Optimization of hydrogen production using a coculture of Chlamydomonas reinhardtii and activated sludge bacteria.

The production of biophotolytic hydrogen (H) relies on the effective management of oxygen (O) levels. Coculturing bacteria with microalgae helps mitigate the excess O produced by algal cells. After depleting O, the bacteria activate the enzyme hydrogenase in microalgae, leading to H production. In this study, Chlamydomonas reinhardtii was cocultured with indigenous bacteria from activated sludge at varying algae-to-bacteria ratios (1:1, 1:1.5, 1:2, 1:2.5, and 1:3 v/v), with an illumination intensity of 2.8 mmol/m/s (31 × 10 lux). The 1:1.5 v/v ratio yielded the highest H volume (1162 mL/L) and the highest O concentration (153.2 mL/L) over a 6-day period. Production of all gaseous components ceased for all ratios as the pH dropped below 4 due to acetate accumulation, and the concentration of acetate reached approximately 1 g/L by the end of each experiment. Gas composition analysis after the first day of coculture revealed that H, CO, N, and O constituted 25%-46%, 20%-40%, 5%-30%, and 1%-10% of the total gas volume, respectively. Glucose (10 g/L) was introduced as an external carbon source for all cultures. After 6 days, the coculture maintained a high total organic carbon (TOC) level of 3.1 g/L, whereas the initial TOC ranged between 3.9 and 4.3 g/L. The findings illustrated a significant correlation between H production, acetate accumulation levels, and O consumption. The algae-activated sludge coculture method substantially enhanced H production compared with previously published methods employing only one or two types of bacterial cultures, underscoring its potential for more efficient biophotolytic H production.