Diversity and Role of Cyanobacteria and Aerobic Heterotrophic Microorganisms in Carbon Cycling in Arid Cyanobacterial Mats

Abstract

Large areas of the intertidal flats of the Arabian Gulf are inhabited by different types of cyanobacterial mats (pustular, flat, pinnacle, convoluted, and gelatinous). These mats vary in their pigmentation, shape, and microbial composition depending on their position and the environmental conditions they are exposed to. The temperature in this region may exceed 50°C in hot summers, salinity ranges between 6% and 20% depending on the closeness to the water line, and the UV and light intensity are high, causing some of the mats to completely desiccate. Using culture-dependent, molecular, and microsensor techniques, the diversity of cyanobacteria and aerobic heterotrophic microorganisms and the role of their key processes (i.e., oxygenic photosynthesis and respiration, respectively) in carbon cycling in the uppermost layer of different mats was studied. The difficulty of obtaining axenic cultures of cyanobacteria may be due to the close association of the two groups. While cyanobacteria fuel mat microorganisms with organics via photosynthesis and fermentation processes, aerobic heterotrophs in the oxic layers of mats utilize these organics and produce carbon dioxide, which is then made available for cyanobacterial photosynthesis. Cyanobacteria in these mats include Microcoleus chthonoplastes, Lyngbya aestuarii, Entophysalis major, Schizothrix splendida, and some unicellular scytonemin-containing cyanobacteria, whereas the aerobic heteroptrophs belonged primarily to Bacteriodetes, Proteobacteria, the Chloroflexus group, and many others. Rates of photosynthesis and light respiration were regulated by salinity and temperature. Both processes remained coupled and their rates decreased with increasing salinities. The mats at the lowest tidal zones exhibited the highest rates of photosynthesis and light respiration. The rates of the two processes also decreased with increasing temperatures. Photosynthesis and light respiration rates are regulated by salinity and temperature in order to maintain the relationship between organic production and decomposition, which determines the accretion rates of mats.