Fatores ambientais moldando a associação entre os genes envolvidos nos ciclos do arsênio e ferro: uma meta-análise

Abstract

The relationships between prokaryotes performing As and Fe redox reactions contributed to delineate geochemical aspects of the primitive Earth. Currently, they have great impact on ecosystems, specially favouring As mobilization and the consequent water and soil contamination. Microorganisms and mechanisms involved in As and Fe speciation are known, nevertheless, correlations between its oxidation and reduction genes, as well its environmental distribution and diversity are not well stablished. This work presents the first meta-analysis of the diversity, relative frequencies and correlations between genes involved in As and Fe biotransformations, and the physical-chemical parameters that influence them. Twenty shotgun sequencing datasets, corresponding to As and/or Fe-rich environments, were obtained from public databases and analyzed. Genomes were assembled de novo and coding sequences (CDSs) were annotated using Uniprot, RefSeq, Pfam and TIGRFAM as reference databases. Finally, genes with well-described involvement in As and Fe speciation were sought within the annotated datasets. From 44 selected genes, 12 were found: aioA (arsenite oxidase), arsC (detoxifying arsenate reductase), arrA (dissimilatory arsenate reductase), ccoN and ccoP (cco operon – cytochrome oxidase Cbb3 complex), cyc1 and rus (rus operon), iro (Fe oxidase), petA, petB and petC (pet operon - cytochrome bc1 complex) and omcB (Fe reductase). The results showed that the environmental distribution, relative frequencies and correlations between these genes correspond to the predicted thermodynamics for the As and Fe speciation, with pH as the most relevant parameter. Genes involved in As cycling were ubiquitous, and ArsC enzyme is likely the main responsible for As(V) reduction in the environment, thus attributing to arsenite resistant microorganisms (ARMs) an important role in As mobilization. Another relevant finding is that arrA is possibly under-represented in databases. Among the genes involved in Fe cycling, cco operon was ubiquitous and the most frequent, suggesting that its hosts might have an increased fitness. iro and cco seem to be implicated in Fe(II) oxidation followed by As(V) immobilization in oxic environments. As mobilization due to Fe(III) reduction by mtr/omc cluster depend on pH and As and Fe environmental concentrations. Finally, a great number of cytochromes with poorly described roles in Fe redox reactions were found. This work provided new relevant data about As mobilization associated with Fe redox reactions, highlighting that As detoxification processes prevail over the metabolic transformations in the environment.