Massive presence of intact microalgal cells in the deep ocean near 5°N of the eastern Indian Ocean

As an extensive and less explored ecosystem on earth, the deep-sea system provides various topographical environments for organisms therein. Here we quantitatively investigated the microphytoplankton and calcified nanophytoplankton community structure in the epipelagic water (0-200 m), middle water (300-500 m), and bathypelagic dark water (1000-3000 m) of the eastern Indian Ocean (EIO) approximately 5 degrees N, which was characterized by high-level eddy kinetic energy. We diagnosed five phytoplankton functional groups (diatoms, dinoflagellates, coccolithophores, diazotrophs, and chrysophytes) in the epipelagic and middle water, and four groups (with the absence of dinoflagellates) in the deep-sea water. A relatively considerable cell abundance and carbon biomass of diatoms (5.2% and 7.3%), in particular centric diatoms, cyanobacteria (5.8% and 5.5%) and living coccolithophores (2.4% and 2%) were recorded in the deep-sea (> 1000 m) water compared to the epipelagic layer (< 200 m). Abundant diazotrophs were observed in the near-surface water of the study area. We highlighted that, the underlying "carbon-nitrogen coupling" organic matter pathway, fueled by nitrogen fixation (mainly > 10 mu m diazotrophs), plays important roles in transporting the high-dominance of diatoms (especially their centric forms) and living calcifying coccolithophores to the deep ocean. Our results imply that photic nutrient deficiency and mineral ballasting (opal and calcite ballasting) are responsible for the massive and fast-sinking aggregates (and fecal pellets) consisting of large and rapidly growing centric diatoms and living coccolithophores. This biochemical factor coupled with physical forcing such as cyclonic eddies, could be a significant process promoting deep-sea biological carbon pump efficiency in the EIO. Our findings may have implications for a better understanding of biological carbon pump mechanisms and biological-physical relevance under the current tropical sites.