Estevez, A. | Search bioRxiv

bioRxiv2026

Lactate:propionate molar ratio determines valerate production in secondary lactate fermentations

Odd-chain carboxylates such as valerate and heptanoate are ecologically relevant metabolites and promising platform chemicals, yet the factors leading to their formation during secondary lactate fermentations remain poorly understood. Here, a continuous anaerobic bioreactor was operated for 297 days under mildly acidic conditions to evaluate how lactate:propionate molar ratios shape product spectrum in lactate fermentations. Valerate was the predominant odd-chain product under all conditions, reaching concentrations up to 110 mM, while heptanoate accumulated only at low levels (<10 mM). At low lactate concentrations (10-20 g/L), product selectivity strongly depended on the lactate:propionate ratio. When lactate:propionate ratios were around 1.2 mol/mol, odd-chain products were favored, whereas higher ratios (up to 4.8 mol/mol) shifted metabolism toward caproate and butyrate formation. However, this trend was not maintained at higher lactate concentrations (30-40 g/L; lactate not fully consumed), where odd-chain selectivities remained high even at lactate:propionate ratios of 4.8 mol/mol. Pathway analysis indicated that under high-lactate conditions up to 30% of lactate was redirected toward propionate and acetate formation, likely via the acrylate pathway. Microbial community analysis revealed a stable dominance of Caproiciproducens spp., that could be correlated to valerate production. Overall, this work provides mechanistic insights into the ecology of lactate fermentations and offers a framework for steering product selectivity in engineered anaerobic systems. HighlightsValerate was the dominant product, reaching up to 110 mM. Lactate:propionate ratios drive product selectivities. High lactate concentrations activated in situ propionate formation pathways. Caproiciproducens dominance was associated with sustained valerate production.

bioengineering↗

bioRxiv2021

Recombinant Fsh and Lh therapy for spawning induction of previtellogenic and early spermatogenic arrested teleost, the flathead grey mullet (Mugil cephalus)

With the expansion and diversification of global aquaculture, efforts continue to develop new bio-technologies for assisted reproduction in species that present reproductive dysfunctions. Flathead grey mullet (Mugil cephalus) held in intensive conditions in the Mediterranean region, display a severe reproductive dysfunction, where males do not produce fluent milt and females are arrested at previtellogenesis or early stages of vitellogenesis. In the present study, weekly injections of species-specific single-chain recombinant gonadotropins (rGths); follicle stimulating hormone (rFsh) (6 to 12 g kg-1 doses) and luteinizing hormone (rLh) (2.5 to 24 g kg-1 doses) were administered to induce vitellogenesis, from previtellogenesis / early vitellogenesis to the completion of vitellogenic growth in females and enhance spermatogenesis to produce adequate volumes of sperm from non-fluent males. During the experiment, all treated females (n = 21) developed oocytes in late vitellogenesis with 603 {+/-} 8 m diameter and all treated males produced fluent sperm. To induce oocyte maturation, ovulation and spawning, females were treated with either (i) a priming dose of 30 g kg-1 of rLh and a resolving dose of 40 mg kg-1 of progesterone (P4), (ii) priming and resolving doses of 30 g kg-1 of rLh, or (iii) priming and resolving doses of 40 mg kg-1 of P4 given 24:05 {+/-} 0:40 h apart. Females were placed in spawning tanks with rGth treated males that had fluent sperm. Spontaneous spawns of fertilised eggs were obtained after inducing with rLh + P4 or rLh + rLh (priming and resolving injections) with a spawning success of the 85% (8 of 9 females) and 100% (n = 6), respectively. The eggs collected from the tanks presented 64 {+/-} 22% fertilization with embryo development and 57 {+/-} 24 % hatching. The treatment P4 + P4 had a lower ovulation success (50 % - 3 of 6 females) and spawning success (17 %) with no fertilised eggs. Success was independent of the initial gonadal stage of females. In comparison, control females did not show any advance in gonadal development from initial stages and control males did not produce fluent sperm. The present results confirm the possibility of controlling oogenesis from previtellogenesis to the completion of maturation and fertilised tank spawning using exclusively rFsh and rLh in a teleost species.

physiology↗