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Planetary protection rules could be a reason not to bring biofilms to Mars, but because several landers and satellites that crashed or landed on Mars were probably not sterilized these rules were broken already many times.

Nostoc Commune is a species of cyanobacteria that develop a biofilm that serves as a protection against harsh conditions.
They use the energy of light to synthesize organic compounds from CO$_2$ and produce O$_2$ this way.

enter image description here Photo taken by YAMAMAYA

Because of this biofilm Nostoc Commune bacteria can survive high levels of UV radiation, are resistant to both heat and cold (-269⁰ C !), repeated patterns of freezing and thawing and can survive long periods without water.
Moreover it can withstand acidity and is salt-tolerant, but it prefers alkalinity (lots of CO$_2$ !).

So why not place these bacteria with their biofilm in a simple plastic cover on a clay-rich, lowland region near the equator so that they are only protected against the dusty winds ?

With a camera to take only one image a day it would be very interesting to see if they can survive the harsh conditions on this planet.

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    $\begingroup$ The only bit of this which is not doable now on Earth is replicating the exact chemnistry of the Martian soils, and the reduced gravity, which seems unlikely to be much of a factor for something this small. Current rovers will give us a lot of information about the soils, and in a few more years, probably return samples, which can be analyzed on Earth to give complete information. Then we can replicate the conditions and do this experiment on Earth. $\endgroup$
    – Steve Linton
    Dec 17, 2018 at 12:11
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    $\begingroup$ I think this question is really unclear. While you're asking "why not?", scientists would rather ask "why should we?" and "what would we learn from it?", and the answers to the latter two questions would exclude placing any biofilms on Mars. I'm not sure what your question is aiming at exactly, but if you want to terraform mars, then that's not where we're heading at the moment. $\endgroup$
    – AtmosphericPrisonEscape
    Dec 17, 2018 at 15:32
  • $\begingroup$ @AtmosphericPrisonEscape Like the investigations of Curiosity are very interesting for geologists, placing biofilms could be very interesting for biologists and especially astrobiologists. It would also be a milestone for life in general and thus an advertising event to the public. $\endgroup$
    – Cornelis
    Dec 17, 2018 at 15:47
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    $\begingroup$ So is your actual question then "What could we learn from placing a biofilm on Mars, that we couldn't learn from placing it on a terrestrial Mars-analogue?" $\endgroup$
    – AtmosphericPrisonEscape
    Dec 17, 2018 at 15:50
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    $\begingroup$ Think about planetary formation, planetary as well as atmospheric evolution, and maybe even the origin of life on Earth. If you erase the surface signature of those histories, then we would loose a major comparison point of Earth with a similar planet that stopped being geologically active very early on. $\endgroup$
    – AtmosphericPrisonEscape
    Dec 17, 2018 at 20:03

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Planetary protection is still in force for Mars, see the special sub class of rules for it. While it can be argued that there have probably been lapses with the crashed orbiters listed at the link, that is different from deliberately flying a carefully nurtured sample there. Even if they were DOA it would mean that every other life detection investigation thereafter would have to be engineered to not get a false positive from this contamination.

The second part is what this experiment would prove on Mars (at great cost) that could not be achieved here on Earth. The combination of near-vacuum and lack of water is going to prevent them doing any breeding/living so it would largely be a question of whether the known hazards would kill them before some unknown thing (such as the potentially reducing nature of the soil) did the job. And that would require much more than a camera since it would be both slow and happening at the cellular level.

There is some valid science here but it would consist of Earth-based testing to find the outer limits of this sort of life, and Mars-based testing of empty greenhouse structures to find low energy ways to get inside those limits by increasing air pressure and temperature, managing UV, controlling water loss and dealing with the chemical makeup of the soil enough to not be immediately lethal.

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