Limestones

Summary

The chapter provides an overview of cyanobacteria in calcareous environments. especially those associated with limestone surfaces. where the organisms may be epilithic or endolithic. The epilithic species of aquatic environments sometimes enhance calcium carbonate deposition. either by trapping particles or by contributing to communities where calcite and aragonite (in freshwaters) or other carbonates are precipitated. Environmental features at a site have an important influence on the process of carbonate precipitation. but photosynthetic activity and also the properties of the sheaths in some species sometimes influence the process. Temporal heterogeneities in water supply and phosphate availability are especially important in determining the species composition.Sites with occasional running water combined with long periods of drought are often dominated by Chroococcales with dark sheaths (e.g. Gloeocapsa), while sites with marked temporal differences in ambient phosphate are often dominated by hair-forming genera (e.g. Rivularia). Calcium carbonate deposition sometimes leads to a permanent increase in the amount of limestone present, as with travertine in freshwaters or stromatolites in both freshwater and, more especially, marine environments.

Endolithic species occur in a variety of situations, but particularly where the limestone is soft and porous. For example, several genera (e.g. Hyella) grow actively into the limestone at sites where removal of surface material tends to occur. Such endolithic communities are best developed on vertical surfaces in the intertidal zone of tropical and subtropical seas, where the more superficial parts of the communities are continually being removed by grazing animals.