Description: The file contains dissolved and total dissolvable trace metal concentrations (Fe, Co, Mn, Ni, Cd, Cu, Pb, Zn and V), hydrogen peroxide (H2O2), Fe(II), and iodide and iodate concentrations of surface water samples and station depth profiles. Trace metal concentrations were measured by ICP-MS after preconcentration (Rapp et al. 2017, Anal. Chim. Acta). Fe(II) and H2O2 were analyzed on-board using chemiluminescence flow injection analysis (Hopwood et al. 2017, Sci. Rep.). Iodide concentrations were analyzed by cathodic stripping square wave voltammetry (Luther et al. 1988, Anal. Chem.) and Iodate concentrations were measured spectrophotometrically (Chapman and Liss 1977, Mar. Chem.).

Description: Surface ocean pH is declining due to anthropogenic atmospheric CO2 uptake with a global decline of ~0.3 possible by 2100. Extracellular pH influences a range of biological processes, including nutrient uptake, calcification and silicification. However, there are poor constraints on how pH levels in the extracellular microenvironment surrounding phytoplankton cells (the phycosphere) differ from bulk seawater. This adds uncertainty to biological impacts of environmental change. Furthermore, previous modelling work suggests that phycosphere pH of small cells is close to bulk seawater, and this has not been experimentally verified. Here we observe under 140 μmol photons/m**2/s the phycosphere pH of Chlamydomonas concordia (5 µm diameter), Emiliania huxleyi (5 µm), Coscinodiscus radiatus (50 µm) and C. wailesii (100 µm) are 0.11 ± 0.07, 0.20 ± 0.09, 0.41 ± 0.04 and 0.15 ± 0.20 (mean ± SD) higher than bulk seawater (pH 8.00), respectively. Thickness of the pH boundary layer of C. wailesii increases from 18 ± 4 to 122 ± 17 µm when bulk seawater pH decreases from 8.00 to 7.78. Phycosphere pH is regulated by photosynthesis and extracellular enzymatic transformation of bicarbonate, as well as being influenced by light intensity and seawater pH and buffering capacity. The pH change alters Fe speciation in the phycosphere, and hence Fe availability to phytoplankton is likely better predicted by the phycosphere, rather than bulk seawater. Overall, the precise quantification of chemical conditions in the phycosphere is crucial for assessing the sensitivity of marine phytoplankton to ongoing ocean acidification and Fe limitation in surface oceans.