The Legend Continues: The Critical Evidence Showing the Bivalve Farming Is a Carbon Sink With a Novel Budget Framework

The inclusion of marine shellfish farming, particularly bivalve cultivation, within the budgetary framework of the blue carbon strategy remains a topic of considerable debate. In a recent review, Fabrice et al. concluded that bivalve farming does not function as a CO2 sink [1]. Established understanding illustrated that bivalve farming contributes minimally to carbon sink [2, 3], while some carbon budget models have alternatively classified bivalve farming as a carbon sink [4, 5]. The traditional carbon sink budget has predominantly attributed to the calcification processes (formation of hard shells). These budget studies are built upon the seawater carbonate chemistry, including dissolved inorganic carbon (DIC), total alkalinity (TA), and pCO2 [1]. Considering the coupled contributions to carbon dynamics, we support the ecosystem-wide evaluation of the carbon budget within the bivalve farming habitat. Our observations of air-sea CO2 flux provide definitive evidence that mussel farming can be characterized as a weak carbon sink, although its effectiveness is constrained by seasonal variations [6]. Based on the field observations, experimental studies and model simulations, a total of 28 studies [1] have been published to support that bivalve farming is a CO2 sink since the report of Tang et al. in 2011 [7]. Feng et al. reported that the carbon sequestration efficiency and intensity of cultivated shellfish are much higher than those of artificial forests in China [8]. Previous studies have elucidated various interactions between shellfish and algae [9, 10], such as impacting on the planktonic structure and nutrient availability [11]. We have advanced an alternative process of carbon sink via an ecologically integrated "3 M" (microalgae–mussel–microbiota) consortium [6]. According to the "3 M" framework (Figure 1), we emphasize the positive contributions of mussels in carbon dynamics, particularly through the continuous consumption of microalgal cells and active recruitment of functional microbes. Consequently, phytoplankton absorb more CO2 from the air and maintain the oceanic primary productivity. Filter-feeding mussels function analogously to a pump, accelerating the turnover of microalgae and facilitating the downward deposition of algae-derived carbon through their feeding activities. Functional microbes convert bioavailable carbon into more stable forms (e.g., recalcitrant carbon, RC), thereby expanding the contributions of mariculture carbon sink. Given the global distribution of bivalves, the challenges of the "3 M" consortium deserve further consideration: quantifying the carbon capture and burial capacity, expanding on the seasonal variability of carbon flux, and clarifying the mechanistic (chemical and biological) pathways of sedimentary RC accumulation in the mussel farming zone. We believe that these studies would shed new light on carbon sinks in mussel farming. Jianyu He: conceptualization, writing – original draft, writing – review and editing, project administration, visualization, funding acquisition. Zhuoyi Zhu: writing – review and editing. Xiaojun Yan: conceptualization, funding acquisition, writing – review and editing. This study was supported by Key R&D projects in Zhejiang Province (Grant No. 2023C03120); National Natural Science Foundation of China (Grant No. 32200083, 42020104009); Zhejiang Provincial Natural Science Foundation of China (Grant No. LTGS23C010001); Science Foundation of Donghai Laboratory (Grant No. DH-2022KF0219). The authors declare no conflicts of interest. Data sharing is not applicable to this article as no new data were created or analyzed in this study.