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, Phaeodactylum tricornutum and Chaetoceros 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.