Unicellular cyanobionts in open ocean dinoflagellates, radiolarians, and tintinnids: Ultrastructural characterization and immuno-localization of phycoerythrin and nitrogenase

Cyanobacterial symbionts (cyanobionts) have been identified forming associations with various open ocean eukaryotic host genera, including two dinophysoid genera, Histioneis sp. and Ornithocercus sp., two radiolarians, Spongastaurus and Dictyocoryne truncatum, sp., and a tintinnid, Codonella sp. The TEM analysis revealed that single individual hosts were closely associated with one to two different cyanobacterial morphotypes (cyanobionts) and two hosts had in addition to cyanobionts, one to two bacterial cell types. Eleven significantly (P < 0.01) different cell types were identified as cyanobionts, with cell diameters ranging 0.5 +/- 0.38-3.7 +/- 0.66 mu m. Using immunogold-labeling techniques coupled to the TEM, four of the five cell types contained phycoerythrin (PE) at high levels (> 71 +/- 28 gold particles.mu m(-2)). Immunolabeling-TEM using nitrogenase antisera demonstrated a significant (P < 0.01) nitrogenase content in cell type four cyanobionts of Histioneis sp. host 1 (39 +/- 34 gold particles.mu m(-2)). The cyanobionts of the radiolarians were of a cell diameter (0.5-0.8 mu m) and showed ultrastructural characters (peripheral thylakoids) reminiscent of Prochlorococcus sp. Also, an open ocean tintinnid, Codonella sp., was shown to contain cyanobacteria as symbionts for the first time. In all cyanobionts, glycogen storage was obvious, no cellular degradation was visible, cells were observed in the process of cellular division, and antisera localization was apparent. These observations suggest that the relationship between host eukaryote and cyanobacteria is an active one, and likely symbiotic.