Relaxation of cellular K gradients by valinomycin induces diatoxanthin accumulation in Cyclotella meneghiniana cells and alters FCPa fluorescence yield in vitro.

Regulation of photosynthetic light harvesting involves all major thylakoid membrane complexes. One important factor is the proton motive force (pmf) driving ATP production. Its proton gradient (ΔpH) component regulates the high energy quenching. Potassium ions largely contribute to the formation of the electric field (ΔΨ). ΔΨ and ΔpH partially compensate each other to form pmf. Whilst in plants considerable progress has been made in analyzing the interplay of H and K gradients, in diatoms knowledge in this field is still scarce. We relaxed cellular K gradients by valinomycin in Cyclotella meneghiniana. We observed a slow decrease of PSII maximum quantum yield in the dark upon valinomycin addition correlating with diatoxanthin accumulation which we attribute to the breakdown of organellar K gradients (either plastid or mitochondria) which might compensate for the loss of the K gradient by adjustment of the thylakoid pH in a secondary step. This response is reversible when ΔpH is relaxed. Similarly, we found higher non-photochemical quenching (NPQ) caused by higher DT accumulation in the steady state in valinomycin-treated cells. In vitro fucoxanthin chlorophyll a (FCPa) antenna complexes in liposomes with natural lipid composition showed a decrease in fluorescence yield if a K gradient is built up. The effect reversed by relaxing the gradient. We interpret these fluorescence changes with surface charge dynamics and FCPa organization in the membrane rather than a direct influence of K gradients on FCPa complexes. Both experiments reveal that K gradients might contribute to fine tuning of light harvesting capacity in relation to pmf in diatoms.