Carbon cycling and physiological traits in phytoplankton functional groups under low pH/low oxygen conditions (CARpHOX). FONDECYT Nº1170065. 2017 – 2021.

On a global scale, there are extensive oxygen minimum zones (OMZ), areas that are expanding due to climate warming and the dynamics of greenhouse gases such as CO₂. These are low pH/high pCO₂ areas, resulting in significant CO₂ emission from the ocean into the atmosphere. Furthermore, they are characterized by a secondary chlorophyll maximum, dominated by Prochlorococcus, where aerobic respiration and photosynthesis can occur under local turbulent mixing, which can occasionally cause intrusion of low pH/low O₂ waters into the euphotic zone. Previous studies suggest that the tolerance and responses of phytoplankton to low pH/low O₂ conditions are highly variable among lineages, therefore improving our knowledge about the influence of low pH/low O₂ on phytoplankton functional groups, and their specific adaptations to these conditions will provide us with information on the impact of the expansion and reduction of O₂ levels and the increase in pCO₂. Thus, CARpHOX aims to address questions such as:

• What is the spatial/vertical heterogeneity of the carbonate system in the OMZ?
• What is the stoichiometry of nutrients, stable isotope signature, and recycling rates of inorganic carbon in low pH/low O₂ waters in the OMZ?
• Are eukaryotic phototrophs occupying different niches in the water column of the OMZ regions exposed to different conditions during carbon fixation?, and
• What are the additive/synergistic physiological effects of low pH/low O₂ conditions on functional groups of phytoplankton inhabiting the OMZ and overlying waters?

Our methodological approach includes expeditions to the OMZ in northern Chile, as well as mesocosm experiments that simulate low pH/low O₂ conditions, investigating metabolic pathways using a multi-proxy approach including stable carbon isotopes and lipid biomarkers. Our approach uses laboratory CO₂/O₂ disturbance experiments to mimic corrosive and hypoxic conditions and to assess their impact on physiological traits, lipid membrane composition, and isotopic fractionation of phototrophic eukaryotes. In this proposal we want to identify and characterize important regulatory processes by studying the physiological and metabolic responses in the context of multiple stressors of climate change, critical information for understanding the carbon cycle in a changing ocean.