Abstract
Alkenones are unusual long-chain neutral lipids that were first identified in oceanic sediments. Currently they are regarded as reliable palaeothermometers, since their unsaturation status changes depending on temperature. These molecules are synthesised by specific haptophyte algae and are stored in the lipid body as the main energy storage molecules. However, the molecular mechanisms that regulate the alkenone biosynthetic pathway, especially the low temperature-dependent desaturation reaction, have not been elucidated. Here, using an alkenone-producing haptophyte alga, Tisochrysis lutea, we show that the alkenone desaturation reaction is catalysed by a newly identified desaturase. We first isolated two candidate desaturase genes and found that one of these genes was drastically upregulated in response to cold stress. Gas chromatographic analysis revealed that the overexpression of this gene, named as Akd1 finally, increased the conversion of di-unsaturated C37-alkenone to tri-unsaturated molecule by alkenone desaturation, even at a high temperature when endogenous desaturation is efficiently suppressed. We anticipate that the Akd1 gene will be of great help for elucidating more detailed mechanisms of temperature response of alkenone desaturation, and identification of active species contributing alkenone production in metagenomic and/or metatranscriptomic studies in the field of oceanic biogeochemistry.
Highlights
Alkenones are long-chain unsaturated methyl- or ethyl- ketones, produced by selected haptophyte algae[1,2,3,4,5]
We present experimental evidences showing that Akd[1] catalyses the desaturation reaction of responsible for converting C37:2 to C37:3 by analysing alkenones in the overexpression mutant strains, Alkenone trans-desaturase Overexpressing Mutant (AtOM)
The function of alkenones as a storage neutral lipid is similar to other neutral lipids such as triacylglycerol (TAG) produced by many other microalgae, but the biosynthetic pathway and physiological response of alkenones to changes in environmental factors are different from TAG
Summary
Alkenones are long-chain unsaturated methyl- or ethyl- ketones, produced by selected haptophyte algae[1,2,3,4,5] In these species, C37-methyl-alkenones, containing two to four trans-type carbon double bonds, are the most prominent molecules found. Desaturation in lipid fatty acyl chains is catalyzed by fatty acid desaturases (FADs) found in almost all organisms[14,15,16] Those enzymes are classified into two large phylogenetically independent families; i.e., soluble FADs and membrane-bound FADs. The soluble FADs, known to be located only in the plastid, form a Δ9-carbon unsaturation bond in stearoyl-ACP. We report the first identification and experimental functional analysis of a novel trans-desaturase which catalyzes desaturation reaction of alkenone
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