Photolithotropic and chemoorganotrophic activity of bacteria and algae as related to beachrock formation and degradation (gulf of Aqaba, Sinai)

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Abstract
Beachrock formation and degradation along the shores of the Gulf of Aqaba (Sinai) were studied in the field and at the laboratory. Different types of beachrock were found, including laminated rocks. Rates of photosynthesis and respiration were studied under high- and low-tide conditions. Epilithic, chasmolithic and endolithic bacteria, cyanobacteria and algae were studied quantitatively. Bacterial contribution to carbonate precipitation and cementation was estimated in thin sections and by laboratory experiments. Precipitation of aragonite and monohydrocalcite was observed at salinities between 28.permill. and 45.permill.. Concentrations of dissolved organic matter (DOM) and particulate organic matter (POM) were high at initial stages of beachrock formation and extremely low in final rock. Anaerobic and later aerobic decay processes initiate carbonate precipitation. A model is suggested that places beachrock as an intermediate between cyanobacterial influenced subtidal and supratidal rocks. Formation, cementation and terminal concentration of organic matter in beachrocks, depend largely on microbial activity, sedimentation rates, energy, O2 supply, evaporation rates and water movement in the intertidal zone. Fossil remains of the biological origin of beachrock are extremely rare and difficult to interpret.