Roots of the wetland plants Typha latifolia and Phragmites australis are inhabited by methanotrophic bacteria in biofilms

Abstract

Wetland plants create partly aerobic conditions in the rhizosphere by releasing oxygen to the waterlogged substrate. The present study was conducted to characterise the arrangement of rhizobacteria, especially those active in methane oxidation, in root-associated biofilms of wetland plants. Root cross sections sampled from Typha latifolia L. (broadleafed cattail) and Phragmites australis (Cav.) Trin. ex Steud. (common reed) were scanned using light and electron microscopy. Methane-oxidising bacteria were identified and quantified by immunological labelling of the α-subunit of the methanol dehydrogenase (α-MDH; encoded in mxaF). On roots of both species there was a diverse subset of bacteria arranged in a microbial biofilm around the roots’ exodermis. Similar bacterial densities in the root-associated biofilm were detected in more basal regions and closer to the root tip. Many microbes carried notable internal membrane systems that are characteristic of methanotrophic bacteria. This morpho-anatomical characterisation was confirmed by immunogold labelling with α-MDH antibodies. Quantification of labelled bacteria revealed that 34–43% of the biofilm bacteria were potentially capable of methane turnover. These findings confirm the presence of methane-oxidising bacteria in the root-associated biofilms of the two common macrophytes T. latifolia and P. australis. This implies that the methanotrophs participate essentially in the microbial processes related to oxygen-releasing roots of wetland plants.