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 base comprises elemental concentrations associated with dissolved organic matter obtained from samples collected on Meteor cruise M147 (May 2018). The data supports the manuscript Trace metal stoichiometry of dissolved organic matter in the Amazon Estuary. Dissolved organic matter (DOM) comprises a distinct component of the Earth's hydrosphere and provides a link between the biogeochemical cycles of carbon, nutrients, and trace metals (TMs). Binding of TMs to DOM is thought to result in a TM pool with DOM-like biogeochemistry. In this manuscript, we determined elemental stoichiometries of aluminium, iron, copper, nickel, zinc, cobalt and manganese associated with a fraction of the DOM pool isolated by solid phase extraction at ambient pH (DOMSPE-amb) from the Amazon estuary. We found the rank order of TM stoichiometry within the DOMSPE-amb fraction was underpinned by the chemical periodicity of the TM. Furthermore, the biogeochemistry of the TMSPE-amb pool was related to the chemical hardness of the TM ion. Thus, the biogeochemistry of TMs bound to the DOMSPE-amb component in the Amazon estuary was determined by the chemical nature of the TM and not by that of the DOMSPE-amb.