UNCOVERING THE ADVANTAGE OF MICROBORING FOR ENDOLITHIC CYANOBACTERIA

Trace fossils of bioerosion by cyanobacteria are found in the geologic record and observed in modern marine carbonate sediments. Some cyanobacteria actively dissolve calcium carbonate mineral substrates to create microbe-sized tunnels that they inhabit, known as “microborings.” The physiological advantage of this lifestyle is unknown, but an advantage must exist because their dissolution mechanism requires non-trivial use of energy—ATP is hydrolyzed to actively pump calcium ions away from the mineral surface. There has also been little systematic characterization of when and where microboring by cyanobacteria takes place. We designed a suite of field incubation experiments to test three hypotheses about the physiological benefit of the endolithic living strategy: 1) living inside microborings provides protection from harmful ultraviolet radiation; 2) dissolving carbonate minerals liberate additional dissolved inorganic carbon that cyanobacteria can use as a carbon source; or 3) mining carbonate minerals provide a source of trace element micronutrients. We conducted field experiments in four locations: Little Ambergris Cay (LAC) in the Turks and Caicos Islands, Coupon Bight in the Florida Keys, Whalebone Bay in Bermuda, and Rookery Bay in southwestern Florida. Seawater and porewater were sampled at each field location to determine the aragonite saturation state (Ω_arag); we hypothesized that microboring intensity would decrease with increasing Ω_arag. We also predicted that microboring intensity would be diminished under attenuated light conditions. The extent of microboring under different conditions was determined using a Scanning Electron Microscope. To determine whether the inorganic carbon is being incorporated into the microbial biomass, we created ¹³C synthetic calcites as a colonization substrate and preserved endoliths in-situ using paraformaldehyde to map the abundance of the ¹³C label using NanoSIMS. Finally, we are working on isolating LAC endolithic cyanobacteria in incubation experiments. Preliminary results reveal differences in microboring intensity among geographic locations and between full light and dark conditions.