GROWTH OF CONIFORM MICROBIAL MATS FROM LABORATORY EXPERIMENTS ON STROMATOLITES FROM CARRIZO CREEK, CALIFORNIA

Laboratory growth experiments on microbial mats from stromatolites forming in Carrizo Creek, Anza Borrego Desert State Park, California, have resulted in coniform microbial mat formation. The stromatolites form on cobbles and boulders in 20-150 cm-deep, ephemeral stream pools. The microbial mat from the stromatolites is dominated by the filamentous cyanobacterium Rivularia. The sheaths are heavily pigmented with scytonamin.

Isolates were made from mats and grown on Bristol's medium, a standard defined freshwater medium. Mats were grown subaerially. This resulted in a shift in dominance to a small, thin-sheathed, as yet unidentified, motile and phototropic oscillatoriacean. Isolates were maintained under controlled conditions of 18^oC at approximately 75 mEinst.m^-2.sec.^-1 in a 12/12 light-dark cycle in Bristol's medium. The light source was overhead. After several weeks, the oscillatoriaceans produced millimeter-sized cones. Compound cones up to 1.5 cm tall also developed when two or more (commonly five or six) cones coalesced. Cones commonly have slender, cylindrical tips extending a millimeter above the cone. “Basal” mat coloration is predominantly a dark brownish-green with patches of orange. Cones are generally orange-colored except many have the upper few millimeters colored dark, brownish-green, including tips. The mats were subjected to higher growth temperatures and died at 40^oC. Ca levels were increased in the medium with no apparent carbonate precipitated. When the light source was from the side, the cones grew subvertically toward the light.

Our preliminary experiments agree with the general understanding of how coniform stromatolites form. Experiments of Walter and others (1976) indicated that coniform stromatolites can be produced by motile, phototropic cyanobacteria. Small, thin-sheathed and large, thick-sheathed cyanobacteria are known to produce coniform stromatolites. Proterozoic Conophyton formed in environments with very low sediment input. No sediment was supplied or precipitated in our experiments. Coniform stromatolites are one of the most distinctive stromatolites known and examples go back 3.45 Ga. Coniform geometry in stromatolites is recognized to be the result of phototropic microbial activity.