## 2 Mechanisms of Acidogenic Fermentation
### Key Microbial Communities Involved in VFA Production
butyricum_ in a mixed culture system increased butyric acid production significantly, from 260 to 3000 mgCOD/L under alkali pH and with dairy industry wastewater as the substrate. This approach also boosted total VFA production from 1500 to 5000 mgCOD/L. The study demonstrated that bioaugmented mixed cultures outperformed pure cultures in butyric acid yield, with efficient production achieved in the first 6 h compared to 14 h in control reactors, highlighting the potential of combined monoculture and mixed culture systems for next-generation biorefineries [49]. _Megasphaera_ has also shown significant potential in butyric acid synthesis through chain elongation under microbial electrosynthesis (MES) conditions. In a study utilizing fed-batch MES cells, _Megasphaera_ played a key role in achieving a selective butyric acid production yield of 78% (carbon basis). When a second MES cell was inoculated with the enriched catholyte containing _Megasphaera_, the butyric acid production rate was maintained, while the lag phase decreased by 82%. This highlights _Megasphaera's_ ability to optimize carbon utilization and chain elongation processes, enhancing butyric acid output under efficient energy conditions and underscoring its potential for advanced bioaugmentation strategies in biorefinery applications [50]. This genus is often found in mixed communities where it helps balance the VFA profile by converting organic acids into butyrate and valerate. The interaction between different microbial species in these communities enhances the overall efficiency of the fermentation process, particularly when dealing with complex substrates like lignocellulosic or protein-rich wastes. In the sustainable bioproduction of isobutyric acid, specific microorganisms have demonstrated potential as alternatives to conventional chemical synthesis. Among them, Escherichia coli has been engineered to convert glucose to isobutyric acid using the Ehrlich pathway. In this process, glucose undergoes metabolic transformations into isobutyraldehyde through enzymes such as acetolactone synthase, keto-acid reductoisomerase, dihydrovacid dehydratase, and keto acid decarboxylase [51]. Using this pathway, engineered _E. coli_ strains have achieved isobutyric acid concentrations up to 12 g/L [52]. Another promising candidate is Pseudomonas sp., which has been engineered to channel isobutyric acid production via isobutyryl-CoA through its valine degradation pathway. By over-expressing the 2-ketoacid decarboxylase enzyme from _Lactococcus lactis_ and performing specific gene deletions, this strain can produce up to 2.3 g/L of isobutyric acid, achieving better selectivity by reducing unwanted byproducts like isobutanol. Unlike other microbes, _Pseudomonas_ sp. offers simpler downstream processing due to its reduced byproduct formation when glucose is used as the primary substrate [52]. In addition to _Escherichia coli_ and _Pseudomonas_ sp., other microorganisms play supporting roles in isobutyric acid pro duction [53; 54]. For instance, under anaerobic or microaerobic conditions, various bacteria such as Bacillus, Corynebacterium, and Streptomyces are capable of producing mixed acids, including isobutyric acid, from pyruvate to maintain redox balance [55]. While these microorganisms are not all specifically engineered for maximizing isobutyric acid production, their metabolic capabilities to process pyruvate and maintain redox balance make them potential candidates for industrial applications, particularly in mixed culture systems. Valeric acid production through acidogenic fermentation involves several key microorganisms, each contributing to the breakdown of organic substrates and aiding in the synthesis of valeric acid. Among the prominent bacteria involved are those from the genera _Clostridium_, _Propionibacterium_, and _Megasphaera_. _Clostridium_ species are known for their robust fermentative pathways, which efficiently convert carbohydrates into a mix of volatile fatty acids, including valeric acid [49].