Friends and foes: competition experiments reveal growth facilitation and interference between cyanobacteria and diatom strains in microbial mats

Community dynamics within microbial mat assemblages are complex, likely involving intricate metabolic linkages. Interactions between microbes shape these communities, particularly in extreme habitats such as those found in the Middle Island Sinkhole (MIS), Lake Huron. Herein, euxinic waters resemble early-Earth ocean conditions, and diverse microbial groups form benthic mats. The goal of this study was to examine interspecific relationships among culturable cyanobacteria (two strains of Anagnostidinema) and diatoms (Craticula cuspidata and Nitzschia palea) isolated from MIS mats in a co-culture factorial experiment. Although most crosses showed limited interactions, a significant increase in abundance of one Anagnostidinema strain was found when grown with Craticula cuspidata compared to other taxa (P < 0.0001). Nitzschia palea growth was significantly reduced when grown with either strain of cyanobacteria (P = 0.0040). Addition of both cells and filtrates from C. cuspidata cultures caused increases in Anagnostidinema percent cover. Unique metabolic capabilities of C. cuspidata may benefit some cyanobacteria, and cyanobacteria may inhibit N. palea growth via interference competition. These interactions may be masked in the mat community in situ by environmental factors or other interspecific interactions, or allow species to survive in low abundance in an otherwise unfavorable habitat.