PHYSIOLOGICAL AND MORPHOLOGICAL STATE OF THE SYMBIOTIC BACTERIA FROM LIGHT ORGANS OF PONYFISH

Symbiotic, bioluminescent bacteria (Photobacterium leiognathi) within and directly removed from the light organs of freshly sacrificed Philippine and Japanese ponyfish (family Leiognathidae) were analyzed for light production, oxygen uptake, morphology, and density. Luminescence averaged 2.4 x 10⁴ quanta·s^-1 · cell^-1 for bacteria from 24 fish (6 species in 3 genera), more than 10 times the maximum luminescences of P. leiognathi grown in culture. Light production (depending on the in vivo quantum yield for luminescence, 0.1 to 1.0) accounted for 1.7 to 17% of the total oxygen utilized by bacteria from the light organ, substantially more than found for P. leiognathi in culture. Bacteria from the light organ were non-motile, non-flagellated coccobacilloid to short rod-shaped cells (1.6 x 3.2 µm), whereas in culture they showed motility and polar flagellation. In situ doubling time for the population of light organ bacteria was estimated to be approximately one day, or 20 to 30 times slower than in culture. Within the tubules of the light organ, the bacteria were solidly packed inside elongate, thinly-walled saccules, with one to 20 saccules tightly filling each light organ tubule. The saccules held the bacteria at a density (calculated from bacterial cell and saccule volumes) of approximately 1 x 10¹¹ cells·ml^-1, which is a density roughly 15 times greater than estimated from total light organ volume. These findings lead to a maximalluminescence, minimal-growth bacterial model of this symbiosis.