CAN FRESHWATER FORAMINIFERA IMPACT STROMATOLITE FABRIC?

Prior observations suggest that marine foraminiferan protists use their reticulopodia to alter microbialite fabric by disrupting laminations of subtidal stromatolites, changing the layered structures into thrombolites in an experimental setting. Because microbialites and foraminifera also occur in non-marine settings, we hypothesized that foraminifera living in lakes could also disrupt stromatolite fabric. With this aim and using a variety of multidisciplinary approaches, we are conducting field surveys and experiments on material from Fayetteville Green Lake (New York State, USA), which has been studied as an analog of the Precambrian ecosystem. The lake is meromictic and alkaline, receives calcium sulfate-rich water in the hypolimnion, and supports a well-developed microbialite platform that provides access to living and relict microbialites. The living microbialites grow from the late spring to autumn forming a laminated mat at the surface, but display a clotted structure at depth. Both binding and trapping of small carbonate particles and active precipitation of CaCO₃ appears to add to the microbialite surface. Active photosynthesis of the cyanobacterial community results in supersaturated (~ 200-400% of air saturation) conditions at 2-3 mm depth. The mats are anoxic below ca. 2 mm early in the day, and permanently anoxic below ca. 5-6 mm. Free sulfide is detected immediately below the oxic-anoxic interface. The alkalinity decreased slightly during the course of the year and the Ca concentration increased some. To date, at least one morphotype of “naked” foraminiferan-like protist is common in FGL microbialite samples. This foraminiferan-like protist has anastomosing pseudopods, as do foraminifera. DNA sequences for 18S rRNA are being used to characterize the taxonomic affinities of this morphotype and examine diversity of rhizarian taxa occurring in the microbialites. We will also present micro-CT scanning results from a preliminary experiment to assess impact of a cultured freshwater foraminifera on FGL stromatolite fabric. Funded by NSF EAR 1561204 and 1561173.