Influence of pH, Mg²⁺, and lipid composition on the aggregation state of the diatom FCP in comparison to the LHCII of vascular plants.

In the present study, the influence of Mg²⁺ ions and low pH values on the aggregation state of the diatom FCP and the LHCII of vascular plants was studied. In addition, the concentration of thylakoid membrane lipids associated with the complexes was determined. The results demonstrate that the FCP, which contained a significantly higher concentration of the negatively charged lipids SQDG and PG, was less sensitive to Mg²⁺ and low pH values than the LHCII which was characterized by lower amounts of SQDG and a higher concentration of MGDG. High MgCl₂ concentrations and pH values below pH 6 induced significant changes of the absorption and 77K fluorescence emission spectra of the LHCII, indicating a strong aggregation of the light-harvesting complex. This aggregation was also visible as a pellet after centrifugation on a sucrose cushion. Although the FCP responded with changes of the absorption and fluorescence spectra to low pH and Mg²⁺ incubation, these spectral changes were less pronounced than those observed for the LHCII. In addition, the FCP complexes did not show a visible pellet after incubation with either low pH values or high Mg²⁺ concentrations. Only the combined action of Mg²⁺ and pH 5 led to FCP aggregates of a size that could be pelleted by centrifugation. The decreased sensitivity of FCP aggregation to Mg²⁺ and low pH is discussed with respect to the differences in the concentration of the lipids surrounding the FCP and LHCII and the different thylakoid membrane organizations of diatoms and vascular plants.