TECHNOLOGY FOCUS - BIOTECHNOLOGY - Preferred Method: (M) further involves generating hydrogen from water and the accumulated substrate using light and a hydrogenase. The nutrient is depleted from the medium to a concentration of 0.5 mM or less. (M) further comprises replacing a head gas with an inert gas, preferably nitrogen, and after incubating and collecting, repeating the steps of growing to accumulate additional substrate, depleting, sealing and incubating for a number of cycles. (M) further involves providing a medium with the depleted nutrient after generating and repeating the steps of growing, depleting, incubating and generating. (I) is selected from green, red, brown, and blue algae such as Chlamydomonas reinhardtii. The substrate is selected from acetate, carbohydrate, lipid and protein. EXAMPLE - Sustained photobiological production of hydrogen gas in Chlamydomonas reinhardtii was as follows. When C.reinhardtii cultures were deprived of inorganic sulfur (less than 100 microM), the light-saturated rates of O2 evolution and CO2 fixation declined significantly within 24 hours in the light, without a proportional loss of chloroplast or thylakoid membrane electron transport components. Analysis indicated that such loss in electron transport activity was due to the conversion of PSII centers from the QB-reducing to QB'-non-reducing form. The activity of photosynthesis, measured from the light-saturated rate of O2 evolution in C.reinhardtii declined biexponentially from 48 mmol O2 (mol Chl)-1 S-1 at t=0 hours to less than 3 mmol O2 (mol Chl)-1 S-1 at t=120 hours. Cellular respiration, measured from the rate of O2 consumption in the dark remained fairly constant at about 13 mmol O2 (mol Chl)-1 S-1 over the 0-70 hour period and declined slightly thereafter. The absolute activity of photosynthesis decreased below the level of respiration in C.reinhardtii after about 24-30 hours of sulfur deprivation. Slower inactivation results were obtained with iron (less than 1.0 microM) or manganese (less than 1.0 microM) deprivation. After about 24-30 hours of sulfur deprivation, a sealed C.reinhardtii culture quickly became anaerobic in the light due to the greater rate of respiration than photosynthesis of the cells. This was confirmed by measurements with a Clark-type O2 electrode. Tests were performed to determine whether the hydrogenase activity of the cells was induced and sustained under these conditions. The results showed that anaerobiosis (but not darkness) was necessary and sufficient for induction of the reversible hydrogenase and for light-induced H2-production activity in C.reinhardtii.