Roles of microzooplankton in the biological carbon pump

Abstract

The biological carbon pump (BCP) is a fundamental component of the Earth’s climate system, enabling the transfer of carbon from surface waters to the deep ocean, where it can be sequestered for centuries. While phytoplankton are the primary agents of carbon fixation in the ocean, the fate of that carbon—whether it is remineralized, respired, or exported—hinges critically on the activity of microzooplankton. These small, mostly unicellular heterotrophic protists are key grazers of primary producers and intermediaries in marine planktonic food webs. This review synthesizes recent findings that reveal how microzooplankton influence BCP efficiency through direct and indirect mechanisms: grazing on phytoplankton and detrital particles, respiration and excretion that regenerate nutrients and carbon dioxide in surface waters, and participation in or interaction with diel vertical migration processes. I highlight how their roles vary across environmental gradients and how size-structured trophic cascades and community shifts affect carbon export potential. Depending on the dominant microzooplankton group, grazing activity may reduce export efficiency by retaining carbon in surface waters, whereas vertical migration—although less studied among protists—may actively contribute to deep ocean carbon transport. By examining field observations, mesocosm experiments, and ecosystem models, I attempt to offer a comprehensive understanding of how microzooplankton act as both gatekeepers and facilitators of the ocean’s carbon economy. Recognizing their dualistic nature—retainers of carbon and subtle agents of sequestration—is vital for improving global biogeochemical models and predictions of carbon–climate feedbacks.