Fig. 1.

Photosynthetic DIC uptake results in an increase in pH at the cell surface. a Light-dependent changes in cell surface pH (upper) and [O₂] (lower) around the large diatom Odontella sinensis measured using microelectrodes. Upon illumination there are very rapid increases in pH and [O₂]. b The increase in cell surface pH is strongly dependent on irradiance. The mean change in cell surface [H⁺] (±s.e.m.) following illumination is shown (n = 12 cells). c Brightfield microscopy image of an O. sinensis cell illustrating spatial variability in cell surface pH in the light. The mean change in [H⁺] (±s.e.m.) following illumination was measured at different positions around the cell in seawater at pH 8.0 (n = 12 cells). The positions of the microelectrode are shown around a representative cell. Bar = 100 µm. d The zone of elevated pH extends significantly away from the cell. For each illuminated cell, pH was recorded at 10 µm increments away from the cell. The change in [H⁺] from the bulk seawater is shown (pH 8.0). n = 7 cells. e Comparison of the light-dependent increase in cell surface pH in four centric diatoms; Thalassiosira weissflogii (approximate length 20–25 µm), Odontella mobiliensis (length 40–60 µm), Coscinodiscus sp. (diameter 140–170 µm) and O. sinensis (length 150–250 µm). The mean light-dependent change in cell surface [H⁺] in seawater at pH 8.0 is shown (±s.e.m.) (n = 12)