Biodiversity & Conservation

Mats of the sulphur reducing, filamentous bacteria Beggiatoa spp. occur at sites of organic pollution, often in areas of soft anoxic mud where there is poor water exchange with the open sea. Beggiatoa mats occur on the surface of the sediment at the hypoxic/anoxic interface. Beggiatoa sp. are characterized by their gliding motility, lack of photosynthetic pigments, and the presence of sulphur globules around the cytoplasm (Hagen & Nelson, 1997). The underlying sediment is primarily depauperate, the low oxygen levels resulting in death or loss of most mega and macrofauna. A few tolerant polychaetes, gastrotrichs, and nematodes may occur (e.g. see Bernhard et al., 2000). The Beggiatoa excludes other heterotrophic bacteria and most other macrofauna, with few elements of the infaunal communities that are found in other muddy biotopes.

Beggiatoa sp. utilize sulphides leaching from the sediment, and oxidize them to sulphate to liberate energy for growth but also require simple organic acids and alcohols for growth (Williams & Unz, 1989; Hagen & Nelson, 1997).The other organisms present (e.g. ciliates, nematodes and euglenoid flagellates) are probably decomposers, feeding on organic matter. However, Bernhard et al. (2000) noted several species of protist contained symbiotic bacteria that were presumably chemoautotrophs. The sediment below the mats is populated by chemoautotrophic bacteria, that remineralize organic matter, producing methane, or sulphides of hydrogen (H₂S), iron or manganese and are probably very similar to microbial communities found at depth in other sediments (for summary see Davies et al., 1996).

The few remaining tolerant species are probably deposit feeders on the microbial rich sediment or scavengers (e.g. crabs, hermit crabs, and starfish) feeding on dead or dying fauna.

The development of Beggiatoa mats are related to environmental conditions such as organic input and oxygen content which may have seasonal trends in some areas. Anoxic conditions may also develop in deep water due to the presence of a thermocline in the summer months e.g. some Fjords (Diaz & Rosenberg, 1995; Gustafsson & Nordberg, 1999) and Aberiddy Quarry (Hiscock & Hoare, 1973).

The biotope has little structural complexity because the surface of the sediment is covered with a mat of the filamentous bacteria reducing access to sediments for infaunal organisms. Scavenging species such as Asterias rubens and Carcinus maenas are typically present where the habitat is not too anoxic but in extreme conditions of anoxia little survives other than the Beggiatoa. The polychaete Ophiodromus flexuosus occurs in high densities at the interface between oxygenated and deoxygenated sediments (in Norwegian fjords). The mats provide habitat for an abundant meiofaunal community (Bernhard et al., 2000) such as nematodes and small ciliates (Spies & Davis, 1979). The combination of anoxic conditions and the related production of sulphides (e.g. H₂S) is highly toxic to most life (see Diaz & Rosenberg, 1995) and the underlying sediment may be effectively abiotic. Where conditions are not anoxic but severely hypoxic, Beggiatoa may be patchy, with an impoverished infauna present.

Productivity in this biotope is limited to the anaerobic chemoautotrophic productivity of infaunal bacteria and of the sulphur-oxidising bacteria Beggiatoa sp. Beggiatoa sp. utilize sulphides leaching from the sediment, and oxidize them to sulphate to liberate energy for growth but also require simple organic acids and alcohols for growth (Williams & Unz, 1989; Hagen & Nelson, 1997). Diaz & Rosenberg (1995) noted that area dominated by bacterial mats, the benthic-pelagic coupling is weakened and the food chain shortened. However, they also noted that bacterial mats may be important sources of organic matter in coastal upwelling oxygen minimum zones.

Bacterial colonies can spread rapidly via asexual reproduction. In many species resting stages, spores and cysts may occur that allow some bacteria to survive for long periods returning to normal growth when conditions are good. Beggiatoa sp. are probably ubiquitous.

Although growth rates of Beggiatoa in natural environments are not known, the generation time for many bacteria is short and growth is usually exponential in optimal conditions (e.g. in some bacteria the population can double in 20 minutes). Therefore, in the right conditions, a Beggiatoa mat is likely to develop rapidly.

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