Scientists Tap Bacterial Strain in Pruning Waste for Sustainable Bioproducts

Summary

Researchers have iden­ti­fied a bac­te­r­ial strain, Rhodococcus sp. 24CO, in olive prun­ing waste that effi­ciently pro­duces neu­tral lipids, par­tic­u­larly tri­a­cyl­glyc­erols, with poten­tial appli­ca­tions in biotech­nol­ogy. The strain demon­strates the abil­ity to con­vert var­i­ous car­bon sources to high-value prod­ucts, such as bio­fu­els, deter­gents, cos­met­ics, and phar­ma­ceu­ti­cal com­po­nents, offer­ing a sus­tain­able solu­tion for uti­liz­ing olive waste and poten­tially reduc­ing envi­ron­men­tal impact.

Researchers from Argentina and Spain have demon­strated the poten­tial of a bac­te­r­ial strain in olive prun­ing waste for biotech­no­log­i­cal appli­ca­tions. 

The study, pub­lished in Fermentation, builds on research pre­sented in 2022 at the XVII Argentine Congress of General Microbiology.

The strain in ques­tion, Rhodococcus sp. 24CO, effi­ciently con­verts var­i­ous car­bon sources to neu­tral lipids, espe­cially tri­a­cyl­glyc­erols, accu­mu­lat­ing up to 47 per­cent of its bio­mass as the dietary lipid. 

Microbial tri­a­cyl­glyc­erol syn­the­sis has received sig­nif­i­cant atten­tion recently due to its poten­tial for pro­duc­ing sus­tain­able high-value prod­ucts from waste mate­ri­als. Such prod­ucts include bio­fu­els, deter­gents, fer­til­iz­ers, cos­met­ics and phar­ma­ceu­ti­cal com­po­nents.

Rhodococcus sp. 24CO, was iso­lated from the leaves of Frantoio cul­ti­vars on the east­ern coast of south­ern Patagonia. Many Rhodococcus species are known for their capac­ity for mit­i­gat­ing per­sis­tent and dan­ger­ous pol­lu­tion such as pes­ti­cides, her­bi­cides and radioac­tive mate­r­ial, as well as their abil­ity to con­vert cheap sub­strate into more valu­able com­pounds.

To assess the strain’s prop­er­ties and capa­bil­i­ties, the researchers sub­jected it to var­i­ous tests.

When cul­tured, 24CO could grow on four of the 50 car­bon sources tested, includ­ing fruc­tose, man­ni­tol, sor­bitol and ara­bitol. The first two, nat­u­rally present in olive leaves, led to tri­a­cyl­glyc­erol accu­mu­la­tions of up to 47 per­cent and 28 per­cent of cel­lu­lar dry weight, respec­tively.

High neu­tral lipid pro­duc­tion from man­ni­tol also occurred in nitro­gen, which the authors believe to be pre­vi­ously unre­ported behav­ior and indus­tri­ally rel­e­vant since it implies that the strain would pro­duce a high bio­mass and, simul­ta­ne­ously, high quan­ti­ties of tri­a­cyl­glyc­erols.

Chemical analy­sis of fresh and dewa­tered leaf extracts of olive prun­ing waste revealed high car­bon-to-nitro­gen ratios, indi­cat­ing its suit­abil­ity for lipid pro­duc­tion. 

Further cul­tures sup­ported this, yield­ing more than 20 per­cent of cell dry weight as neu­tral lipids with the dewa­tered infu­sion and eight per­cent with the fresh infu­sion. Triacylglycerols pro­duced with the for­mer resem­bled those from man­ni­tol cul­ti­va­tion.

In addi­tion, both cul­tures detected small amounts of poly­hy­drox­y­bu­tyrate, a poly­mer of inter­est for biodegrad­able plas­tic pro­duc­tion. Genome analy­sis of Rhodococcus sp. 24CO con­firmed its poten­tial for syn­the­siz­ing these poly­mers.

Analysis fur­ther revealed sig­nif­i­cant genetic dif­fer­ences between 24CO and Rhodococcus sp. RHA1, one of the best-stud­ied mem­bers of the genus. 24CO lacked sev­eral genes linked to redox and nitro­gen metab­o­lism present in RHA1, which are typ­i­cally acti­vated under lipid-pro­duc­ing con­di­tions. The absence of these genes did not appear to hin­der 24CO’s lipid accu­mu­la­tion, sug­gest­ing alter­na­tive reg­u­la­tory mech­a­nisms.

Although 24CO was found to pos­sess fewer wax ester enzymes than RHA1, its lipid yields were equal to or supe­rior to those of the for­mer. This sug­gests that the enzymes respon­si­ble for tri­a­cyl­glyc­erol biosyn­the­sis in this strain may be more effi­cient. Kennedy path­way enzymes were fully present, how­ever, and had high redun­dancy.

After assess­ing suit­able envi­ron­men­tal con­di­tions for the species, the researchers con­cluded that it could sur­vive and repro­duce between 4 °C and 30 °C, with an opti­mal tem­per­a­ture of 28 °C. 

Salinity and pH exper­i­ments showed that it could tol­er­ate up to five per­cent weight per vol­ume of sodium chlo­ride with a pH value from six to ten, or slightly acidic to basic. 

The authors also note that leaf sur­faces in Patagonia rep­re­sent a harsh envi­ron­ment, sub­ject to high ultra­vi­o­let radi­a­tion, low nutri­ent lev­els and des­ic­ca­tion.

Although many ini­tia­tives aimed at reduc­ing and reusing olive waste are under­way, most olive prun­ing waste is still burned. 

In Spain alone, approx­i­mately 1.25 mil­lion met­ric tons of leaves are gen­er­ated from prun­ing annu­ally. The researchers believe that Rhodococcus sp. 24CO is a viable can­di­date for trans­form­ing this enor­mous bio­mass into sus­tain­able and valu­able prod­ucts, pri­mar­ily via tri­a­cyl­glyc­erol biosyn­the­sis.

• Conicet
• Microbiology