Ananyev et al. [39 ] recently demonstrated that in the hypercarbonate-requiring filamentous cyanobacterium Arthrospira (Spirulina) maxima anaerobic H2-production in the dark occurs in two temporal phases, involving two distinct metabolic processes. In this organism H2 evolution represents a major pathway for energy (ATP) production during fermentation, by regenerating NAD+ essential for the glycolytic degradation of glycogen, and the catabolism of other substrates. Moreover, the removal of nitrate during fermentative H2 evolution produces an immediate and large stimulation of the rate of H2 evolution, as nitrate is a competing substrate for the consumption of NAD(P)H. Environmental and nutritional conditions that increase anaerobic ATP production and the intracellular reduction potential (NADH/NAD+ ratio) are key variables for increased H2 evolution.