Yeasts from acidic aquatic environments: towards an ecological understanding

RUSSO GABRIEL; LIBKIND DIEGO; CHAVES SANDRA; GADANHO MÁRIO; BASTOS ELISABETE; TENREIRO ROGERIO; VAN BROOCK MARÍA R.

Lisboa

Conferencia; III International Conference on Environmental, Industrial and Applied Micorbiology; 2010

The study of microbial diversity in extreme environments, such as acid and/or heavy metal polluted water, is important from both scientific and biotechnological points of view. The comparison of microbial communities of such environments from different geographic areas is of interest for the detection of highly adapted species, and also important for physiological and genetic studies aiming the elucidation of mechanisms that allow the colonization of such habitats. These studies may also reveal new biocatalysts, such as extremozymes, or inspire novel bioremediation strategies. We compared novel yeasts of the genus Cryptococcus inhabiting acidic aquatic environments of anthropogenic (São Domingos Mines, Portugal - MSD) and volcanic origin (Rio Agrio-Caviahue lake, Argentina - RAC). Strains were studied by conventional and molecular techniques (PCR fingerprinting, AFLP, rRNA gene sequencing), and their tolerance to heavy metals was assessed (Cd2+, Co2+, Cu2+, Li+, Ni2+ and Zn2+). On the one hand, strains of an undescribed Cryptococcus species common of both acidic environments, isolated from the most extreme conditions, were found to be phenotypically similar and with identical D1/D2 and ITS sequences. The species showed unique acidophilic characteristics and was considered the most adapted species inhabitant in both studied sites. However, when strains of MSD where exposed to RAC water, the viability of the formers was considerably lower, revealing different survival capabilities and hence physicochemical adaptations. In addition, tolerance to heavy metals was not identical between MSD and RAC strains. However, genetic differentiation of the Argentinean and Portuguese populations was only achieved when studying highly differentiating genomic fingerprinting methods as PCR-fingerprinting and Amplified Fragment Length Polymorphisms (AFLP). Considering the large geographic distances and the different histories of both acidic environments, the genetic differences between the two Cryptococcus spp. populations were low. Differences were found only after applying MSP-PCR, RAPD and AFLP techniques, known to be good genomic tools for intraspecific differentiation. The peculiar acidophilic characteristics suggest a high dependence for acidic environments and probably lead to a yet unknown specific ecological role of this species. Due to the high selection pressure imposed by the physicochemical conditions of the acidic environments studied, it is possible that this yeast species presents a much conserved genome, but further studies should be carried out to confirm this hypothesis. On the other hand, four related Cryptococcus species were compared phylogenetically because they showed interesting physiological characteristics suggesting that they were also adapted to acidic habitats. Three of them were isolated from Portugal, described as C. aciditolerans, C. ibericus and C. metallitolerans, and one from Argentina, named C. agrionensis. The latter species was the most divergent one. These species formed a well-separated phylogenetic cluster within the Filobasidiales of the Agaricomycotina (Basidiomycota). Considering that these new species also shared a peculiar ecology, being able to thrive in extreme environmental conditions characterized by very low pH and high concentrations of heavy metals, this combination of phylogenetic and ecological characteristics was designated as an ecoclade. The members of the ecoclade probably share a common ancestor that beared less extreme conditions than the acidophilic species, but was also autochthonous of acidic environments.