Endolithic Cyanobacteria and Microbial Bioerosion

Organisms that colonize the interior of rocks and other hard substrates are known as endoliths. The endolithic mode of life includes several different ecological niches: chasmoendoliths and cryptoendoliths occupy preexisting fissures and structural cavities in the rocks, whereas euendoliths penetrate soluble carbonatic and phosphatic substrates (Golubic et al. 1981).

Specialized euendolithic cyanobacteria penetrate and colonize the interior of solid limestone and dolomite rocks as well as loose sand-size particles, shell hash and other skeletal fragments. Vast surfaces of shifting sands in shallow tropical seas, from ooid shoals of the Bahama carbonate platform and Arabian/Persian gulf, to coral rubble and sands on the Pacific atolls, appear macroscopically barren - but each calcareous sand grain, if exposed to light and nutrients, is potentially a cyanobacterial habitat. Microbial endoliths are instrumental in destruction of carbonate sand grains, creating so called micritic envelopes. The figure left shows an ooid grain from the Bahama carbonate platform perforated by microbial endoliths

Oolitic grains (see the frame picture above and klick it for details) and sand-size shell fragments often contain entire assemblages of euendolithic cyanobacteria now as in the Precambrian. They can be visualized in resin-cast preparations of modern ooids. These organisms are often exposed to changes in light and nutrients as the sand grains are moved and churned by waves and tidal currents. Several species of coccoid cyanobacteria of the genus Hyella inhabit ooid grains. Hyella stella Al-Thukair et Golubic is characterized by dichotomous branching of its boring pseudofilaments. Similar organism described as Eohyella dichotoma Green et al. These ancient cyanobacteria inhabited ooid grains 800 million years ago, and evolved fossil assemblages that show similar species diversity as their modern counterparts (Green et al. 1986; Al-Thukair & Golubic 1991a). The oldest known endolithic cyanobacterium Eohyella campbellii Zhang et Golubic penetrated lithified stromatolite surfaces 1500 million years ago in northern China marking the earliest known occurence of bioerosion.

Intertidal and supratidal ranges of carbonate coasts are today the sites of most intensive bioerosion which effectively destroys rocky shores and contributes to fine grain sediment production at a geologically significant scale. Epilithic and endolithic cyanobacteria are the principal primary producers in these ranges and the ultimate cause of coastal bioerosion. Filamentous cyanobacteria of the genus Scytonema, for example, endure extreme salinity fluctuation and complete desiccation within the wave spray zone. Temporary rock pools are home of the coccoid cyanobacterium Solentia. These rockpools heat up and evaporate during low tides or are washed in rain, but they are periodically recharged with seawater by tides and waves.

Endolithic cyanobacteria support a variety of grazing animals, equipped with hard rock-scraping mouth parts. As compared with the formidable destructive capacity of these grazers, the bioerosion effect of microscopic cyanobacteria alone appears modest. However, the significance of cyanobacterial colonization in these intertidal ranges is in providing the nutritional base of a complex and diversified trophic pyramid. As a consequence of settling preferences, indurance, and competitivness of endolithic cyanobacteria, coastal rocks are also biodegraded selectively, resulting in peculiar geomorphological features called biokarst which include elaborate sharp-edged karstic rock forms (lapies in French; Karren in German), flat-bottom rock pools and, in wave-protected areas, a deep bioerosional intertidal notch. These processes act as positively re-enforcing mechanisms. They produce sheltered microbial habitats with improved local water retention which, in turn, foster bioerosion and maintain and increase biological diversity.

Publications on Microbial Bioerosion
Publications on Ecology and Diversity of Cyanobacteria
Publications on Microbial Role in Biosedimentology

e-mail: golubic@bu.edu

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(The background color approximates
that of cyanobacterial pigmentation)

updated July 2, 1999