Intracellular pH Regulation in the Giant-Celled Marine AlgaChaetomorpha darwinii

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Abstract
The vacuolar pH (pHv) and the cytoplasmic pH (pHc) of the marine giant-celled green alga Chaetomorpha darwinii were measured by pH microelectrode techniques on extracted vacuolar sap, and by the [I4C]DMO distribution method respectively. Equilibration of DMO occurred with a half-time of about 2 h, with an apparent PDMO of 3.6 × 10−5 cm s−1, but the vacuolar concentration of free, undissociated DMO was always less than the external concentration. The explanation offered for freshwater giant-celled algae of net DMO− leakage across the plasmalemma cannot apply to Chaetomorpha darwinii, since electrically-driven DMO− exit from the cytoplasm should be similar across the plasmalemma and the tonoplast in these cells with large, vacuole-positive potential differences across the tonoplast. pHc was accordingly computed assuming either tonoplast or plasmalemma equilibration of DMO, with correction for DMO metabolism within the cell. pHc was 8.0–8.3 in the light in artificial seawater (pHo about 8.0), was some 0.5 units lower in the dark, and was slightly lower with an external pH of 7. Vacuolar pH was 6.5–6.9, without consistent effects of illumination or of external pH of 7 rather than 8. While δμH+ at the tonoplast was similar to that in giant-celled freshwater algae (although with a greater contribution from δψ relative to ΔpH), δμH+ at the plasmalemma was less than 8 kJ mol−1, i.e. less than one-third of the value in freshwater green algae. ΔμNa+ was some 13 kJ mol−1 at the plasmalemma. The possibility that the primary active transport process at the plasmalemma of Chaetomorpha darwinii (and certain other marine algae) is Na+ efflux rather than H+ efflux is discussed.