AMMONIUM STIMULATION OF DARK CARBON FIXATION AS AN INDICATOR OF NITROGEN DEFICIENCY IN PHYTOPLANKTON: POTENTIAL ERRORS CAUSED BY AMMONIUM-OXIDIZING BACTERIA1

Stimulation of dark fixation of carbon by NH₄⁺ is often used as an indicator of phytoplankton N deficiency. This assay is based on the influence of available NH₄⁺ on anaplerotic CO₂ fixation by algae. However, carbon fixation by chemoautotrophic NH₄⁺-oxidizing bacteria may also be stimulated by NH₄⁺ enrichment, a process that can mask the algal response in natural communities. NH₄⁺ addition enhanced dark carbon fixation up to 300%, relative to unamended controls, in organisms collected on a 0.7-μm retention filter in oligotrophic Flathead Lake, Montana, but the effect was not detectable in the presence of nitrapyrin, an inhibitor of NH₄⁺-oxidizing bacteria. Dark carbon fixation was enhanced with addition of NH₄⁺ in organisms retained on 2-μm filters (which should allow passage of most bacteria). NH₄⁺ stimulated dark carbon fixation in N-deficient axenic cultures of Chlamydomonas reinhardtii Dang but not in N-replete cultures in both the presence and absence of nitrapyrin. Application of nitrapyrin or size fractionation treatments, to separate the processes of dark carbon fixation by nitrifiers and phytoplankton, may improve the efficacy of assays using NH₄⁺ stimulation of dark carbon fixation to specifically indicate N deficiency in natural algal communities.