# Microbial Biosynthesis of Lactones: Gaps and Opportunities towards Sustainable Production
## 4 Microbial Production
### De Novo Biosynthesis  
It is known that the biosynthesis of lactones can be quite different between microorganisms, for instance regarding the types of fatty acids and hydroxy acids formed. As stated above, microbial hosts able to biosynthesize lactones from non-hydroxy fatty acids and perform de novo biosynthesis from substrates such as sugars or glycerol would enable economically competitive and more sustainable bioproduction of lactones. If these substrates were renewable or derived from industrial wastes all the biorefinery requisites towards a process contributing to a circular bioeconomy would be fulfilled. Therefore, it is urgent to identify microorganisms with the ability to de novo biosynthesise lactones or engineer that capacity in industrial models. To biosynthesize lactones a microorganism should be able to convert sugars or other non-lipid substrates into fatty acids, perform post-modifications to introduce a hydroxy group to these fatty acids and perform their chain shortening through \(\beta\)-oxidation. It is not therefore surprising that de novo biosynthesis of lactones is a rare natural trait among microorganisms, namely among fungi. Tables 1 and 2 display the lactones produced by de novo biosynthesis of microorganisms and the production levels obtained from different microorganisms and substrates, respectively. As for other industrial biotechnological processes, engineering of the microbial host to achieve a competitive process will always be necessary. Some authors consider that the production of \(\gamma\)-decalactone by the genus Sporidiobolus has been known since the 1930s [47]. This yeast was able to produce \(\gamma\)-decalactone from different carbon sources such as glucose, sucrose, fructose and mannitol, while cis-6-\(\gamma\)-dodecenolactone was produced in glucose, sucrose and fructose, but not in mannitol [38]. In liquid cultures, the production profile of \(\gamma\)-decalactone was also associated with the growth phase and a maximum of 4.5 mg/L was obtained from mannitol. In turn, 1.9 mg/L of cis-6-\(\gamma\)-dodecenolactone was detected in cultures using fructose as a carbon source [38]. The effect of media composition on lactone de novo biosynthesis by _S. salmonicolor_ was also investigated [40]. The main differences of the media were related to the presence or absence of amino acids and yeast extracts. Absence of these two components led to slower growth but increased lactone biosynthesis (12 mg/L vs. 3 mg/L) after 12 days of growth in liquid cultures at 25 \({}^{\circ}\)C. Therefore, this indicated an inversely proportional correlation between growth rate and lactone production in this yeast. From the five \(\gamma\)-lactones detected, \(\gamma\)-octalactone and \(\gamma\)-nonalactone were reported for the first time, but the major lactones produced were cis-6-\(\gamma\)-dodecenolactone (medium with amino acids and yeast extract) and \(\gamma\)-decalactone (medium without amino acids and yeast extract) [40]. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Cultured** & \multirow{2}{*}{**Lactones Produced**} & \multirow{2}{*}{**Substrate**} & \multirow{2}{*}{**Production Levels**} & \multirow{2}{*}{**Culture Time**} & \multirow{2}{*}{**Reference**} \\
**Microorganisms** & & & & & & \\ \hline _A. gossypii_ & & & & & & \\ \hline \multirow{9}{*}{Non-engineered strains} & \(\gamma\)-decalactone & & Up to 18 mg/L & & & \\  & \(\gamma\)-nonalactone & & Up to 0.2 mg/L & & & \\  & \(\gamma\)-dodecalactone & & Up to 82 ug/L & & \\  & \(\gamma\)-undecalactone & & Up to 40 ug/L & 2 days & \\  & \(\gamma\)-octalactone & & Up to 24 ug/L & & \\  & \(\gamma\)-valerolactone & & Up to 12 ug/L & & \\  & \(\gamma\)-caprolactone & & Up to 3.5 ug/L & & \\ \hline Engineered strains & \(\gamma\)-decalactone & & Up to 31 mg/L & & \\  & \(\gamma\)-octalactone & & Up to 0.5 mg/L & & \\ \hline \multirow{9}{*}{_F.