Nitrogen fixation (acetylene reduction) in nonheterocystous cyanobacterial mats from the Dampier Archipelago, Western Australia

Abstract

Discs punched from non-heterocystous cyanobacterial mats, one containing Microcoleus chthonoplastes, Oscillatoria sp. and Phormidium sp., the other Phorrnidium sp. and Aphanocapsa sp., were incubated for 23 days in artificial sea-water of salinity 0 to 140 g L^-1. The chlorophyll a content of both mats increased over this salinity range, with lower increases above 100 g L^-1. There was little change in the species composition of mats at salinities 240 g L^-1; ≥ 40 g L^-1, mats produced essentially monospecific thalli containing small quantities of the other species.

Acetylene reduction ranged from zero at the highest salinity to a maximum of 1100-1500 pmol C₂H₂ reduced m-2 h^-1 at 20-60 g L^-1. Maximum fixation rates were two orders of magnitude higher than in situ measurements (8-60, mean 16 pmol C₂H₂ reduced m^-2 h^-1). The salinity range observed in the field was 40-60 g L^-1, but maximum fixation rates in the field (60 Fmol C₂H² reduced m^-2 h^-1) were much lower than those observed in the laboratory.