Microalgae Synthesize Hydrocarbons from Long-Chain Fatty Acids via a Light-Dependent Pathway

Abstract

Microalgae are considered a promising platform for the production of lipid-based biofuels. While oil accumulation pathways are intensively researched, the possible existence of a microalgal pathways converting fatty acids into alka(e)nes has received little attention. Here, we provide evidence that such a pathway occurs in several microalgal species from the green and the red lineages. In Chlamydomonas reinhardtii (Chlorophyceae), a C17 alkene, n-heptadecene, was detected in the cell pellet and the headspace of liquid cultures. The Chlamydomonas alkene was identified as 7-heptadecene, an isomer likely formed by decarboxylation of cis-vaccenic acid. Accordingly, incubation of intact Chlamydomonas cells with per-deuterated D 31 -16:0 (palmitic) acid yielded D 31 -18:0 (stearic) acid, D 29 -18:1 (oleic and cis-vaccenic) acids, and D 29 -heptadecene. These findings showed that loss of the carboxyl group of a C 18 monounsaturated fatty acid lead to heptadecene formation. Amount of 7-heptadecene varied with growth phase and temperature and was strictly dependent on light but was not affected by an inhibitor of photosystem II. Cell fractionation showed that approximately 80% of the alkene is localized in the chloroplast. Heptadecane, pentadecane, as well as 7-and 8-heptadecene were detected in Chlorella variabilis NC64A (Trebouxiophyceae) and several Nannochloropsis species (Eustigmatophyceae). In contrast, Ostreococcus tauri (Mamiellophyceae) and the diatom Phaeodactylum tricornutum produced C 21 hexaene, without detectable C 15 -C 19 hydrocarbons. Interestingly, no homologs of known hydrocarbon biosynthesis genes were found in the Nannochloropsis, Chlorella, or Chlamydomonas genomes. This work thus demonstrates that microalgae have the ability to convert C 16 and C 18 fatty acids into alka(e)nes by a new, lightdependent pathway.

Beyond oil degradation: enzymatic potential of Alcanivorax to degrade natural and synthetic polyesters

“…It has been suggested, although, that Alcanivorax spp. may persist in pristine environments by using alkanes released by marine cyanobacteria (Coates et al, 2014;Lea-Smith et al, 2015) and other hydrocarbon-producing eukaryotic algae (Sorigué et al, 2016(Sorigué et al, , 2017. The hydrolysis of aliphatic polyesters such as poly(caprolactone) (PCL), poly(hydroxybutylate) (PHB) and poly(butylene succinate) (PBS) by some Alcanivorax isolates has also been shown (Sekiguchi et al, 2011;Zadjelovic et al, 2019), although the underlying molecular mechanisms behind the ability of these strains to degrade such polyesters remained unknown.…”

Hydrocarbons Are Essential for Optimal Cell Size, Division, and Growth of Cyanobacteria

“…Homologs of FAR/FAD or Ols are not present in other bacteria or plant and algal species. However, C15-C17 alkanes and alkenes, synthesized by an alternate, uncharacterized pathway, were recently detected in a range of green microalgae, including Chlamydomonas reinhardtii, Chlorella variabilis NC64A, and several Nannochloropsis species (Sorigué et al, 2016). In C. reinhardtii, hydrocarbons were primarily localized to the chloroplast, which originated in evolution from a cyanobacterium that was engulfed by a host organism (Howe et al, 2008).…”

Biocatalysis Fueled by Light: On the Versatile Combination of Photocatalysis and Enzymes

“…Comparison of proteomic data revealed a glucose‐methanol‐choline (GMC) oxidoreductase as the most likely candidate responsible for the decarboxylation activity. In parallel, a homologous enzyme was identified in C. reinhardtii 137C, a microalga that had previously been shown to convert long‐chain fatty acids into alkanes or alkenes in a light‐dependent fashion . The authors proposed the new name “fatty acid photodecarboxylase” (FAP) for this enzyme family.…”