Carbon is a keystone element in global biogeochemical cycles. It plays a fundamental role in biotic and abiotic processes in the ocean, which intertwine to mediate the chemistry and redox status of carbon in the ocean and the atmosphere. The interactions between abiotic and biogenic carbon (e.g., CO 2 , CaCO 3 , organic matter) in the ocean are complex, and there is a half-century-old enigma about the existence of a huge reservoir of recalcitrant dissolved organic carbon (RDOC) that equates to

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... he magnitude of the pool of atmospheric CO 2 . The concepts of the biological carbon pump (BCP) and the microbial loop (ML) shaped our Downloaded from https://academic.oup.com/nsr/advance-article-abstract/doi/10.1093/nsr/nwy074/5053813 by University of East Anglia user on 31 July 2018 understanding of the marine carbon cycle. The more recent concept of the microbial carbon pump (MCP), which is closely connected to those of the BCP and the ML, explicitly considers the significance of the ocean's RDOC reservoir and provides a mechanistic framework for the exploration of its formation and persistence. Understanding of the MCP has benefited from advanced "omics", and novel research in biological oceanography and microbial biogeochemistry. The need to predict the ocean's response to climate change makes an integrative understanding of the BCP, ML and MCP a high priority. In this review, we summarize and discuss progress since the proposal of the MCP in 2010 and formulate research questions for the future. Keywords: biological carbon pump, microbial loop, microbial carbon pump, ocean carbon cycle, global climate change IDENTIFICATION AND QUANTIFICATION OF THE COMPOSITION OF RDOM According to Hansell et al. (2009) less than 1% of the DOC in the ocean is labile and 94% is refractory, while the remaining 5% is classified as semi-labile (note: Hansell, 2013 divided the DOC into labile, semi-labile, semi-refractory, refractory and ultra-refractory). Much of the RDOC production in the ocean can be attributed to microbial activities (e.g., Ogawa et al., 2001) . Kaiser and Benner (2008) estimated that 25% of the total organic carbon (including both POC and DOC) was of bacterial origin. Based on the estimates of Hansell et al. (2009) and Kaiser and Benner (2008) , Benner and Herndl (2011) calculated that about 10 Pg of semi-labile DOC and 155 Pg of refractory DOC are of bacterial origin. Hansell (2013)