Microscale Vertical Profiles of N 2 Fixation, Photosynthesis, O 2 , Chlorophyll a , and Light in a Cyanobacterial Assemblage

Abstract

Profiles of 15 N 2 fixation, O 2 production (gross photosynthesis), O 2 concentration, chlorophyll a concentration, and photon fluence rates were measured with 50-μm resolution in colonies of the heterocyst-forming cyanobacterium Nostoc parmelioides . Microelectrode measurements were made after 20 h of incubation under 15 N 2 gas. Colonies were frozen, and 50-μm sections were prepared by using a freezing microtome and analyzed for 15 N enrichment and chlorophyll a concentration. Colonies exhibited steep spatial gradients in rates of gross photosynthesis, O 2 concentration, and irradiance, with the highest values generally occurring at the surface. O 2 concentration, photosynthesis, and irradiance all showed positive correlations, but chlorophyll a concentrations varied independently of photosynthesis and irradiance. Forty-four percent of the variation in 15 N incorporation was explained by gross photosynthesis (a positive correlation) when incorporation of 15 N was expressed per unit of biomass (chlorophyll a ).