How Vibrio fischeri copes with the antimicrobial properties of the squid's blood pigment is an unanswered question, but agents with selective antimicrobial activity are not unknown: "People tend to assume that whenever they encounter an antimicrobial, it is meant to kill everything," Ruby notes. "Here, we know some bacteria are being courted."
The second role played by the hemocyanin protein in helping to establish symbiosis is more in keeping with its traditional function of ferrying oxygen. The oxygen-transporting properties of hemocyanin are exploited by the squid as the symbiotic population of Vibrio fischeri requires lots of oxygen to fuel the chemical reaction that causes the microbe to light up in the dark. The animal seems able to direct its symbionts to modulate the acidity of the crypts where they take up residence, creating oxygen rich niches at night while suppressing the flow of oxygen during the day when the squid has no need of its companions' glow.
"At night the squid is creating an environment that is more acidic, where oxygen is more easily dumped," says Ruby.
"Oxygen is really pivotal," adds McFall-Ngai. "There is a lot of energy that goes into making light."
The new findings, according to the Wisconsin biologists, help reveal some of the hidden rules of symbiosis, processes that are also likely occurring in higher animals, including humans, who also depend on microbes to perform critical services.
"There is a dynamic interplay in symbiosis," says McFall-Ngai, who designed and performed the study with UW-Madison post-doctoral fellow Natacha Kremer, the lead author of the Proceedings of the Royal Society B report. "In humans, there is an ecological succession in microbiota. What we are looking for in
|Contact: Margaret McFall-Ngai|
University of Wisconsin-Madison