Photosynthesis and structure of benthic microbial mats: Microelectrode and SEM studies of four cyanobacterial communities1

The microzonation of photosynthetic organisms in four cyanobacterial mats of Solar Lake, Sinai, was studied by light and scanning electron microscopy. The zonation was compared to the distribution of photosynthesis and of O 2 , H 2 S, and pH. Microelectrodes were used to measure the chemical gradients as well as the photosynthetic rates by a newly developed technique which allows a spatial resolution of 100 µ m. In one of the mats, the photic zone was only 0.8 mm thick and there was a maximum photosynthetic rate of 50 µ mol O 2 ·cm ‒3 ·h ‒1 at 0.3–0.4‐mm depth. Three other mats had photosynthesis down to 2.5, 4.5, and >10 mm. Photosynthetic rates in the shallow water mats ranged from 13.3 to 17.6 mmol O 2 ·m ‒2 ·h ‒1 . Two‐dimensional maps of oxygen and photosynthesis distribution at the mat surface demonstrated a strong heterogeneity with rapid oxygen production within dense diatom tufts of 0.2–0.5‐mm diameter overgrowing the cyanobacteria. The photosynthesis in the tufts reached 100 µ mol O 2 ·cm ‒3 ·h ‒1 , which led to an oxygen turnover time of only 23 s . Gas bubbles within the mat served as dynamic reservoirs for oxygen during light‐dark cycles and buffered against extreme diurnal variations between oxygen and sulfide.