Vibrio fischeri

Ecology

V. fischeri can be located in the microhabitats of its hosts’ light organs, but can also be found living freely as ‘marine snow’, in fecal pellets, as saprohytes (Herring, Bioluminescence), and amongst the microbial flora in the guts of marine animals. The bacterium is distributed in the pelagic zone of temperate and subtropical waters (Ruby, Oxygen). However, higher cell populations of V. fischeri is achieved when it is associated with its host. The symbiosis benefits E.scolopes by using the bioluminescent bacteria in a camouflage strategy called ‘counter-illumination’. The light produced by the bacterium is projected ventrally by the squid, which mimics down dwelling moonlight when viewed from below. The squid effectively projects no shadow, and it can regulate the amount of bacterial illumination with the light organ (Ruby, Oxygen). The squid provides housing and nutrients, especially carbon and nitrogen, in the form of proteins and peptides inside the crypts of the light organ (Serc). In providing a nutrient rich environment, the bacteria population increases to 1.09 cells of which 90-95% are expelled every morning by the squid. The remaining bacteria in the light organ are replenished daily, and the squid becomes a vehicle to produce an abundance of V. fischeri (Ruby, Oxygen). The symbiotic relationship between strains of V. fischeri and their particular hosts is highly specific (McFall-Ngai, Negotiation). Newly hatched squids are born without the bacterium and must acquire them from the surrounding seawaters. In waters inhabited by E.scolopes, the abundance of V. fischeri constitutes only 0.1% of total microbes per mL of seawater, yet only the specific strain of V. fischeri can effectively colonize and remain in the light organ of the squid (McFall-Ngai, Negotiation). The bacterium is housed inside the crypts of the light organ located in the squid’s mantle cavity (serc). Once the bacterium is acquired and colonizes the crypts, morphogenesis occurs for both the bacterium and its host (McFall-Ngai, Negotiation). The bacteria lose the flagella, and the cells decrease in size. As for the host, the ciliated fields used to obtain the bacterium undergo programmed cell death, and the light organ swells (McFall-Ngai, Negotiation). Colonization by the bacterium concurrently alters gene expression in the squid and the bacterium (Uconn).