Shedding Light on the Bioluminescence “Paradox”

T he fascinating biochemistry, genetics, and cell density-dependent regulation of bacterial bioluminescence provoke a challenging question. What good is it to bioluminescent bacteria? There may be no single answer to this seemingly simple question. However, two recent advances shed new light on the problem. First, studies of the symbiosis between the bioluminescent bacterium Vibrio fischeri and the Hawaiian bobtail squid bring an ecologically relevant niche for a bioluminescent bacterium into focus under the discerning lens of controlled laboratory experimentation. Second, progress in understanding the genetics of V. fischeri, including genomic sequencing of a squid symbiont, is enabling researchers to analyze how luminescence integrates into the physiology of this bacterial species and to test specific hypotheses about what advantage light production confers on the bacteria. Understanding how bioluminescence aids V. fischeri in a squid light organ will likely not be the final word on how bioluminescence benefits bacteria. Differences among bioluminescent bacteria intimate that this ancient system confers varied selective advantages. For example, “cryptically luminescent” Vibrio pathogens, such as Vibrio salmonicida, produce luciferase, the enzyme responsible for bioluminescence, but little or no aldehyde substrate, and may use luciferase to form “dark reaction” hydrogen peroxide as a host-damaging virulence factor. In contrast, some strains of Vibrio logei produce an accessory Y1 protein that shifts their emitted light to a yellow wavelength, which may have special significance for this bacterium. Bioluminescence offers many such puzzles, and it is unlikely a single solution will solve them all. Nonetheless, it is an exciting moment in bioluminescence research, with recent advances offering the promise of answering the longstanding question, “how can bioluminescence help bacteria?” Although researchers learned nearly a century ago that luminescence reduces oxygen and that symbiotic bacteria inhabit the light organs of squids, what was unimaginable until very recently is our ability to analyze the V. fischeri genome sequence and to combine this knowledge with the ability to genetically manipulate these bacteria and observe them under controlled laboratory conditions in the ecologically relevant environment of a natural squid host.