TRANSFER OF N 2 AND CO 2 FIXATION PRODUCTS FROM ANABAENA OSCILLARIOIDES TO ASSOCIATED BACTERIA DURING INORGANIC CARBON SUFFICIENCY AND DEFICIENCY 1

Transfer of N 2 and CO 2 fixation products from the bloom forming blue‐green alga, Anabaena oscillarioides Bory, to attached and free swimming bacteria is common during active growth of the former. Incubation with 15 N 2 and 14 CO 2 followed by size fractionation filtration reveals that: i) magnitudes of fixed N and C excretion, relative to N 2 and CO 2 fixation, are dictated by dissolved inorganic carbon (DIC) availability for A. oscillarioides photosynthetic production, ii) associated bacteria exhibit preferences for recently fixed excreted N compounds, iii) bacterial utilization of excreted N is independent of ambient light conditions, and iv) lag times between N 2 fixation and detectable bacterial assimilation of excreted fixed N compounds are ca. 1–2 h. Both 14 NH 4 Cl dilution and Hg(NH 3 ) 2 Cl 2 precipitation techniques indicate that NH 3 is a major excretion product from A. oscillarioides, particularly during DIC limited growth. Active N and C excretion and transfer to associated bacteria are features of viable A. oscillarioides filaments. Hence, transfer of these metabolites reflects complex mutualistic, and possibly symbiotic associations rather than solely signaling senescence.