Although MTB were isolated from different habitats such as ponds and lakes, sewage-treatment ponds, rivers, estuaries salt marsh and seas (Mann et al., 1990; Bazylinski et al., 1988) their ecology and distribution in sediments and stratified water columns is not very well known. In an early report, Stolz (1992) showed that whereas magnetotactic cocci could be detected only in the oxic and microoxic zone, diverse morphotypes were abundant both in the microoxic and anoxic zone in the presence of up to 2 mM sulfide. Therefore it can be concluded that the distribution of different MTB is determined by different optima in sulfide and oxygen gradients. This behavior could be important in terraformation for building up microcosms in which MTB should be able to reversibly move from the oxic to the anoxic zone in search of optimum conditions for growth and multiplication.

Another study investigated the vertical distribution of magnetite- and greigite-producing MTB (Bazylinski et al., 1995). Generally, more magnetite producers were found at and above the oxic-anoxic transition zone (OATZ), whereas more greigite-producing MTB were located in the anoxic sulfidic zone. Similar observations were reported for a stratified water column of a brackish water pond (Bazylinski and Moskowitz, 1997).

Another paper (Petermann and Bleil, 1993) showed that the majority of MTB were found in the anoxic zone (in the upper 10 cm of South-Atlantic deep-sea sediments), where nitrate was available, suggesting that most MTB might reduce nitrate as the terminal electron acceptor.

Although these studies have indicated that MTB are major constituents of microbial communities in certain zones of aquatic habitats, the biogeo-chemical interactions controlling their occurrence in stratified sediments have remained poorly understood. In the studies of microcosms performed by the Flies et al. (2005), in all samples analyzed, the occurrence of MTB was restricted to a narrow layer in the upper sediment located closely to the OATZ. As most of the cultivated MTB strains are known to behave as typical micro-aerophiles, it was surprising that in all microcosms most MTB were detected in the suboxic zone immediately below the OATZ. Maximum numbers were between 9.7 105 and 1.5 107/cm3, thus accounting for at least 1% of the total cell numbers in this region. The maximum MTB numbers in their study were considerably higher than MTB numbers estimated for environmental samples (103-104 MTB/cm3) (Blakemore et al., 1979; Blakemore, 1982) but were in the same range as previously reported for other laboratory enrichments (Blakemore et al., 1979; Spring et al., 1993; Moench and Konetzka, 1978; Petersen et al.,

1989). It has also been suggested that in microcosms and possibly in natural ecosystems, MTB can exists in a resting or metabolically inactive state which occurs in the absence of higher concentrations of organic substrates (Flies et al., 2005), which represent an important aspect for terraformation. If MTB can exist in a resting or metabolically inactive state, then we could speculate that MTB together with other bacteria having resting or metabolically inactive states (but able to recover) could have potential in terraformation by switching off to an inactive state while the nutrients (organic and/or inorganic) are not available in appropriate concentrations and by switching on as nutrients become available.

As already suggested (Flies et al., 2005), further studies regarding in situ measurements of metabolic activity of MTB in undisturbed sediments from marine and freshwater habitats and the interactions between MTB and other (micro)organisms are urgently required, before actually using MTB for terrafor-mation of Mars or other planets.

Little is known about the relationships between MTB and other microorganism on Earth. Bazylinski et al. (2000) described some magnetic protists which may possible ingest MTB. If the iron from ingested magnetosomes can be used by the protistan cells as an iron source, then these protists could play a role in iron cycle by making it available to other microorganisms that consume them. These putative trophic relations could be useful for terraformation.

0 0

Post a comment