Phytoplankton Directly Release Recalcitrant Dissolved Organic Carbon as a Hidden Pathway in Ocean Carbon Sequestration

Recalcitrant dissolved organic carbon (RDOC) constitutes a major long-term carbon reservoir in the ocean and is widely thought to form through microbial reprocessing of organic matter via the microbial carbon pump. Whether phytoplankton can directly produce RDOC has remained untested. Here, we show that two ecologically contrasting diatoms, <i>Phaeodactylum tricornutum</i> and <i>Chaetoceros</i> sp. DS1, intrinsically release RDOC that persists against microbial degradation. Between 21 and 36% of direct diatom-released DOC is biorefractory (operationally defined as DOC resistant to microbial degradation over 90 days), molecularly enriched in sulfur-containing compounds, and distinct from microbially reprocessed RDOC. Comparative analyses against a global ocean DOC molecular data set reveal that diatom-released RDOC signatures are ubiquitous throughout the water column, from surface to abyssal depths, accounting for a substantial fraction of oceanic DOC. We roughly estimate that direct diatom RDOC production can contribute 0.2-2.2 Pg C yr^-1 in global oceans, representing an important and previously underappreciated contribution to oceanic carbon storage. These findings confirm phytoplankton as direct and globally significant contributors to recalcitrant carbon storage, expanding the framework of ocean carbon sequestration.