Streptomycetes Are a Prominent Population in Heavy Metal Contaminated Soils

Streptomycetes are prominent in all soils. This is even evident from the smell of fresh soil - the volatile substances associated with fresh earth are geosmins, which are produced by streptomycetes. Streptomycetes have a very active secondary metabolism and can produce a wide variety of chemicals. Among these chemicals are antibiotics; up to 80% of the antibiotics used by man originated from streptomycete metabolites (Haferburg et al. 2009; Hopwood 2006). These antibiotics are thought to provide the population that produces them with a competitive edge compared to other taxa in the soil.

In normal soils, streptomycetes - or more generally, actinobacteria - comprise about 20% of the bacterial population. Filamentous growth, the formation of hyphae, can be interpreted as an adaptation to living in soil, where nutrients may be unequally distributed spatially. The formation of a mycelium in which nutrients can be transported from one area to another enables growth under conditions where some of the nutrients needed by single-celled bacteria may be available in their immediate surroundings while others are not, and can also lead to a more constant water supply in an environment prone to drying out and rewetting. The same mechanism is utilized by fungi, which also form a high proportion of the soil biomass.

An additional adaptive advantage under dry conditions is the production of spores. This gives Gram-positives with endospore-forming bacilli and clostridia, as well as actinomycetes, a competitive edge when growing in the soil. Actinomycetes do not form endospores, but are able to differentiate spores from the aerial mycelium, either in spore chains (e.g., streptomycetes) or in sporangia (e.g., actinoplanetes). These spores are not as resistant to chemicals or heating as the endospores of Gram-positive bacteria with low genomic G+C contents (bacilli and clostridia), but they are nevertheless able to withstand heat and lack of moisture, which are conditions that can be found in natural soils. Upon the return of moist and temperate conditions, the spores germinate, forming new substrate mycelium for feeding and, subsequently, aerial mycelium for dispersing spores and spreading them to new environments.

These specific features of streptomycetes - the highly active secondary metabolism as well as the mycelial growth and spore production - appear to be mechanisms that also provide advantages under other adverse conditions, including heavy metal contamination of terrestrial environments. In a former uranium mining site near Ronneburg in Thuringia, Germany, the population of soil microbes was analyzed by cultivation-dependent and cultivation-independent DNA-based methods (Haferburg et al. 2007; Schmidt et al. 2005). It was found that, in contrast to uncontaminated soils from temperate regions, the population was highly enriched in Gram-positive bacteria, with bacilli and streptomycetes clearly dominating over Gram-negative proteobacteria, which often form large parts of the population in normal soil (Fig. 10.1). Thus, the advantages described above can be assumed to aid growth in poor soils contaminated with metals, as observed in the former mining district at Ronneburg.

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Body Detox Made Easy

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