BACKGROUND Anaerobic digestion (AD) could be designed as a source of volatile fatty acids (VFAs). However, acidogenesis optimization for novel substrates such as Chlorella vulgaris biomass needs to be investigated considering parameters such as temperature (T), organic loading rate (OLR), hydraulic retention time (HRT) and the adaptation of the sludge to temperature and substrate. RESULTS The best organic matter conversion into VFAs (CODVFA/CODin; COD, chemical oxygen demand) was achieved with HRT = 8 d, adapted anaerobic sludge (AAS) and OLR = 1.5 gCOD L-1 d(-1) (CODVFA/CODin = 39.8 +/- 1.0% and productivity VFA = 0.5 +/- 0.1 g L-1 d(-1)). Acetic and butyric acids represented 50% of the total VFAs. The microbiota related to acidogenesis and acetogenesis (Firmicutes 55% of the operational taxonomic units in R5) and the low archaeal population resulted in VFA accumulation at 25 degrees C. CONCLUSIONS The use of low HRT and temperatures promoted VFAs production especially when AAS was employed. Microbial communities were strikingly different to the ones often found in AD targeted at biogas production. The relevance of the Firmicutes phylum (<= 55% in R3 and R5) and euryarchaeota absence at 25 degrees C contributed to VFA accumulation. The use of AAS reported an increase in Actinobacteria species. (c) 2019 Society of Chemical Industry