"Long before it's in the papers"
February 09, 2016


That slime can see you—somewhat

Feb. 9, 2016
Courtesy of eLife
and World Science staff

Af­ter more than 300 years of look­ing, sci­en­tists say they have fig­ured out how some slimy bac­te­ria “see”—and that they do it in a sur­pris­ingly si­m­i­lar way to us.

Cer­tain mi­crobes known as cya­no­bac­te­ria bas­ic­ally use their whole ti­ny, glob­u­lar bod­ies as eye­balls, ac­cord­ing to the re­search. In that way, they at­tain a vi­sion that’s blur­ry, but suit­able for their pur­poses.

How bacteria are optical objects, in a diagram provided by the the publisher of a new study (courtesy eLife)

“The idea that bac­te­ria can see their world in bas­ic­ally the same way that we do is pret­ty ex­cit­ing,” said lead re­searcher Con­rad Mul­l­i­neaux, a bi­ol­o­gist at Queen Mary Un­ivers­ity of Lon­don.

“The fact that bac­te­ria re­spond to light is one of the old­est sci­en­tif­ic ob­serva­t­ions of their be­hav­ior,” he added. But the fact that they use their bo­dies as eye­balls, he said, though “pret­ty ob­vi­ous with hind­sight... we nev­er thought of it un­til we saw it. And no one else no­ticed it be­fore ei­ther, de­spite the fact that sci­en­tists have been look­ing at bac­te­ria un­der mi­cro­scopes for the last 340 years.”

His team of Brit­ish and Ger­man re­search­ers de­scribes in the jour­nal eLife how bac­te­ri­al cells act as the equiv­a­lent of a mi­cro­scop­ic eye­ball or the world’s old­est, small­est cam­era eye.

Cya­no­bac­te­ria live in huge num­bers in wa­ter bod­ies or can form a slip­pery green film on rocks. The spe­cies used in the stu­dy, Sy­ne­cho­cys­tis, lives in freshwa­ter lakes and riv­ers. Cyanobac­te­ria evolved an es­ti­mat­ed 2.7 bil­lion years ago. The fact that they can pro­duce ox­y­gen and con­vert car­bon di­ox­ide to or­gan­ic ma­te­ri­al us­ing en­er­gy from the sun—the pro­cess of pho­to­syn­the­sis—thought to have caused mass ex­tinc­tions and the old­est known ice age.

As pho­to­syn­the­sis is cru­cial to these bac­te­ri­a’s sur­viv­al, sci­en­tists have sought to un­der­stand how they sense light. Pre­vi­ous stud­ies have shown that they con­tain light-sens­ing mo­le­cules and that they can per­ceive and ap­proach a light source, a pro­cess called pho­to­tax­is.

The new study found they can do this be­cause the cell body acts like a lens. As light hits the round sur­face, it re­fracts, or bends in­to a point on the oth­er side of the cell. This trig­gers move­ment by the cell away from the fo­cused spot. With­in min­utes, the mi­crobe grows ti­ny tentacle-like things called pi­li that reach out to­wards the light. As they at­tach to the sur­face that they’re on, they re­tract and pull the bac­te­ria along.

Sy­ne­cho­cys­tis serves as a spher­i­cal lens but the team thinks that rod-shaped bac­te­ria can al­so trap light and sense the di­rec­tion it is com­ing from us­ing re­frac­tion, act­ing like an op­ti­cal fi­ber.

The sci­en­tists called the find­ings a likely ex­am­ple of con­ver­gent evo­lu­tion—where two or more or­gan­isms evolve si­m­i­lar struc­tures, but us­ing dif­fer­ent ge­net­ic means.

“The phys­i­cal prin­ci­ples for the sens­ing of light by bac­te­ria and the far more com­plex vi­sion in an­i­mals are sim­i­lar, but the bi­o­log­i­cal struc­tures are dif­fer­ent,” said co-author An­negret Wilde from the Un­ivers­ity of Frei­burg in Ger­many.

A Sy­ne­cho­cys­tis cell is about half a bil­lion times smaller than the hu­man eye. As with the ret­i­na in the hu­man eye, the im­age on the rear of the cell will be up­side down. But its res­o­lu­tion will be much low­er, so only a blurred out­line of any ob­ject can be per­ceived. The abil­ity of op­ti­cal ob­jects to dis­tin­guish fi­ne de­tail is de­ter­mined by “an­gu­lar res­o­lu­tion.” In the hu­man eye this is an im­pres­sive 0.02 de­grees; in Sy­ne­cho­cys­tis, an esti­mated 21 de­grees.

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After more than 300 years of looking, scientists say they have figured out how some slimy bacteria “see”—and that they do it in a surprisingly similar way to us. Certain microbes known as cyanobacteria basically use their whole tiny, globular bodies as eyeballs, according to the research. In that way, they attain a vision that’s blurry, but good enough for their purposes. “The idea that bacteria can see their world in basically the same way that we do is pretty exciting,” said lead researcher Conrad Mullineaux, a biologist at Queen Mary University of London. His team of British and German researchers describes in the journal eLife how bacterial cells act as the equivalent of a microscopic eyeball or the world’s oldest and smallest camera eye. Cyanobacteria live in huge numbers in water bodies or can form a slippery green film on rocks. The species used in the study, Synechocystis, lives in freshwater lakes and rivers. Cyanobacteria evolved an estimated 2.7 billion years ago. The fact that they are able to produce oxygen and convert carbon dioxide to organic material using energy from the sun—the process of photosynthesis—is thought to have caused mass extinctions and the oldest known ice age. As photosynthesis is crucial to these bacteria’s survival, scientists have sought to understand how they sense light. Previous studies have shown that they contain light-sensing molecules and that they can perceive and approach a light source, a process called phototaxis. The new study found they can do this because the cell body acts like a lens. As light hits the round surface, it refracts, or bends into a point on the other side of the cell. This triggers movement by the cell away from the focused spot. Within minutes, the microbe grows tiny tentacle-like things called pili that reach out towards the light. As they attach to the surface that they’re on, they retract and pull the bacteria along. “The fact that bacteria respond to light is one of the oldest scientific observations of their behavior,” said Mullineaux. “Our observation that bacteria are optical objects is pretty obvious with hindsight, but we never thought of it until we saw it. And no-one else noticed it before either, despite the fact that scientists have been looking at bacteria under microscopes for the last 340 years,” he said. Synechocystis serves as a spherical lens but the team thinks that rod-shaped bacteria can also trap light and sense the direction it is coming from using refraction, acting like an optical fiber. The scientists called the findings a likely example of convergent evolution—where two or more organisms evolve similar structures, but using different genetic means. “The physical principles for the sensing of light by bacteria and the far more complex vision in animals are similar, but the biological structures are different,” said co-author Annegret Wilde from the University of Freiburg in Germany. A Synechocystis cell is about half a billion times smaller than the human eye. As with the retina in the human eye, the image on the rear of the cell will be upside down. But its resolution will be much lower, so only a blurred outline of any object can be perceived. The ability of optical objects to distinguish fine detail is determined by “angular resolution.” In the human eye this is an impressive 0.02 degrees. The team estimate that in Synechocystis it is about 21 degrees.