Effects of Photosynthesis Inhibitors on H₂ Production in Microalgae and Cyanobacteria

Photobiological hydrogen (H₂) production by microalgae and cyanobacteria is widely seen as a promising and sustainable alternative to H₂ produced from fossil fuels. However, its efficiency remains limited because the enzymes responsible for H₂ production are highly sensitive to oxygen (O₂), while photosynthesis itself generates O₂ that can suppress their activity. This mini-review explores how different photosynthesis inhibitors affect H₂ production in these microorganisms, with a focus on their molecular targets and their physiological effects. In both microalgae and cyanobacteria, compounds such as DCMU, atrazine, DBMIB, CCCP, and KCN influence H₂ metabolism by altering electron transport, O₂ release, proton gradients, and cellular redox balance. The reviewed studies indicate that complete inhibition of photosynthetic electron flow is usually unfavorable for sustained H₂ production because it reduces the electron supply required by H₂-evolving enzymes. Therefore, anaerobiosis is more reliably established by physiological or cultivation-based strategies, whereas photosynthetic and respiratory inhibitors are best used as mechanistic probes or as auxiliary modulators under carefully optimized conditions. Their effects are strongly context-dependent, reflecting the balance between O₂ suppression, residual electron transport, respiratory O₂ consumption, and competing electron sinks.