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It is commonly known that phytoplankton have a pivotal role in marine biogeochemistry and ecosystems as carbon fixers and oxygen producers, but their response to deoxygenation has scarcely been studied. Nonetheless, in the major oceanic oxygen minimum zones (OMZs), all surface phytoplankton groups, regardless of size, disappear and are replaced by unique cyanobacteria lineages below the oxycline. To develop reasonable hypotheses to explain this pattern, we conduct a review of available information on OMZ phytoplankton, and we re-analyze previously published data (flow cytometric and hydrographic) on vertical structure of phytoplankton communities in relation to light and O-2 levels. We also review the physical constraints on O-2 acquisition as well as O-2-dependent metabolisms in phototrophs. These considerations, along with estimates of the photosynthetic capacity of phytoplankton along OMZ depth profiles using published data, suggest that top-down grazing, respiratory demand, and irradiance are insufficient to fully explain the vertical structure observed in the upper, more sunlit portions of OMZs. Photorespiration and water-water cycles are O-2-dependent pathways with low O-2 affinities. Although their metabolic roles are still poorly understood, a hypothetical dependence on such pathways by the phytoplankton adapted to the oxic ocean might explain vertical patterns in OMZs and results of laboratory experiments. This can be represented in a simple model in which the requirement for photorespiration in surface phytoplankton and O-2-inhibition of OMZ lineages reproduces the observed vertical fluorescence profiles and the replacement of phytoplankton adapted to O-2 by lineages restricted to the most O-2-deficient waters. A high O-2 requirement by modern phytoplankton would suggest a positive feedback that intensifies trends in OMZ extent and ocean oxygenation or deoxygenation, both in Earth's past and in response to current climate change.

Anoxic marine zones (AMZs) constitute pelagic systems distinguished from the oxygen minimum zones (OMZs) by the complete absence of detectable oxygen and the accumulation of nitrite in mid-waters. At the top of the oxygen-depleted layer and below the oxycline, nutrients are abundant; light intensity is very much reduced (13CPOC in the top of the oxygen-depleted layer. This data showed significant differences among stations with and without the development of a SCM, being 3.0‰ heavier when a SCM is developed, and indicating photosynthetic activity and/or remineralization in the top of the AMZ. More depleted δ13CPOC values were also found when no SCM was present indicating stronger chemoautotrophic activity, potentially driven by anammox and sulfur-oxidizing bacteria activity. Assimilation rate data show that when sufficient light and Prochlorococcus are present, photosynthesis exceeds chemoautotrophic carbon fixation, and can exceed heterotrophic assimilation of glucose or acetate. However, in the majority of the stations, assimilation rates of both glucose and acetate exceeded carbon fixation rates under light stimulation, suggesting that often the SCM is still a net heterotrophic system.

The vertical distribution of phytoplankton is of fundamental importance in the structure, dynamic, and biogeochemical pathways in marine ecosystems. Nevertheless, what are the main factors determining this distribution remains as an open question. Here, we evaluated the relative influence of environmental factors that might control the coexistence and vertical distribution of pico-nanoplankton associated with the OMZ off northern Chile. Our results showed that in the upper layer Synechococcus-like cells were numerically important at all sampling stations. Pico-nano eukaryotes and phototrophic nanoflagellates (PNF) also showed high abundances in the upper layer decreasing in abundance down to the upper oxycline, while only Prochlorococcus showed high abundances under oxycline and within the oxygen-depleted layer. Statistical analyses evidenced that temperature, oxygen, and carbonate chemistry parameters (pH and dissolved inorganic carbon, DIC) influenced significantly the vertical distribution of phototrophic pico-nanoplankton. Additionally, we experimentally-evaluated the combined effect of low pH/low O