Mono- and digalactosyldiacylglycerol composition of glaucocystophytes (Glaucophyta): A modern interpretation using positive-ion electrospray ionization/mass spectrometry/mass spectrometry

Abstract

SUMMARY Glaucocystophytes are freshwater algae that possess an almost-intact cyanobacterium, referred to as a cyanelle, as their photosynthetic organelle. Because the cyanelle represents an intermediate state in plastid evolution, glaucocystophytes have been the subject of several studies to characterize the genetics and biochemistry of their cyanelles. However, only a small handful of older studies exist on the composition of their lipids, particularly two major plastid lipids, mono- and digalactosyldiacylglycerol (MGDG and DGDG, respectively), found in all photosynthetic life. Our study has used a modern mass spectrometry approach, namely positive-ion electrospray ionization/mass spectrometry/mass spectrometry, to provide a fresh interpretation of the MGDG and DGDG composition of the species, Cyanophora paradoxa Korshikov and Glaucocystis nostochinearum Itzigsohn, representing two glaucocystophyte genera. We have found that the major forms of MGDG and DGDG (with sn-1/sn-2 regiochemistry) are 20:5/16:0 MGDG, 20:5/20:5 MGDG, 20:5/16:0 DGDG, and 20:5/20:5 DGDG. A comparison of these four forms, along with other more minor forms of MGDG and DGDG, to two examples of cyanobacteria has revealed that glaucocystophytes do not share intact forms of MGDG and DGDG with extant cyanobacteria, but may have maintained certain C16 and C18 cyanobacterial fatty acids.