Temperature-dependent modulation of the methylation degree of (tetra)ester-linked membrane-spanning lipids in an Acidobacterium

Ester-linked membrane-spanning lipids, such as iso-diabolic acid (iso-DA), are known components of some bacterial cell membranes, e.g., several species of Acidobacteria. However, the underlying biosynthetic mechanism and the physiological function of iso-DA remain elusive. In this study, we identified ester-linked membrane-spanning lipids, iso-diabolic-based tetraesters (iso-DTs) and their derivatives including C-6 methylated homologues, synthesized by the Acidobacterium Candidatus Koribacter versatilis Ellin345. We demonstrate that in this bacterium iso-DA is derived as a hydrolyzed or degradation product of iso-DT rather than being a building block for membrane-spanning lipids, as previously proposed. Moreover, we observed a positive correlation between the growth temperature of Ca. Koribacter versatilis and the C-6 Methylation index of iso-Diabolic Tetraesters (MDT6ME)/Acids (MDA6ME) in our cultures (grown between 10 and 25 °C). These correlations were like those observed between temperature and the degree of methylation of branched glycerol dialkyl glycerol tetraethers (e.g., MBT′5ME) observed in cultures of another Acidobacterium as well as environmental samples. These results suggest a similar temperature-dependent physiological characteristic between ester- and ether-linked membrane-spanning lipids. Overall, this study provides insight into the physiological modulation for the methylation of different types of bacterial membrane-spanning lipids and further suggests its potential as a temperature indicator in environmental investigations.