with a layered structure similar to a stromatolite. Different algal patches, which appear along with seasonal changes (especially discharge, temperature and light), characterize the stromatolite. This structure has a great capacity for organic matter utilization, as indicated by the high extracellular enzymatic activities (b-glucosidase, b-xylosidase and phosphatase) measured in the stromatolitic algal patches over an annual cycle. However, each patch showed a particular ability in the use of organic matter since a different hydrolytic potential capacity was measured. The highest β-glucosidase and b-xylosidase activities were measured in the mixed community (cyanobacterial crust with a sparsely developed overstorey), indicating that the understorey of this stromatolitic crust is highly active. In the Zygnema-Spirogyra community, autotrophic activity might enhance b-glucosidase activity in spring, whilst in the diatom bloom (which appeared in May-June), polysaccharides released by algae and mucilagenous material from the diatom stalks might regulate the ectoenzymatic activities. The adaptation of the Rivularia community to oligotrophic conditions occurring in summer, autumn and winter is shown by the extremely high phosphatase activity, related to the low phosphorus concentration in stream water. The appearance and progressive substitution of the different patches can be interpreted as the adaptive response to the most available organic matter source, as well as a way of surviving the drastic environmental changes characteristic of Mediterranean streams.

KEY WORDS: Organic matter · Stromatolite · Ectoenzymatic activities · Bacteria · Algae · Mediterranean streams