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.

Description: The main component of this data set comprises calculated inorganic iron concentrations (Fe' = sum of iron hydroxide species). Inorganic iron is the most bioavailable chemical form of Fe in the ocean. Concentrations of Fe' were calculated according to two models, which we refer to as the discrete ligand model and the continuous binding site model. The discrete ligand model, which is currently applied to calculate Fe speciation in global biogeochemical models, combines dissolved Fe concentrations, conditional stability constants and ligand concentrations to obtain inorganic iron, whilst the continuous distribution model uses the NICA-Donnan model to obtain Fe'. The data supports the manuscript "Climate change decreases biologically available iron pool in the surface ocean." In this manuscript we use the continuous binding site model to show that surface ocean Fe' is sufficient for Fe-replete phytoplankton. We apply new estimates of Fe' to a simple phytoplankton growth model to show that both Fe' and relative growth rates will decrease under the high-end future climate scenario (SSP5-8.5) in all Fe-limited ocean regions, and will mitigate current projections of increased primary productivity in Fe-limited high latitudes regions such as the Southern Ocean. Overall, we demonstrate that Fe-binding site heterogeneity is critical for iron speciation, and must be considered when predicting the response of marine primary producers to ongoing changes in ocean chemistry.