Ecology, taxonomy, culture, and recent to fossil comparisons of microbial endoliths in carbonate sand grains of the Arabian Gulf

Nine new species of endolithic cyanobacteria were discovered within carbonate ooid sand grains in moving shoals of the Arabian/Persian Gulf, Saudi Arabia. They are (in descending order of frequency): Hyella reptans, H. inconstans, H. racemus, H. stella, H. immanis, Solentia sanguinea, H. conferta, H. salutans, and H. arbuscula. These species show morphological characters and boring patterns that are different from other known endolithic taxa. Natural populations were studied after carbonate grains were dissolved with dilute acid. The distribution of these species is relatively constant irrespective of depth or site of collection.;The Arabian Gulf assemblage was compared with endoliths in ooid grains of the Bahama carbonate platform, and found to be different in species composition, with only one common species: H. racemus. Seven endolith strains have been isolated and cultured, and six of these identified with taxa found in natural populations. A modified culturing protocol was used to assure the identity of isolated strains with natural populations. The endolithic properties of isolated strains were tested, by adding calcite crystals, ooids and conch shell fragments to liquid cultures. All cultured strains penetrated the carbonate substrates and maintained their endolithic habit as well as their specific borehole morphology. Rates of colonization and carbonate removal under culture conditions were measured for H. immanis. Turbulence proved to be important in enhancing frequency of borings, and to a lesser extent boring rates. This result is consistent with the field observations, showing that ooids in high energy environments are intensively bored. Approximately 50% of randomly sampled ooids contain microbial borings, and about 25% are inhabited by active endoliths at any time.;Fossil morphological counterparts of four of the newly described species, Hyella reptans, H. conferta, H. racemus, and H. immanis have been identified in silicified oolites from 700-800 Ma old Upper Proterozoic rocks of Greenland. The Arabian Gulf region supports a distinct suite of taxa and may represent a speciation center separate from that of the Atlantic region. Thus, the hypothesis that morphological similarities between modern and fossil endolithic cyanobacteria are based on direct descendence appears unlikely.