Diversity of bacterial laccase-like multicopper oxidase genes in forest and grassland Cambisol soil samples

Laccases- or laccase-like multicopper oxidases (LMCO) catalyze the oxidation of various substrates, such as phenols, diamines and metals, coupled with the reduction of molecular oxygen to water. Compared to studies on function and diversity of LMCO in plants and fungi, little is known about this enzyme type in bacteria and especially on their possible implication in degradation of organic matter in soils. This study presents a molecular investigation of the diversity and distribution of bacterial LMCO genes among three upper horizons of a forest Cambisol and in a grassland Cambisol. Some culture strains of soil bacteria were also analyzed at the molecular level and for their capability to oxidize naturally occurring 2,6-dimethoxyphenol, a LMCO substrate. A high LMCO gene diversity was found in the Cambisol soil samples with 16 distinct sequence type clades, of which approximately one half was not matching with any reference sequence of known bacteria. The highest richness of bacterial LMCO genes was observed in the organic horizon of the forest soil, which is concomitant with a previous analysis of the diversity of fungal laccase genes and corresponding soil laccase activity. Some clusters of sequence types showed a specific distribution in one of the soils or in horizons, while others appeared more ubiquist. Multiple bacterial LMCO genes were described in Agromyces salentinus and Sinorhizobium morelense, what so far was only known from fungi.