Deep ocean prokaryotes are healthier under highly productive surface waters

Prokaryotes, which comprise both bacteria and archaea, are one of the most important members of oceanic ecosystems, representing a large fraction of biomass among marine organisms and driving global biogeochemical cycles. While prokaryotes live at varying depths in the ocean—some deeper, some shallower—they are not isolated: the constant and gentle fall of marine snow transports particles and cells downwards, providing, among other things, organic matter essential to fuel the metabolism of those cells living deep. Indeed, deep sea prokaryotes tend to be more abundant under highly productive surface waters, because higher productivity provides more sinking organic matter. However, in order to understand how deep ocean prokaryotes respond to surface conditions it is important to address the physiological status of individual cells because not all prokaryotes are active all the time. One way to do so is determining the viability of cells, that is, whether their cell membranes are intact (indicating alive and possibly active cells) or compromised (dead or injured cells). In this study we measured how the viability of prokaryotic cells changed along a surface productivity gradient in the Atlantic Ocean. We found that, besides the well-known positive relationship between surface productivity and deep-ocean prokaryotic abundance, surface productivity was also positively correlated with cell viability down to 3500 m. This suggests that, although viability decreases with depth owing to harsher conditions in deep waters, the vertical transport of organic matter has a positive effect in deep ocean prokaryotes, allowing their survival. The results we report are especially relevant as viable, healthy cells represent the fraction of the community able to contribute to metabolic processes and, hence, to the cycling of elements.

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Gómez‐Letona, M., Arístegui, J., Hernández‐Hernández, N., Pérez‐Lorenzo, M., Álvarez‐Salgado, X. A., Teira, E., & Sebastián, M. (2022). Surface productivity gradients govern changes in the viability of deep ocean prokaryotes across the tropical and subtropical Atlantic. Limnology and Oceanography.

Text written by Markel Gómez-Letona and edited by Clara Ruiz and Félix Picazo