Benthic foraminifera populate a diverse range of marine habitats. Their ability to use alternative electron acceptors—nitrate (NO₃ ⁻ ) or oxygen (O₂ )—makes them important mediators of benthic nitrogen cycling. Nevertheless, the metabolic scaling of the two alternative respiration pathways and the environmental determinants of foraminiferal denitrification rates are yet unknown. We measured denitrification and O₂ respiration rates for 10 benthic foraminifer species sampled in the Peruvian oxygen minimum zone (OMZ). Denitrification and O₂ respiration rates significantly scale sublinearly with the cell volume. The scaling is lower for O₂ respiration than for denitrification, indicating that NO₃⁻ metabolism during denitrification is more efficient than O 2 metabolism during aerobic respiration in foraminifera from the Peruvian OMZ. The negative correlation of the O₂ respiration rate with the surface/volume ratio is steeper than for the denitrification rate. This is likely explained by the presence of an intracellular NO₃⁻ storage in denitrifying foraminifera. Furthermore, we observe an increasing mean cell volume of the Peruvian foraminifera, under higher NO₃⁻ availability. This suggests that the cell size of denitrifying foraminifera is not limited by O₂ but rather by NO₃ ⁻ availability. Based on our findings, we develop a mathematical formulation of foraminiferal cell volume as a predictor of respiration and denitrification rates, which can further constrain foraminiferal biogeochemical cycling in biogeochemical models. Our findings show that NO₃⁻ is the preferred electron acceptor in foraminifera from the OMZ, where the foraminiferal contribution to denitrification is governed by the ratio between NO₃⁻ and O₂.