Diverse, uncultivated bacteria and archaea underlie the cycling of dissolved protein in the ocean

Research Brief Integrated Microbial Biodiversity 20.06.2017

This study seeks to better understand the organisms responsible for transforming high molecular weight (HMW) dissolved organic nitrogen in the upper ocean. In particular, it explores whether microbial taxa previously implicated in HMW dissolved organic matter turnover could be specifically involved in HMW dissolved organic nitrogen (protein) cycling.

BACKGROUND

Dissolved organic nitrogen (DON) is the second most abundant form of fixed nitrogen in the ocean. The total DON pool consists of many low and high molecular weight compounds and compound classes of varying concentration and bioavailability. Microbial heterotrophy can control the flux and composition of DON, yet the specific organisms responsible for DON transformations and their biochemical mechanisms are poorly understood.

Various forms of phytoplankton cell death are a major source of DON. The liberated dissolved proteins and oligopeptides become refractory because of processes such as abiotic complexation with existing dissolved organic matter (DOM). Dissolved proteins are recycled faster relative to the bulk HMW-DON pool.

Protein in dissolved and particulate fractions can be readily degraded in seawater by microbial activity. LMWDON, such as dissolved free amino acids released by protein hydrolysis, is assimilated by bacteria in aquatic systems and can support up to around 50% of bacterial production in the oceans. Studies linking some microbial groups to LMW-DON uptake have demonstrated great seasonal and spatial variation in DON utilization. However, due to the need to select target groups beforehand, these studies have not revealed the full diversity of bacteria and archaea involved in DON cycling.

A large number of bacterial groups in the ocean, however, have been implicated in HMW-DOM turnover, and some specifically in the cycling of HMW-DON and proteins including Flavobacteria, the y-Proteobacteria subgroups Arctic96B16, Ant4D3 and SAR86, and the y-Proteobacteria clade. The uncultivated Marine Group II (MGII) Euryarchaeota are also thought to have a role in DON transformation, based on the recovery of protein degradation pathways in metagenomics data. However, direct evidence of their role in using dissolved proteins has not been demonstrated, and interactions between archaea and HWM organic matter are largely unexplored. A recent study found that some MGII were physically associated with particles, and their genome content suggested

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