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I found some popular articles (e.g. nbcnews and iflscience) that bacteria can "see," but I highly doubt it's in the same way as people do just from looking at the limitations in the vision of small animals like insects. So what exactly are bacteria capable of "seeing" or what are the limitations of their vision?

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  • $\begingroup$ Can you please link to some of these articles? $\endgroup$ – Remi.b Mar 27 '18 at 2:38
  • $\begingroup$ Well I mean google can take care of that, for instance nbcnews.com/science/science-news/… or just iflscience.com/environment/… $\endgroup$ – John Joe Mar 27 '18 at 2:39
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    $\begingroup$ @JohnJoe you are asking people here to help you and then say "google missing information". That is not very nice. Meanwhile, please post a quote that you need help understanding, right now question is unclear. $\endgroup$ – aaaaa says reinstate Monica Mar 27 '18 at 2:50
  • $\begingroup$ I'm not asking for people to verify the sources of what I said though. The way the articles phrase it is pop science anyway, not real science, so there's not even a point to referencing that information. However, the general topic is still relevant. Multiple news sources would not all lie about the same scientific subject, it is only that they do not portray it accurately. $\endgroup$ – John Joe Mar 27 '18 at 2:51
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Simple answer is that many simple organisms, including bacteria, carry light-sensitive molecules. One example is halorhodopsin (not bacterial but archaeic). This is a molecule, light-gated ion pump, that reacts to light, thus allowing organism to react to photons by changing concentration of certain ions inside the cell.

Study from Stanford described similar molecule found in bacteria.

In conclusion, if by "vision" you understand ability to react to light in the visible part of the spectrum, then a lot of organisms possess such ability. However, they lack other crutial parts of human vision.

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  • $\begingroup$ I think that makes sense, thank you. So just to be clear, you're saying bacteria have the ability to sense simply "that" there is light in different levels almost like an on-off switch, but not the detail we see in larger animals? Can bacteria in any way distinguish either different frequencies of light, or, any kind of 2D or 3D structure? $\endgroup$ – John Joe Mar 27 '18 at 2:58
  • $\begingroup$ @JohnJoe Most likely they don't. The most they can do is differentiate between high and low light. That too not by neural processing, rather by differential activity of light gated ion pumps. $\endgroup$ – Roni Saiba Mar 27 '18 at 3:42
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Because bacteria are extremely small, the principles of optics prevent them from having lenses or other organs capable of determining the direction from which light is incident. On the other hand, an entire spherical bacterium can potentially act as a lens, concentrating light from a given direction onto a corresponding position on the opposite side of the bacterium, and thus forming a very crude image. Bacteria are certainly capable of responding to the frequency of incident light, if they contain molecules that selectively absorb light in specific frequency ranges. And, they should be able to respond to the brightness or intensity of incident light, if those molecules return to their normal state shortly after absorbing a photon. Of course, additional molecular machinery would need to be present in a bacterium to translate absorption of a photon into any sort of meaningful response.

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    $\begingroup$ @xusr, that paper makes a pretty convincing case that I was too hasty in rejecting the possibility that a relatively large, spherical bacterium could form enough of an image to "perceive" the direction of light. The directional response to light described in the paper is, indeed, plausible. $\endgroup$ – S. McGrew Mar 27 '18 at 18:51
  • $\begingroup$ Is there some way to combine both answers? They both have decent points. $\endgroup$ – John Joe Mar 27 '18 at 19:44
  • $\begingroup$ I edited the answer accordingly. $\endgroup$ – S. McGrew Mar 27 '18 at 21:28
  • $\begingroup$ Great, the only thing you're missing is credible references. $\endgroup$ – John Joe Mar 28 '18 at 14:16
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    $\begingroup$ The reference given by @xusr, Cyanobacteria use micro-optics to sense light direction [elifesciences.org/articles/12620] is very good. If bacteria were large and spherical - on the order of a millimeter in diameter- and if their refractive index could be on the order of 2 rather than ~1.4 to ~1.6, then they would be able to form a high-quality image by this means: parallel rays from any given direction would come to a fine focus on the surface on the opposite side of the bacterium. However, with a refractive index of 1.4, the focus lands far beyond the surface, hence a low res image. $\endgroup$ – S. McGrew Mar 28 '18 at 16:11
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I'm surprised at the focus on light in all these answers! To my knowledge, light is absolutely not the way that some 'bacteria' may be able to see.

Try looking on youtube for a macrophage (white blood cell) chasing down a bacteria.

You can see it actually following the bacteria - it's crazy. How on earth does it know where it is?

Chemical signalling, of course!! Sort of like a neurotransmitter. They secrete chemicals and follow the trail - it could be really simple, such as, the transmitter converts to something else once it comes into contact with a phospholipid (of course indicating a cell membrane). I haven't researched that bit, but I hope it will allow you to direct your own train of thought/research.

Macrophages also have things called 'Toll-Like Receptor' proteins on their membranes, and they produce these things called 'pseudopods' that are essentially feelers/protrusions of the cell membrane. The Toll Like Receptor proteins recognise antigens by detecting Pathogen Associated Molecular Patterns that are common on the surface of pathogens/non-self objects (funnily enough). This is how your most basic innate immune system works- if the TLR protein on the pseudopod (wavy, baddie-finding arms) comes into contact with something it recognises as 'bad', it binds to it and draws it in for the white blood cell to engulf.

But it doesn't actually destroy/digest it at that point, because the TLR protein isn't perfect and sometimes there's some friendly-fire. A T Cell (I think T-Helper) comes along and 'double checks' that the macrophage hasn't picked up a friendly object. If it HAS picked up a friendly object, the T Cell will actually kill the macrophage for its mistake (usually).

OMG FRED, WHAT HAVE YOU DONE

Or at least that's how I like to imagine it goes down.

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    $\begingroup$ Chemical signals aren't exactly vision though, they aren't responding to photons. $\endgroup$ – John Joe Mar 29 '18 at 2:14

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