Physiological ecology of cyanobacteria in microbial mats and other communities

Abstract

summary In this review some aspects of the physiological ecology of cyanobacteria are discussed by taking a microbial mat as an example. The majority of microbial mats are built and dominated by cyarsobacteria which are primary producers at the basis of the microbial foodweb in microbial mats. These micro‐scale ecosystems are characterized by steep and fluctuating physico‐chemical gradients of which those of light, oxygen and sulphide are the most conspicuous. Light is strongly attenuated in the sediment, and owing to constant sedimentation, the mat‐forming cyanobacteria have to move upwards towards the light. However, at the sediment surface, light intensity, particularly in the u.v. part of the spectrum, is often deleterious. The gliding movement of the cyanobacteria, with photo‐ and chemotaxis, allows the organism to position itself in a thin layer at optimal conditions. The organic matter produced by cyanobacterial photosynthesis is decomposed by the ruicrobial community. Sulphate‐reducing bacteria are important in the end‐oxidation of the organic matter. These organisms are obligate anaerobes and produce sulphide. Gradients of sulphide and oxygen move up and down in the sediment as a response to diurnal variations of light intensity. Cyanobacteria, therefore, are sometimes exposed to large concentrations of the extremely toxic sulphide. Some species are capable of sulphide‐dependent anoxygenic photosynthesis. Other cyanobacteria show increased rates of oxygenic photosynthesis in the presence of sulphide and have mechanisms to oxidize sulphide while avoiding sulphide toxicity. Iron might play an important role in this process. Under anoxic conditions in the dark, mat‐forming cyanobacteria switch to fermentative metabolism. Many species are also capable of fermentative reduction of elemental sulphur to sulphide. The gradients of sulphide and oxygen are of particular importance for nitrogen fixation. Very few microbial mats are formed by heterocystous cyanobacteria, which are best adapted to diazntrophic growth. However, these organisms probably cannot tolerate greater concentrations of sulphide or anoxic conditions or both. Under such conditions non‐heterocystous cyanobacteria become dominant as diazotrophs. These organisms avoid conditions of oxygen supersaturation. In the ecosystem, nitrogen fixation and photosynthesis might be separated temporally as well as spatially. In addition, non‐heterocystous diazotrophic cyanobacteria have mechanisms at the subcellular level to protect the oxygen‐sensitive nitrogenase from inaction.

Carbon and nitrogen cycling on intertidal mudflats of a temperate Australian estuary. III. Sources of organic matter

“…The enriched cyanobacterial fraction was dominated by Oscillatoria spp., which are known to be highly mobile in cyanobacterial mats (Stal 1995). In a hot-spring microbial mat, O. terebriformis was found to move to the surface of the sediment in the morning, move back into the sediment during the middle of the day, return to the surface in the afternoon, and move back into the sediment at night (Stal 1995). The motility of Oscillatoria spp.…”

“…In some areas, intertidal sediments may be dominated by cyanobacteria where high rates of nitrogen fixation may supply a significant amount of nitrogen to the benthic community (Stal et al 1984, Bautista & Paerl 1985, Stal 1995.…”

“…Most studies to date have shown that the MPB on mudflats is dominated by diatoms (Cariou-Le Gall & Blanchard 1995, Barranguet et al 1997, Brotas & Plante-Cuny 1998, Lucas & Holligan 1999, Cahoon & Safi 2002, but this may shift seasonally to chlorophytes and cyanobacteria (Barranguet et al 1997). In some areas, intertidal sediments may be dominated by cyanobacteria where high rates of nitrogen fixation may supply a significant amount of nitrogen to the benthic community (Stal et al 1984, Bautista & Paerl 1985, Stal 1995.…”

Detection and Capturing of 14C Radioactively-Labeled Small Subunit rRNA from Mixed Microbial Communities of a Microbial Mat Using Magnetic Beads

“…Microbial mats are composed of physiologically different groups with oxygenic (i.e., cyanobacteria) and anoxygenic phototrophs (sulfur-oxidizing bacteria and chloroflexi) capable of converting CO 2 into organic carbon [1]. As a result, low molecular weight compounds are excreted directly in the mats and can be utilized by aerobic as well as anaerobic heterotrophic bacteria [2].…”

“…As a result, low molecular weight compounds are excreted directly in the mats and can be utilized by aerobic as well as anaerobic heterotrophic bacteria [2]. Different types of dissolved organic compounds are produced in the dark under anoxic conditions by fermenting storage compounds like glycogen [1,[3][4][5]. These compounds as well as cyanobacterial extrapolymeric substances (EPS) and the organics released after cell death and lysis can become available to heterotrophic bacteria.…”