NITROGEN-FIXATION IN THE SUB-ANTARCTIC MACQUARIE ISLAND

The N2-fixing biota of Macquaire Island are dominated by cyanobacteria growing epiphytically or symbiotically with plants or lichens. Highest rates of C2H2-reducing activity were found in the leafy lichen Peltigera sp. colonizing herbfields and short grasslands and in the coastal angiosperm Colobanthus muscoides. Significant rates of C2H2 reduction were also found to be associated with the liverwort Jamesoniella colorata, commonly occurring in coastal and plateau mires, in a moss-bed of Dicranella cardotii colonizing a land-slip face on the grassland slopes at 100 m altitude and within polsters of the mosses Ditrichum strictum and Andreaea sp. found in exposed localities on the plateau at 200-300 m altitude. It was concluded that the common feature of plants supporting active N2 fixation in dry habitats was the dense packing of stems and leaves, enabling water translocation to the cyanobacterial zone by wick action. Epiphytic cyanobacterial C2H2 reduction in wet habitats was wide-spread and not restricted to any particular plant species. Notable N2-fixing lichens of the plateau were Pseudocyphellaria delisea and Stereocaulon sp., although both were also occasionally found in coastal herbfields. No significant N2-fixing activity was associated with any of the dominant grasses tested. Heterotrophic N2 fixation was also found to be insignificant in the various habitats tested, however N2-fixing Bacillus (B. macerans or B. polymyxa) were universally present in coastal, grassland slope, or plateau samples, including moss polster samples. A N2-fixing Clostridium sp. was isolated in only one instance, from soil in the vicinity of a seal wallow on the coast.