Nitrogenase Activity by Hapalosiphon flexuosus Associated With Sphagnum erythrocalyx Mats in the Cloud Forest on the Volcano La Soufriere, Guadeloupe, French West Indies

Abstract

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 C2H4 production in situ by entire plants with the associated cyanobacterium was 19.1 nmol C2H4 gdwh-I (range 0.20101) with a nitrogenase activity for the uppermost green tips 4.5 times greater than that measured for the basal portions. The highest rates for C2H2 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-ha-l year-'. THIS PAPER REPORTS THE RESULTS OF A STUDY OF NIT-ROGEN FIXATION by epiand endophytic cyanobacteria on Sphagnum to the nitrogen economy of the cloud forest orobiome on the tropical volcano La Soufriere. Also discussed is the relationship between the rates of acetylene reduction and the degree of habitat damage following the 1976-1977 eruption of the volcano La Soufriere. La Soufriere (alt. 1467 m), a recently active volcano located on the island of Guadeloupe in the French West Indies (16?40'N, 60?10'E), supports an orobiome (1000-1467 m) termed savane d'altitude (Stehle 1935, 1979), tropical doud forest (Howard 1970), or cloud-shroud ecosystem (sensu Odum 1970). The prevailing doud cover is only occasionally broken by periods of high insolation subjecting the flora, which is adapted to the prevailing low temperatures and insolation values, to unfiltered sunlight and high temperatures (Fritz-Sheridan & Coxson 1988a). Tropical cloud forests, characterized by persistent cloud cover, promote colonization by poikilohydric plants such as mosses and lichens (Grubb 1977, La Bastille & Pool 1978, Weaver et al. 1973, Weaver et al. 1986). The prevailing high moisture and low temperatures reduce the mineralization rate (Grubb 1971, 1977), and mineral loss is accelerated by high rainfall on the steep slopes of La Soufriere, mnaking nitrogen input by fixation a potentially important part of the mineral cycle. I Received 30 January 1990, revision accepted 28 September 1990. 2 Formerly Fritz-Sheridan. The eruptions in 1976 and 1977 disturbed the pre-eruption flora by fumigation with hydrogen sulfide gas and the deposition of ash and boulders, effectively dividing the dome into four distinct posteruption vegetation zones (Howard 1962, Howard et al. 1980, Sastre et al. 1983). This cloud forest supports a diverse group of nitrogen-fixing cyanobacteria, and primary succession in lahar flows on the east flank is by the nitrogen-fixing lichens Dictyonema glabratum and Stereocaulon virgatum (Coxson 1987; Fritz-Sheridan 1987, 1988; Fritz-Sheridan & Coxson 1988a, b; Fritz-Sheridan & Portecop 1987). The occurrence and nitrogenase activities of nitrogen-fixing microepiphytes on dominant species of vascular plants involved in posteruption revegetation (e.g., Clusia mangle, Norantia spiciflora, Tibouchina ornata) have been described by Fritz-Sheridan & Portecop (1987). Sastre et al. (1983) divided the dome of La Soufriere into quadrats and 30 percent of these were occupied by Sphagnum erythrocalyx Hampe.