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.
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
Publications on Microbial Role in
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(The background color approximates
that of cyanobacterial pigmentation)
updated July 2, 1999