Microscopic and Genetic Characterization of Bacterial Symbionts with Bioluminescent Potential in Pyrosoma atlanticum
The pelagic tunicate, Pyrosoma atlanticum, is known for its brilliant bioluminescence, but the mechanism that causes this bioluminescence is unknown. This study identifies the bacterial symbionts of P. atlanticum individuals collected in the northern Gulf of Mexico believed to be responsible for the luminescence using various methods such as electron microscopy, light microscopy, and molecular genetics. The bacteria are localized within a specific pyrosome light organ. Bioluminescent symbiotic bacteria have now been identified and composed >50% of taxa in tunicate samples (n=13?) by utilizing current molecular genetics methodologies. While searching for bacterial lux genes in (n=2) tunicate samples, we serendipitously generated tunicate mitochondrial sequences which were used for P. atlanticum pyrosome identification. Furthermore, *** a total of 396K MiSeq16S rRNA reads provided pyrosome microbiome profiles to determine bacterial symbiont taxonomy. After comparing with the Silva rRNA database, a 99% sequence identity matched a Photobacterium sp. R33-like bacterium dominated to the most abundant bacteria within P. atlanticum samples. Photobacterium is known as one of three luminescent genera in the family Vibrionaceae. Specific 16S rRNA V4 probes for fluorescence in situ hybridization (FISH) verified what we now term Photobacterium-Pa1 location around the periphery of each luminous organ of the pyrosome. Furthermore, scanning and transmission electron microscopy (SEM, TEM respectively) examination confirmed a rod-like bacterial presence in the light organ, which also appear distributed intracellularly. This intracellular bacteria localization is unique and may represent a bacteriocyte formation found in other invertebrates.