Brown algae produce hydrocarbons via fatty acid photodecarboxylase-dependent and fatty acid photodecarboxylase-independent pathways.

Across the tree of life, diverse organisms synthesize hydrocarbons from fatty acids using various enzymes. In plants and insects, hydrocarbons are typically excreted (eg waxes, pheromones), whereas in algae, they seem to be mostly intracellular and membrane-associated. To date, all studied organisms have been found to possess only one hydrocarbon-forming pathway. In most green algae, unesterified fatty acids are converted into hydrocarbons by the algal-specific photoenzyme fatty acid photodecarboxylase. Other green algae lacking the fatty acid photodecarboxylase are able to synthesize hydrocarbons via the ECERIFERUM 1/3 (CER1/3) protein. Here, using heterologous expression in E. coli, we show that 6 fatty acid photodecarboxylase homologs belonging to diverse groups of brown algae (Phaeophyceae) are functional. We also demonstrate that Ectocarpus species 7 and Saccharina latissima produce 2 intracellular hydrocarbons during vegetative growth: n-pentadecane (15:0 hydrocarbon) and n-heneicosahexaene (21:6 hydrocarbon), which is distinct from the n-heneicosahexaene isomer found in some green algae. Through genome editing, we show that 15:0 hydrocarbon, likely localized in chloroplast membranes, is synthesized via a fatty acid photodecarboxylase-dependent pathway in Ectocarpus, while the 21:6 hydrocarbon is produced through an unidentified alternative pathway. Our results support the hypothesis that fatty acid photodecarboxylase plays a conserved biological role in brown algae. Furthermore, this work provides the first example of a group of organisms harboring a second, distinct hydrocarbon-forming pathway, which may fulfill a unique biological function.