Intertidal mussel-symbiont associations act as CO₂ sinks during daily emersion

Human activities have disrupted the global carbon cycle, reducing carbon dioxide (CO_₂) uptake by tidal wetlands and submerged vegetation. This exacerbates climate challenges, including rising temperatures and ocean acidification. Coastal systems such as mangroves and seagrasses serve as key carbon sinks, promising for CO_₂ removal (CDR). Growing attention is being given to bivalves, whose calcification and reef-building activities shape coastal carbon dynamics. Most studies reduce bivalve impacts to a balance between individual CO_₂ emissions and the carbon stored in their shells and tissues, often overlooking species interactions-such as symbioses-that may modulate carbon fluxes. Here, we examined the mussel-symbiont holobiont using Mytilus edulis under emersion in a controlled chamber to quantify CO_₂ exchange. Mussels hosting cyanobacterial symbionts exhibited net atmospheric CO_₂ uptake during daily air exposure, a critical phase of the tidal cycle. To evaluate the potential significance at larger ecological scales, we combined laboratory-derived CO_₂ uptake data with field-based estimates of symbiont prevalence to model carbon fluxes at the mussel bed scale and compared them with values of established blue carbon systems. This research highlights the importance of species interactions in coastal carbon cycling and underscores the need to incorporate the mussel-symbiont holobiont into CDR models.