Abstract
Photobiological hydrogen production by nitrogen-fixing unicellular cyanobacteria has long been considered to be an environmentally sound and very promising method for the future supply of renewable clean energy. Using six Korean nitrogen-fixing unicellular cyanobacterial strains and the Synechococcus sp. strain Miami BG043511 we performed cultivation experiments to find out the strain-specific optimal temperature for population growth and <TEX>$H_2$</TEX> production. Under <TEX>$20^{\circ}C$</TEX> the population growth of all the tested strains was significantly retarded in contrasts to the faster and higher growth under 25, 30 or <TEX>$35^{\circ}C$</TEX>. The highest growth rates in all the 7 strains were measured under <TEX>$30^{\circ}C$</TEX> while the maximal biomass yields were under <TEX>$30^{\circ}C$</TEX> (strains CB-MAL 026, 054, and 055) or <TEX>$35^{\circ}C$</TEX> (strains 002, 031, 058, and Miami BG043511). The difference between the maximal biomass yields at <TEX>$30^{\circ}C$</TEX> and <TEX>$35^{\circ}C$</TEX> was not greater than 10%. The quantity of photobiologically produced <TEX>$H_2$</TEX> was only slight larger under <TEX>$35^{\circ}C$</TEX> than that under <TEX>$20^{\circ}C$</TEX>. Our result may suggest a two-step process of <TEX>$H_2$</TEX> production which includes rapid and sizable production of biomass at <TEX>$30^{\circ}C$</TEX> and the following high <TEX>$H_2$</TEX> production at <TEX>$20^{\circ}C$</TEX> by the test strains of marine nitrogen-fixing unicellular cyanobacteria.
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