Decomposition of soil polymeric organic matter by Bacteroidetes and Clostridia:progress and perspectives

Abstract:

The decomposition of soil organic matter requires the coordinated function of microbial guilds,including the preliminary breakdown by upstream microorganisms,the further decomposition by midstream guilds and the final metabolism by downstream organisms.The initial breakdown of complex organic polymers is a key step,which requires the activity of microbial exoenzymes.Significant progress has been achieved recently in the research on the organic matter decomposition in gut.In this review,we compared the organic matter decomposition in human intestine,ruminant rumen and soil,with a focus on the specific mechanisms of polysaccharide transport in Bacteroidetes and Clostridia.Bacteroidetes secrete a cocktail of extracellular hydrolases into the outer membrane via type IX secretion system (T9SS) and require a series of gene loci termed polysaccharide utilization loci (PUL) to metabolize glycans.Polysaccharides are degraded into oligosaccharides and then transported into the periplasmic space via Sus complex located in the outer membrane.Oligosaccharides are further degraded into simple sugars like glucose and imported into the cell via inner membrane transporter.Clostridia utilize cellulosomes to directly degrade glycans in the extracellular space.The monosaccharides are captured by sugar-binding proteins and transported into the cell via ABC transporters.The intracellular sugar concentration is regulated by two-component systems (TCSs) and carbon catabolite repression (CCR) mechanism.At the end of this review,we put forward the perspectives of the future research on the decomposition of complex organic matter in soil.