Natural fermentation as an inoculation strategy for dark fermentation of Ulva spp. hydrolysate

Abstract

Dark fermentation from Ulva spp. hydrolysate, a third-generation feedstock, can be exploited as sustainable alternative for biofuel production as it possesses a high carbohydrate content, contributes to decarbonisation and allows the simultaneous production of bioH2 and chemicals. However, it requires microorganisms with enzymatic capability to metabolise the sugars released by the macroalgae. In the present study, the natural fermentation process exploring the microbial community growing attached onto the Ulva spp. was for the first time assessed as an alternative inoculation method for bioH2 production from Ulva spp. hydrolysate. Inoculum enrichment from Ulva spp was tested at 4, 30 and 55 °C in a batch experiment. Then, the liquid phase was assessed as inoculum of a continuous up-flow anaerobic structured bed reactor (AnSTBR) fed with diluted acid-thermal Ulva spp. hydrolysate at 30 and 55 °C. At initial pH 6.5 the sulfidogenic pathway was predominant, however decreasing the pH to 5.5 stimulated the bioH2 production, with higher production at 55 °C, resulting in a median volumetric hydrogen production rate (VHPR) of 297.5 NmL L−1 d−1. The main soluble metabolites (SM) at 55 °C were butyric acid (HBu) and acetic acid (HAc), reaching medians of 912.9 and 483.4 mg L−1. At 30 °C, the primary pathway was solventogenic, resulting in ethanol (EtOH) and HAc production at a ratio 2.5:1. Therefore, the natural fermentation strategy was found to be a suitable strategy to inoculate the dark fermentation reactor.