2 Optimizing Metabolic Capacity and Flux through Environmental Cues to Maximize 3 Hydrogen Production by Cyanobacterium Arthrospira maxima 4

8 9 Abstract (243 words) 10 Environmental and nutritional conditions that optimize the yield of hydrogen (H2) from 11 water using a two-step photosynthesis/fermentation (P/F) process are reported for the 12 hypercarbonate-requiring cyanobacterium Arthrospira maxima. Our observations lead to four 13 main conclusions broadly applicable to fermentative H2 production by bacteria: 1) Anaerobic 14 H2 production in the dark from whole cells catalyzed by a bidirectional (NiFe)-hydrogenase is