Microbial biofilms are held together by extracellular polymeric substances (EPS), which can be secreted by many organisms. EPS production can facilitate intercellular communication and inter-guild microbial mutualisms, intraspecific gamete exchange, nutrient sequestration, and desiccation resistance. Benthic microbial mats (periphyton) of the Florida Everglades and other karstic wetlands contain abundant EPS generated by mat-producing filamentous blue-green algae and many other species of mat-dwelling algae, cyanobacteria, and bacteria, including the most abundant Everglades diatom, Mastagloia calcarea. The benthic diatom genus Mastogloia is characterized by several morphological and physiological features that foster production of a ‘halo’ of EPS around the frustule, but the cellular and environmental triggers of EPS production are unknown. In this study, I examined how conductivity, pH, periphyton total phosphorus, hydroperiod, reproduction, and the presence/absence of bacteria affect the presence and area of the EPS halo around M. calcarea cells. Field and cultured samples were microscopically examined, and individual M. calcarea cells with and without halos were measured and analyzed to determine environmental and population correlates to EPS production. We found that small, paired (likely copulating) cells were more likely to have EPS halos, and these were more abundant at low ionic conductivity, higher pH, and low periphyton total phosphorus concentrations. We also found evidence for reduced halo presence and size and lower population turnover when bacteria were removed from M. calcarea cultures. These results indicate that EPS production by mat-dwelling diatoms is maintained at high levels in ultraoligotrophic, carbonate-rich wetland conditions where beneficial associations with bacteria support rapid population turnover and sexual reproduction.
