CO sub(2) gas exchange of benthic microalgae during exposure to air: a technique for the rapid assessment of primary production

A method of measuring CO sub(2) gas exchange (caused, for example, by microalgal photosynthesis on emersed tidal mudflats) using open flow IR gas analyzers is described. The analyzers are integrated in a conventional portable photosynthesis system (LI-6400, LI-COR, Nebraska, USA), which allows manipulation and automatic recording of environmental parameters at the field site. Special bottomless measuring chambers are placed directly on the surface sediment. Measurements are performed under natural light conditions and ambient CO sub(2) concentrations, as well as under different CO sub(2) concentrations in air, and various PAR radiation levels produced by a LED light source built into one of the measurement chambers. First results from tidal channel banks in a north Brazilian mangrove system at Braganca (Para, Brazil) under controlled conditions show a marked response of CO sub(2) assimilation to CO sub(2) concentration and to irradiance. Photosynthesis at 100 mu mol mol super(-1) CO sub(2) in air in one sample of a well-developed algal mat was saturated at 309 mu mol photons m super(-2) s super(-1), but increased with increasing ambient CO sub(2) concentrations (350 and 1000 mu mol mol super(-1) CO sub(2)) in the measuring chamber. Net CO sub(2) assimilation was 0.8 mu mol CO sub(2) m super(-2) s super(-1) at 100 mu mol mol super(-1) CO sub(2), 5.9 mu mol CO sub(2) m super(-2) s super(-1) at 350 mu mol mol super(-1) CO sub(2) and 9.8 mu mol CO sub(2) m super(-2) s super(-1) at 1000 mu mol mol super(-1) CO sub(2). Compensation irradiance decreased and apparent photon yield increased with ambient CO sub(2) concentration. Measurements under natural conditions resulted in a quick response of CO sub(2) exchange rates when light conditions changed. We recommend the measuring system for rapid estimations of benthic primary production and as a valuable field research tool in connection with certain ecophysiological aspects under changing environmental conditions.