Sometimes in our daily lives as scientists we lose sight of what attracted us to science in the first place. It is easy to get lost in the demands and deadlines and never stop to marvel at the amazing complexity of it all. I am as guilty as anyone of forgetting the things that made me fall in love with biology so long ago. A case in point: squids.
What about squids is so inspiring, you might wonder? Well for one thing they light up, and that by itself rates pretty high on the coolness scale. Maybe you already knew that they could light up, but did you know that they use bacteria to do it? Or that the squid and bacteria exist symbiotically in an amazingly orchestrated balance? The relationship is much more complex than just bacteria living in the the light-emitting organs of squid, it actually allows the squid to control the size (and perhaps light output) of the bacterial population.
In a study published in the Proceedings of the National Academy of Sciences, researchers followed the symbiotic relationship between the Hawaiian bobtail squid (Euprymna scolopes) and the luminescent bacterium (Vibrio fischeri) that inhabit the light-emitting organs of these tiny creatures (1). The squid use the light to fool predators that might be hunting at night when the squid are most active. The bacteria get food and lodging from the squid.
Scientists already knew that these creatures followed a daily ritual where upon at dawn they expelled roughly 90% of the V. fischeri that colonized their light organs. In addition, the light intensity of the bacterial also follows a cycle, producing the brightest light at night when the squid need it most. In the PNAS study, Wier, et al. looked at the molecular choreography of this symbiotic cycle.
By analyzing the timing of up regulation of genes in the squid light organ tissues, Weir et al. found that in the hours before dawn, the squid turns on genes that control cytoskeleton production. This causes the tissue of the light organs to change dramatically. At around the same time, the bacteria ramp up expression of genes required for catabolism of chitin and GlcNAc. Thus, at the time when the squid needs light production to be at its highest, the bacteria are using chitin fermentation to generate energy.
Immediately after expulsion of the bacteria, the squid turn down the cytoskeleton genes and their tissues rapidly regain their normal characteristics, with microvilli interfacing with the bacterial population. Similarly, just after the dawn expulsion, the remaining bacterial population also changes gene expression, turning on genes associated with anaerobic respiration (versus fermentation) of glycerol. This change allows the bacteria to use the vascular membranes as food, and once this nutrient supply is exhausted, to incorporate the fatty acids freed from the vascular membranes directly in to their membrane lipids. So, when light production is not as important as re-establishing the bacterial population, the squid trigger a metabolism shift in the bacteria away from fermentation to anaerobic respiration.
For me, the article was a good reminder about how wondrously complex is the world around us. And how exciting it can be when science brings just a bit of that complexity into sharper focus.
Wier AM, Nyholm SV, Mandel MJ, Massengo-Tiassé RP, Schaefer AL, Koroleva I, Splinter-Bondurant S, Brown B, Manzella L, Snir E, Almabrazi H, Scheetz TE, Bonaldo Mde F, Casavant TL, Soares MB, Cronan JE, Reed JL, Ruby EG, & McFall-Ngai MJ (2010). Transcriptional patterns in both host and bacterium underlie a daily rhythm of anatomical and metabolic change in a beneficial symbiosis. Proceedings of the National Academy of Sciences of the United States of America, 107 (5), 2259-64 PMID: 20133870
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