TRANSFER OF N2 AND CO2 FIXATION PRODUCTS FROM ANABAENA OSCILLARIOIDES TO ASSOCIATED BACTERIA DURING INORGANIC CARBON SUFFICIENCY AND DEFICIENCY1

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Abstract
Transfer of N2 and CO2 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 15N2 and 14CO2 followed by size fractionation filtration reveals that: i) magnitudes of fixed N and C excretion, relative to N2 and CO2 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 N2 fixation and detectable bacterial assimilation of excreted fixed N compounds are ca. 1–2 h. Both 14NH4Cl dilution and Hg(NH3)2 Cl2 precipitation techniques indicate that NH3 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.