Neutral lipid production in Alcanivorax borkumensis SK2 and other marine hydrocarbonoclastic bacteria

Abstract

Triacylglycerols (TAG) and wax esters (WE) constitute together with polyhydroxyalkanoates (PHA) the major storage lipophilic compounds in prokaryotes. Recently, the production of neutral lipids such as TAG and WE has been reported in species of the genus Alcanivorax, which belongs to the group of obligate hydrocarbonoclastic bacteria (OHCB). We analyzed the production of such neutral lipids by different marine hydrocarbonoclastic bacteria growing on pyruvate or hexadecane as sole carbon source, and compared it to other bacteria such as Rhodococcus opacus strain PD630 and Acinetobacter baylyi strain ADP1, which are two well-studied strains for production of neutral lipids. Alcanivorax borkumensis SK2 synthesized mainly TAG when cells are cultivated on pyruvate, while biosynthesis and accumulation of WE was mainly observed in cells growing on hexadecane. Alcanivorax jadensis T9 synthesized and accumulated mainly WE if cells were cultivated with hexadecane, while both TAG and WE were observed if cells were cultivated with pyruvate as sole carbon source, respectively. Predominantly production as well as export of WE was observed in Marinobacter hydrocarbonoclasticus SP17 growing on pyruvate or hexadecane as sole carbon source. The chemical structures of TAG and WE produced by A. borkumensis SK2 were analyzed by gas chromatography and/or mass spectrometry, and first studies to investigate the influence of different C/N ratio (7, 50 or 150) on the production of neutral lipids were performed.

Differences in Substrate Specificities of Five Bacterial Wax Ester Synthases

“…Although the production of neutral lipids such as wax esters as a potential storage compound is rare among bacteria, WS/DGAT homolog enzymes are found in a broad range of bacteria (12,13,22,25). Here, we confirm the production of wax esters in the psychrophilic bacterium P. cryohalolentis K5 (and have additionally confirmed their presence in Psychrobacter arcticus 273-4 [results not shown]), each of which are strains of bacteria isolated from polar environments.…”

“…The wax ester accumulation pathway, which represents a potential mechanism for reduced carbon storage, is limited to only a small percentage of bacteria overall, and the presence of neutral lipids such as wax esters or TAGs has been confirmed in only a small number of bacterial species (1,7,10,13). In contrast, alternative methods for carbon storage such as the synthesis of polyhydroxybutyrate, starch, or glycogen seem to be a predominant reduced carbon storage mechanism for most bacteria (22,25).…”

“…Cells of each organism grown under the lipid accumulating conditions described above were used to determine the composition of native neutral lipids. The approach utilized to optimize the medium composition is similar to what has been reported by others (10,13). Cells from each organism were harvested by centrifugation, frozen, and dried by lyophilization.…”

Acyltransferases in Bacteria

“…Although the WS activity is about four times the DGAT activity in wild-type cells, they primarily accumulate TAG when cultivated with pyruvate. WE, besides a greater proportion of TAG, are synthesized only when hexadecane is provided (26,91). This inability is due to the lack of a fatty acyl-CoA reductase, so that during growth with unrelated carbon sources, there are no fatty alcohols present that could serve for WE synthesis.…”

“…For example, Alcanivorax jadensis (formerly Fundibacter jadensis) and Thalassolituus oleivorans are known to form intracellular lipid inclusions (TAG, WE, or wax diesters), and they were even reported to export WE when cultivated with n-alkanes (26,29,91,92). However, the responsible acyltransferase has not yet been identified.…”