Traditional oceanographic theories suggest that waters below 200 m, including the oceanic twilight and aphotic zones, only contain sinking phytoplankton detritus and cannot support the survival of photosynthetically active phytoplankton. Based on deep-sea surveys, this study confirms that intact, viable phytoplankton widely inhabit deep waters from 200 m to 4000 m, as well as the hadal zone. Multiple deep-sea phytoplankton groups, mainly diatoms and green algae, were isolated, challenging the conventional view that no active photosynthetic plankton exist in the deep ocean.
We systematically investigated the adaptive mechanisms of the deep-sea diatom Chaetoceros sp. DS1, which was isolated from 1000 m depth. Under extreme deep-sea conditions, the strain maintains cell membrane fluidity by upregulating unsaturated fatty acid synthesis and enhances antioxidant capacity via elevated activities of glutathione S-transferase and peroxiredoxin. It can survive for more than three months in continuous darkness and low temperature, showing stronger environmental tolerance than surface phytoplankton. Notably, core genes for photosynthesis and pigment synthesis remain highly expressed under dark and high-pressure conditions, forming a latent photosynthetic system that can be rapidly activated by faint light.
Laboratory experiments under in-situ high pressure and weak blue light (~0.2 μmol photons m^-2 s^-1) verified that the strain initiates photosynthetic metabolism and nutrient uptake through cryptochrome and rhodopsin blue-light receptors. This diatom adopts a facultative mixotrophic strategy to adapt to variable deep-sea light environments. It sustains cell survival by consuming intracellular carbon reserves during long-term darkness. Under weak blue light, it upregulates organic transporter genes to activate photoheterotrophy and utilize ambient dissolved organic carbon. Public metagenomic and metatranscriptomic data from the Tara Oceans expedition support our experimental results. Chaetoceros is abundant among eukaryotic picophytoplankton in the twilight zone, with higher in-situ populations than surface waters at multiple sampling sites. The active expression of blue-light receptor and protein synthesis genes in twilight-zone Chaetoceros demonstrates that these phytoplankton are indigenous active communities, rather than passively sinking surface-derived detritus.
In summary, this study identifies the deep ocean (below 200 m) as an unrecognized cryptic niche for phytoplankton. These findings update the understanding of deep-sea plankton distribution, provide new insights into deep-sea biological carbon pump processes, and offer theoretical and experimental support for the exploration and utilization of deep-sea algal resources.
 

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