A low oxygen threshold for physiological responses and trace metal quotas of <i>Prochlorococcus</i> and <i>Synechococcus</i>

Abstract Continued human activity is expected to accelerate ocean deoxygenation, leading to the expansion and shallowing of oxygen‐deficient zones (ODZs). This decline in oxygen may impact both phytoplankton growth and trace metal uptake. We conducted culture experiments with Prochlorococcus MIT9313 and Synechococcus XM‐24, two numerically dominant genera of marine picocyanobacteria, under varying O 2 levels (36–242 μ M). These experiments provide some of the first data on trace metal quotas in phytoplankton under varying O 2 conditions in culture. Moderate deoxygenation, above hypoxic levels, did not negatively impact the growth and trace metal quotas of either Prochlorococcus MIT9313 or Synechococcus XM‐24 (80 and 85 μ M O 2 , respectively). Indeed, some parameters (e.g., growth rate, carbon fixation and chlorophyll fluorescence) suggest slight growth enhancement at these moderate levels of deoxygenation. However, hypoxic conditions (36 μ M O 2 ) significantly inhibited the growth of Prochlorococcus MIT9313, suggesting a minimum O 2 threshold needed to prevent disruptions in O 2 ‐dependent processes. Synechococcus XM‐24 exhibited no significant response to elevated p CO₂ (1054 μ atm), indicating possible adaptation to variable coastal CO₂ environments. Changes in trace‐metal quotas (metal‐to‐phosphorous ratios; metal/P) were also observed when Prochlorococcus MIT9313 was cultured under hypoxia, including up to 48% increases in Fe/P, 48% decreases in Ni/P, and 27% decreases in Co/P, compared to oxic conditions. These results contrast with elevated metal/P ratios in autotrophic‐associated particles from ODZs, suggesting either distinct metal uptake strategies by phytoplankton in low‐O 2 regions or that elevated particulate metals are influenced by other plankton types such as heterotrophs.