Microbial mats

Microbial mats are formed by cyanobacteria, organisms which thrive in environments where extreme fluctuations in conditions occur. Two types of cyanobacterial mat are found in close vicinity on the intertidal flats near Guerrero Negro, Baja California, Mexico.

In view of their intense physiological activity, microbial mats, are ideal study objects in a geophysiological context.

The pustular mat

The heterocystous cyanobacterium Calothrix sp. forms a pustular mat. This is a nitrogen-fixing organism. The terminal heterocysts contain the nitrogenase molecules. In fact, the pustular mat is one of the few examples of a microbial mat formed by a heterocystous species.

The pustular mats are located on sites which are rarely, if ever, inundated. The mat owes its name to its pustular appearance. The porous structure allows oxygen to penetrate readily into the sediment. The mat is thus well-aerated. Anoxic conditions prevail seldom, if ever, in the pustular mat.

The mat turns anoxic only in the dark, and only when inundated. Therefore, anaerobic bacteria are almost entirely absent from these mats. Such anaerobes include the sulphate reducers that produce sulphide. Sulphide is toxic to all organisms. However, heterocystous species are thought to be particularly susceptible to sulphide toxicity.

The structure of the pustular mat becomes apparent up close. The cyanobacteria constitute the top layer. This cyanobacterial layer is rather loose and only 2-4 mm thick. Underneath this layer, white sand is visible. The cyanobacterial mass is mixed with sediment particles.

The smooth mat

The non-heterocystous, filamentous cyanobacterium Lyngbya aestuarii forms the smooth type of mat. This is also a nitrogen-fixing organism. However, nitrogenase activity in this species is confined mostly to the dark period, in contrast to Calothrix, which fixes nitrogen in the light.

Lyngbya excretes a thick polysaccharide sheath. The trichomes are embedded in a dense matrix of mucilage. The smooth mats are found where the intertidal flat is inundated regularly. Conspicuous cracks form during low tide, when the mat is exposed to the air as well as to intense sunlight. Such conditions lead to desiccation. The dense structure of the smooth mat hampers oxygen diffusion. In the light, oxygen supersaturation occurs. In the dark, the mat turns anoxic.

Anaerobic bacteria, such as sulphate reducers, thrive in the layers beneath the Lyngbya layer. Thus, a strongly reduced black layer of FeS is formed. This black FeS horizon is visible in the picture on the right. The top green layer is made up by a thin but very dense meshwork of Lyngbya trichomes. Between the thin (1-2 mm) cyanobacterial layer and the FeS horizon a purple layer of sulfur bacteria can be discerned.

source: Lucas J. Stal: Physiological ecology of cyanobacteria in microbial mats and other communities, New Phytologist (1995), 131, 1-32