Marine methylotrophs revealed by stable-isotope probing, multiple displacement amplification and metagenomics

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The concentrations of one-carbon substrates that fuel methylotrophic microbial communities in the ocean are limited and the specialized guilds of bacteria that use these molecules may exist at low relative abundance. As a result, these organisms are difficult to identify and are often missed with existing cultivation and gene retrieval methods. Here, we demonstrate a novel proof of concept: using environmentally-relevant substrate concentrations in stable-isotope probing (SIP) incubations to yield sufficient DNA for large-insert metagenomic analysis through multiple displacement amplification (MDA). A marine surface-water sample was labelled sufficiently by incubation with near in situ concentrations of methanol. Picogram quantities of labelled 13C-DNA were purified from caesium chloride gradients, amplified with MDA to produce microgram amounts of high-molecular-weight DNA (≤ 40 kb) and cloned to produce a fosmid library of > 10 000 clones. Denaturing gradient gel electrophoresis (DGGE) demonstrated minimal bias associated with the MDA step and implicated Methylophaga-like phylotypes with the marine metabolism of methanol. Polymerase chain reaction screening of 1500 clones revealed a methanol dehydrogenase (MDH) containing insert and shotgun sequencing of this insert resulted in the assembly of a 9-kb fragment of DNA encoding a cluster of enzymes involved in MDH biosynthesis, regulation and assembly. This novel combination of methodology enables future structure–function studies of microbial communities to achieve the long-desired goal of identifying active microbial populations using in situ conditions and performing a directed metagenomic analysis for these ecologically relevant microorganisms.

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