Impact of pH and organic loading rate on carboxylate production and microbial communities during semi-continuous anaerobic fermentation of macroalgae

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Macroalgae waste accumulating in coastal regions has become an increasingly significant ecological issue worldwide. Anaerobic fermentation offers an economical and environmentally friendly alternative to landfilling or incineration. This study evaluates the impact of pH (6.0 and 7.5) and organic loading rate (OLR, 1.5 and 3.0 g volatile solids (VS)·L −1 ·d −1 ) on carboxylate production, mainly short-chain fatty acids (SCFA), and microbial communities during semi-continuous anaerobic fermentation of macroalgae. Macroalgae acidification reached 40% at low OLR under both pH condition. At pH 7.5, the dominant SFCA were acetic acid (51 ± 3%) and propionic acid (35 ± 2%), whereas at pH 6.0, the dominant SCFA were acetic acid (37 ± 7%), propionic acid (17 ± 4%), n-butyric acid (25 ± 2%), and n-caproic acid (11 ± 6%). Increasing the OLR decreased acidification at pH 6.0, but diversified the SCFA profile, leading to heptanoic acid production. In contrast, both acidification and the SCFA profile remained stable at pH 7.5. The microbial population was more influenced by pH than by OLR, exhibiting greater diversity at pH 7.5. Strong correlations between microbial populations and SCFA production were observed, particularly involving members of the phylum Firmicutes , Actinobacteriota and Proteobacteria. Overall, this study demonstrates that maintaining a neutral pH enhances process stability and microbial diversity, while acidic conditions promote chain elongation and the production of medium-chain carboxylates.