This study examined hydrogen metabolism in Nostoc sp. strain Cc, a nitrogen-fixing cyanobacterium isolated from Cycas circinalis. Researchers investigated how nickel ions affect hydrogen production and consumption in photoheterotrophic cultures (growing with glucose and light). Cultures without added nickel produced hydrogen, but when nickel was supplemented, hydrogen consumption increased ten-fold compared to controls. When exogenous hydrogen was provided to nickel-supplemented photoheterotrophic cultures, it significantly increased nitrogenase activity (nitrogen fixation measured by acetylene reduction) by approximately 15% and increased the total nitrogen content of biomass by 13%, along with enhanced ammonia excretion. Notably, hydrogen uptake did not affect growth rates or glucose consumption. The authors concluded that the cyanobacterium's hydrogen-uptake capability may improve nitrogen fixation efficiency in plant-cyanobacteria symbioses, suggesting that nickel availability in plant roots could enhance nitrogen assimilation in natural symbiotic associations like the Cycas-Nostoc partnership.

Key findings

• Nickel ions are essential for hydrogen-consuming ability in Nostoc sp. strain Cc; nickel-supplemented cultures showed ten-fold higher hydrogen consumption rates than controls
• Exogenous hydrogen uptake increases nitrogenase activity and nitrogen incorporation into biomass by 13-15% without affecting growth or glucose consumption
• Hydrogen metabolism provides approximately 10% of the theoretical energy equivalent to glucose consumption under photoheterotrophic conditions
• The cyanobacterium maintains hydrogen uptake activity during photoheterotrophic growth, contrary to typical patterns in other bacteria where organic carbon suppresses hydrogenase expression
• Enhanced nitrogen fixation efficiency through hydrogen recycling may improve nitrogen supply to plant hosts in Cycas-cyanobacteria symbiotic associations