Fig. 2: Photosynthetic carbon fixation and nonphotochemical quenching (NPQ) of natural phytoplankton assemblages grown under different O₂ and CO₂ treatments.

(a) Net photosynthesis per volume of seawater (μmol C L⁻¹ h⁻¹), and (b) per Chl a (μmol C (μg Chl a)⁻¹ h⁻¹) measured at day 1, 3, 5, 10, as well as (c) gross photosynthesis per volume of seawater (μmol C L⁻¹ h⁻¹), and (d) per Chl a (μmol C (μg Chl a)⁻¹ h⁻¹) measured at day 3, 5, 10. In a–d, values are presented as % of rates under ambient CO₂ and O₂ levels, and the absolute values for the rates are shown in Supplementary Table 3. (e) Nonphotochemical quenching (NPQ) measured during the noon period at day 4, 8, 10. Black symbols represent ambient O₂ (AO, ~213 μM) and red symbols low O₂ (LO, ~57 μM); Circles represents ambient CO₂ (AC, ~13 μM); triangles represent high CO₂ (HC, ~27 μM). Mesocosms were incubated under incident sunlight and natural levels of temperature (Supplementary Fig. 1), and all data were obtained under growth conditions. Detailed information for the mesocosms experimental features are given in Supplementary Figs. 1 and 2. The values are the means with error bars indicating standard deviations of independent biological replicates (n = 3 mesocosms). Light-colored symbols are individual data corresponding to the treatments. Blue * and red * indicate significant differences (P < 0.05, LSD test) due to low O₂ under ambient (LOAC) and elevated CO₂ levels (LOHC), respectively, compared to the control treatment (AOAC).