Bacteriology of extremely cold soils exposed to hydrocarbon pollution

RUBERTO LAM, VAZQUEZ SC, MAC CORMACK WP

Microbiology of Extreme Soils

Springer-Verlag Berlin and Heidelberg GmbH & Co. KG

Lugar: Berlin; Año: 2008; p. 247 - 274

Extremely-cold soils are present on both hemispheres and comprise mainly the polar areas which are ice-free during a few months in the spring-summer period, exposing the underlying substrate.  In addition to the Antarctic and Arctic regions showing this characteristic, several high mountains regions (as Alps, Andes and Himalayas) and high plateaus also present extremely-cold soils. All these regions are characterized by the scarce human presence, the Arctic being the only one having permanent and stable human settlements. In Antarctica, on the other hand, the logistic and scientific stations are the unique examples of human presence. Although some of these stations have all-year activity they have not permanent crews. In this chapter, we focus our attention on the bacteriology of Antarctic soils and the changes occurring by hydrocarbon contamination caused by such human activity. In addition, the bacteriological aspects of the different bioremediation studies carried out in order to reduce hydrocarbon levels in Antarctic soils are reviewed. Finally, results from the Antarctica are continuously compared with those reported from the Arctic, which has been, at the present days, more deeply analysed.  Extremely-cold soils have a clear vertical discontinuity represented by the presence of permafrost. The permafrost table, located between 0.5 and 3 m under the soil surface represents a low permeability barrier to liquid water as well as to the vertical migration of contaminants. In this aspect, permafrost represents a relevant factor conditioning the bacterial populations able to colonize the active layer. Above the permafrost table, the active layer suffers continuous cycles of freezing and thawing that also acts as a limiting factor selecting those living organisms able to tolerate these drastic soil changes. The seasonal changes in soil temperature cause or induce dramatic changes in the number of soil microorganisms, which are related with changes in water and nutrients availability.  As the low temperature significantly impacts on the biodiversity, including plants, and since the activity of soil microorganisms is closely related to the presence of plants, reduction in microbial diversity is more connected with the low-temperature mediated decreasing in plant diversity than with the decrease in the soil temperature “per se”.     Despite the harsh environmental conditions and differing with what happens with the higher organisms, current studies using novel culture-independent molecular techniques have shown a great abundance and a relevant bacterial diversity in both, Arctic and Antarctic soils. On the other hand, using culture-dependent techniques a huge number of different bacteria inhabiting Antarctic soils have been reported. Among them, Coryneform-related microorganisms (Arthrobacter, Cellulomonas, Brevibacterium, Corynebacterium) as well as representatives of Gracilicutes, specially Bacteroidetes (Flavobacterium, Cytophaga) and Gamma proteobacteria (Pseudomonas, Stenothrophomonas, Burkhordelia, etc) were predominant. At a less extent, Gram-positive microorganisms have also been frequently found (Bacillus, Nocardia, Micrococcus, Streptomyces). These widely distributed genera were also found in Arctic soils. At the present days, there is no bacterial genus considered as exclusively polar, because using polyphasic identification approaches, members of genera formerly described as “polar” were reported from non-polar environments. Most of these non-polar sites are deep-sea environments, characterized by low temperatures and supporting the life of psychrotolerant and psychrophilic bacterial strains only. However, it is not possible to refer to these strains as exclusively polar. Less than 0.3% of the Antarctic surface is ice-free during the summer period. However, is in this small portion of the continent where the main biological activity takes place. Although Antarctic soils differ in the presence and type of plant cover, climate conditions of the area where they are located, composition and topographic position, all of them are members of one of two major groups: coastal ice-free areas and the cold desert soils (like the Dry Valleys). In spite of the bacterial diversity that characterizes the different pristine Antarctic cold-soils, a dramatic decrease in biodiversity has been reported when soils are exposed to hydrocarbon contamination. Under this condition, only a few group of bacteria adapted to use hydrocarbons as sole carbon and energy source predominate, representing a significant fraction of the total microbiota. Although basal levels of naturally occurring hydrocarbons have been reported, the main source of hydrocarbons in both, Antarctic and Arctic soils are derived from human activity. Hydrocarbon contamination is frequently associated to accidental spills during transporting and storage of fuels in Antarctica. In Arctic soils accidental spillage from pipelines represents other frequent source of hydrocarbons contamination in soils. Diesel generators, incinerators, vehicles and some scientific activities are minor (but not negligible) contributors to the bulk of hydrocarbon pollution in cold soils. These sources determine, in the affected areas, the presence of massive amounts of aliphatic as well as significant levels of aromatic hydrocarbons. Even though there are abundant reports concerning the hydrocarbon levels in Arctic soils, these environmental monitoring studies are really scarce for Antarctic areas. Total hydrocarbons values near 30000 ppm as well as PAHs concentration of 100 ppm were frequently reported for Antarctic soils near active stations (Scott Base, Marble Point, etc.). In addition, values as high as 345 ppm were detected in soil samples from old Palmer Station. The presence of high levels of aliphatic hydrocarbons seems to cause the enrichment of extremely-cold soils in members of certain bacterial genera, as Acinetobacter, Pseudomonas, Sphingomonas and Rhodococcus. These genera were also reported as dominant in several studies from Arctic and Alpine cold-soils, suggesting that although a diverse group of genus can be detected in low proportions, all hydrocarbon-contaminated cold soils tend to select a similar set of dominant psychrotolerant alkane-degrading strains. In PAHs-contaminated cold soils, Pseudomonas and related genera also seem to be the dominant group. However, Acinetobacter as well as Rhodococcus (and other strains belonging to the Actinomyccetales group) have not been frequently reported as members of the PAHs-degrading bacterial community in cold soils. In the last years, the use of the catabolic activities of microorganisms naturally present in the contaminated environments has been considered as an adequate tool to reduce contaminants concentration in soils. Under some circumstances a variety of physical, chemical and biological processes act without human intervention to reduce the presence concentration of contaminants in soil. This natural attenuation processes imply the activity of the indigenous microflora. However, in polar soils and other extremely cold environments, the harsh climate conditions turn this type of bioremediation processes not much efficient. In these areas, biostimulation of the natural microflora with the adequate addition of nutrients as well as the bioaugmentation with highly efficient hydrocarbon-degrading strains and consortia seem to be the most effective strategies. Both, biostimulation and bioaugmentation strategies involve a transient or permanent alteration of the indigenous microbiota. Even in polar soils, several works have demonstrated the efficiency of biostimulation with different sources of nutrients and applying several dosage strategies. On the contrary, bioaugmentation showed ambiguous results. Most published results reinforce the idea that bioaugmentation is not an adequate strategy to improve bioremediation of long-term contaminated soils.  As mentioned above, in these soils the indigenous microbiota is well adapted to the presence of hydrocarbons, and their activity is enough to effectively reduce the amount of contaminants when the soil is adequately prepared and C:N:P ratio is balanced.  Recent reports suggest that, under the competition with the natural microbiota, the added bacteria are not capable of survival at levels that significantly improve the biodegradation rate. The opposite situation occurs in an acute contamination event. In this case, the indigenous microbiota is not adapted to the presence of contaminants and thus, addition of “in vitro” cultivated hydrocarbon-degrading bacteria improves the efficiency and reduces the time required for the bioremediation processes. In the particular case of Antarctic continent indigenous hydrocarbon-degrading bacteria are an absolute prerequisite for the development of new bioremediation processes. This fact is not only related to the close adaptation of these microbes to the strict environmental conditions of the soils to be treated but also due to the limitations to the introduction of non-indigenous species imposed in the Antarctic Treaty and their Protocol signed in Madrid. Finally, it is expected that in the near future the novel techniques used for detection and identification of microorganisms will help us to understand the changes produced in the autochthonous microbial community during biostimulation as well as to demonstrate the survival and fate of the added microorganisms during bioaugmentation in polar soils.