Heterocysts are microaerobic specialized nitrogen fixing cells formed in some filamentous cyanobacteria and having the structural and physiological capacity to create and install internal anaerobic environment. The findings presented herein suggest that the auxin herbicide 2,4-D (2,4-dichlorophenoxyacetic acid) might be used for decreasing the culture and/or cellular oxic environment in Nostoc sp SAG 2306 that is favorable for enhancing the hydrogenases activities. This is achieved via induction and enrichment of heterocyst frequency. Treatment of the filamentous heterocyst-forming cyanobacterium Nostoc sp. SAG 2306 grown in nitrogen-deprived medium with different concentrations of 2,4-D has increased heterocyst frequency up to 19%.  Furthermore, 2,4-D induced heterocyst formation in the presence of combined nitrogen (nitrate) up to 90 µM. An increase in respiratory oxygen uptake and lowered photosynthesis/respiration (P/R) ratio, observed in 2,4-D treated Nostoc cultures, was associated with an increase in hydrogenases activities. 2,4-D caused a decrease in phycocyanin/chlorphyll a ratio which was associated with a decrease in photosynthetic oxygen evolution suggesting that the decrease in phycocyanins which are a major component of phycobilisomes, the light-harvesting complex of photosystem II in cyanobacteria, due to 2,4-D treatment has a negative impact in oxygen evolution and a positive impact on hydrogenases activities.
Lowered values of phycocyanin/chlorphyll a ratio, decreased oxygen evolution, photosynthesis/respiration (P/R) ratio with enhanced respiration and heterocyst formation all have been induced by 2,4-D cooperated to install anaerobiosis for hydrogen metabolism.