Recalcitrant dissolved organic matter and its major production and removal processes in the ocean

Dissolved organic matter (DOM) in the ocean accounts for nearly 662 Pg of carbon, most of which is formed into a long-lived carbon pool that plays an important role in carbon sequestration. This carbon resists rapid biological decomposition and is widely known as recalcitrant DOM (RDOM). The recalcitrant nature of deep-sea DOM has been addressed for more than half of a century, studies examining the chemical composition, production, distribution, and removal of RDOM are progressively increasing in recent decades and could add to our understanding of its cycling; however, they need to be better synthesized. A concept proposed over a decade ago, the microbial carbon pump (MCP), attributes the main mechanism of RDOM formation to successive microbial processing of organic matter. This ultimately leads to inaccessibility due to the intrinsic recalcitrant nature of DOM under certain environmental conditions, or to the organic concentration falling below the threshold for microbial reutilization, further explaining the recalcitrant nature of RDOM. This review focuses on those DOM molecules with intrinsic recalcitrant nature and collates studies on the following three aspects: (i) proxies of intrinsic RDOM, (ii) ecological processes that affect RDOM production within the conceptual framework of the MCP, and (iii) distribution and removal processes of RDOM in the ocean. Throughout this extensive review, we examine the vital role of microbes in the production and maintenance of RDOM in the ocean and conclude that the MCP is essentially associated with the sequestration of RDOM in the ocean.