Novel Insights into Agar Metabolism in Marine Heterotrophic Bacteria

Abstract

Agar is a key structural polysaccharide of red macroalgae which provides a rich habitat for marine heterotrophic bacteria in marine ecosystems. Marine agarolytic bacteria, that can use agar as the sole carbon source, are distributed in diverse marine environments from open coastal waters to the gut of marine herbivores. Agarolytic bacteria employ a combination of carbohydrate-active enzymes (CAZymes) for the depolymerization of agar. Extensive studies on the genomic architecture of the agarolytic bacteria suggested that genes encoding these CAZymes are arranged in polysaccharide utilization loci (PUL). Agar hydrolyzing enzymes (agarases) are categorized into; β-agarase (GH16, GH50, GH86, and GH118), α-agarase (GH96), neoagarooligosaccharide hydrolase (GH117), and agarolytic β-galactosidase (GH2). The molecular functionality, structural elements, and catalytic mechanisms of agarases belonging to different GH families show unique characteristics. L-AHG, one of the main constituents in agar, is a rare monosaccharide and its metabolic pathway is exclusively found in marine agarolytic bacteria. Recent trends in the agarolytic systems are mostly focused on the sequence data to visualize the universal agarolytic enzyme repertoire and the evolution of the agarolytic pathway in marine heterotrophic bacteria. In addition, increasing attention is paid to understanding the oligosaccharide transport mechanisms and transcriptional regulation of genes in PUL. In this review, we will cover a comprehensive overview of genomic architecture, structural and functional analysis of agar hydrolyzing enzymes, and agar metabolism in marine heterotrophic bacteria.