Nitrogenase activity by Hapalosiphon flexuosus associated with Sphagnum erythrocalyx mats in the cloud forest on the volcano La Soufriere, Guadeloupe, French West Indies

Acid mats of Sphagnum erythrocalyx Hampe. growing on the dome of the tropical volcano La Soufriere, Guadeloupe, French West Indies, support associated populations of the nitrogen-fixing cyanobacterium Hapalosiphon flexuosus Borzi. The acetylene reduction technique was employed to measure nitrogenase activity. The mean rate of C$_2$H$_4$ production in situ by entire plants with the associated cyanobacterium was 19.1 nmol C$_2$H$_4\cdot gdw ^{-1}\cdot h^{-1}$ (range 0.20-101) with a nitrogenase activity for the uppermost green tips 4.5 times greater than that measured for the basal portions. The highest rates for C$_2$H$_2$ reduction were recorded for plants damaged by high winds and for those plants established within vegetation zones recovering from severe damage resulting from the 1976-1977 phreatic eruption of this volcano. The estimated rate of the annual contribution of nitrogen by nitrogen fixation is 4.02 kg $N\cdot ha^{-1} \cdot$ year$^{-1}$