Further evidence supporting the existence and accumulation of triacylglycerol and acylplastoquinol in cyanobacteria

Triacylglycerol (TAG) has been frequently reported in cyanobacteria; however, unlike in plants and algae, the isolation of TAG as a pure substance and its subsequent chemical characterization have proven challenging. The slr2103 gene in Synechocystis sp. PCC 6803, which encodes a putative acyltransferase involved in TAG biosynthesis, has been considered evidence for the existence of TAG. However, the identification of acylplastoquinol (APQ) as the major component of the "TAG fraction" obtained through thin-layer chromatography (TLC) has raised questions about the actual presence of TAG in cyanobacteria. To address concerns regarding potential chemical and biological contamination in the detection of TAG in cyanobacteria, we developed 1D- and 2D-TLC methods to separate submicromole quantities of TAG and APQ from Synechocystis cells. Both compounds were convincingly identified using NMR and LC/MS. TAG levels depended upon culture conditions. Well-aerated cyanobacterial cultures exhibited minimal TAG levels, while TAG accumulation reached approximately 1% of total lipids in static or slowly swirled senescing cultures, where photosynthetic activity had declined substantially. Under these conditions, we observed numerous lipid globules, approximately 72 nm in diameter, located at the periphery of the cells. These findings provide critical insights into TAG and APQ accumulation in cyanobacteria, elucidating the role of lipid globules and offering perspectives on TAG biosynthesis in cyanobacteria, as well as the potential function of APQ in photosynthesis.