Abstract
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.