For decades, marine phytoplankton have been regarded as organisms confined to the sunlit surface ocean, with cells detected in deeper waters generally interpreted as sinking detritus. In this presentation, I will present evidence challenging this paradigm by showing that certain phytoplankton remain metabolically active in the ocean twilight zone and contribute directly to long-term carbon sequestration. Using a diatom (Chaetoceros sp. DS1) isolated from 1000 m depth, we demonstrate that prolonged survival under darkness and hydrostatic pressure is supported by membrane lipid remodeling, enhanced antioxidant defenses, and maintenance of a latent photosynthetic apparatus that can be rapidly reactivated under dim blue light characteristic of the mesopelagic zone. This metabolic flexibility enables continued photosynthesis, nutrient acquisition, and organic carbon utilization under twilight-zone conditions. Global metatranscriptomic analyses further reveal that Chaetoceros-like populations are widespread and transcriptionally active throughout the mesopelagic ocean. 
Building upon these findings, I will show that living diatoms directly release substantial amounts of recalcitrant dissolved organic carbon (RDOC) without requiring prior microbial transformation. Between 21% and 36% of freshly released dissolved organic carbon resists microbial degradation, exhibits a distinctive sulfur-rich molecular composition, and shares molecular signatures with RDOC distributed throughout the global ocean. These observations identify a previously overlooked pathway by which metabolically active phytoplankton contribute directly to the long-term storage of carbon in the ocean interior.
Together, these findings redefine the ecological role of phytoplankton beyond the euphotic zone, establish the ocean twilight zone as a previously unrecognized habitat for certain metabolically flexible algae, and reveal an important new mechanism linking deep-living phytoplankton to persistent dissolved carbon sequestration in the global ocean.