Work: via the application of a newly developed pH-sensing nano-probe, which allowing high spatial and temporal resolution quantification (down to 50 nm spatial resolution and 2 ms response time), we investigated whether the phycosphere pH of small cells differs from bulk seawater. By determining the phycosphere pH of model marine diatoms, green algae and coccolithophores under different environmental conditions, via changing light, seawater pH and buffering capacity, we studied the underlying mechanisms of phycosphere pH regulation and its responses to ambient environmental changes. Moreover, using newly derived proton and Fe binding constants for marine dissolved organic matter, we investigated the influences of the phycosphere on Fe speciation and its availability to phytoplankton. Finally, we investigated the influences of phycosphere pH, reacitive oxygen species and associated microbes on Fe availability to in situ plankton assemblages.
Main results: under 140 μmol photons·m-2·s-1 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 better predicted by the phycosphere, rather than bulk seawater. The field data indicate the phycosphere might have little effect on the Fe bio-uptake.
Main dissemination: One research paper has been published in an international leading journal ISME J (i.e. nature.com/articles/s41396-022-01280-1) and the new findings were presented at several international conferences including The Association for the Sciences of Limnology and Oceanography, the Royal Society meeting as well as world-leading universities including Imperial College London.
Data availability and for further exploitation:
The datasets from this project are available in the Figshare repository via the link
https://doi.org/10.6084/m9.figshare.19576477.v1(s’ouvre dans une nouvelle fenêtre).
