You might wonder, okay we are required by the Outer Space Treaty to protect Earth from harmful contamination from Venus. But does it really matter if life from Venus gets established on Earth, or genetic material gets transferred to Earth archaea via GTA's.? Would it indeed be harmful if this happens? If we can show that it is not harmful, there is no cause for concern, and also, perhaps we don't need to worry about the OST either (I'm not saying this as a lawyer, but after all, the clause refers to "harmful contamination", so if you could prove it is not harmful - would that not be enough to say it is okay to return life from Venus clouds).
So, just to go over it quickly, as some of the things that could happen are similar to those for Mars.
Life returned to Earth from another planet may well be harmless, but there are many ways that it could cause harm, also. We can't know with reasonable certainty until we know something about the form of life and how it works.
Finally, there is the possibility that Venusian life is not based on DNA but some other basis such as XNA (change of backbone) or something more radical than that. If so then we can't really generalize from DNA to capabilities of XNA.
Rotating DNA animation. Could life on Venus have a different backbone from DNA , using PNA, HNA, TNA, GNA or other XNA?
Here XNA is a general term for nucleic acid analogues - with the same bases as DNA but a different "backbone", in place of the Deoxyribose of DNA. These include HNA, PNA, TNA or GNA (Hextose, Peptide, Therose or Glycol NA).
The PNA world hypothesis for instance suggests that life on Earth went through an earlier stage where it used PNA (peptide nucleic "acid") before it started to use RNA or DNA. That's because DNA and RNA are so complex it is a little hard to see how they arose from non living chemicals alone.
Life on Venus could have done the same, but maybe didn't end up as DNA. It may still use PNA or perhaps it evolved to use some different form of XNA.
That raises the possibility that XNA based life could be better at coping with Earth conditions than DNA itself. This could be possible, if it is really a completely different form of life with different metabolism, cell machinery, etc. and has never had any previous contact with the Earth environment.
The Venusian clouds indeed might give us one of our best chances of finding XNA in our solar system - in the remote case where there is life there. That's because for hundreds of millions of years, and possibly for billions of years it has been almost impossible for Earth life to be transferred to Venus. The surface of Venus is so hot that Earth life would be destroyed soon after it got there, if it made it all the way to the surface of Venus. So, it's hard to see Earth life reaching the upper Venus atmosphere. Any particles light enough to be captured without damage would surely be thoroughly sterilized by UV and solar storms and cosmic radiation on the voyage from Earth to Venus.
The other way around also, then it is almost impossible for the cloud top life of Venus, if it exists, to be ejected through the thick atmosphere as the result of meteorite impacts on the surface of Venus. A huge asteroid impact on Venus would disturb the cloud deck for sure, but could even a giant impact send significant amounts of the high Venusian atmosphere into space? And if it did, again you have the issue - what could protect the life so that it survives the journey all the way to Earth?
Chandra Wickramasinghe has put forward a controversial theory that the solar wind could transfer microbes from the upper Venus atmosphere (high above the cloud decks) to Earth at times when the planets are aligned. See Microbes Could Travel from Venus to Earth However other scientists find his research unconvincing, so far, with many details to be filled in. For instance, it doesn't seem that the solar wind would have enough energy to remove a microbe from the Venus gravity well, since it is far heavier than the ions it can transport. Also, any dormant microbes that did get ejected from Venus would also be vulnerable to cosmic radiation and high levels of UV, which they might not be adapted to.
I don't know of any other suggestion of ways that Venus life could be transferred to Earth in its current state. It could have happened in the earlier solar system when its atmosphere was thinner. But it would still be hard for life to be transferred to Earth - with the gravity not that much less than that of Earth itself. If it also had a thick atmosphere, then there would be the same problems that you have for ejection of microbes from Earth that the easiest way for this to happen would be after huge asteroid impacts large enough to punch a hole in the atmosphere. And how robust was the life back then in the late heavy bombardment?
So - it seems at least possible that life could have evolved independently on Venus, and has been there ever since. If so, it would probably be a form of XNA (as it could only evolve DNA through convergent evolution) depending how likely it is for DNA / XNA to evolve. Another possibility is that Venus, unlike Earth, could have retained multiple different systems of biochemistry simultaneously. Perhaps it had both XNA and DNA, and the cloud life is based on XNA, or some of it is.
In that case, if Venus life is based on XNA, all bets are off as far as planetary protection of the Earth.
We can't say much by analogy with DNA life even about its size, or its properties or its adaptability to different environments. There are other places that could have XNA, including Mars, or comets.
If Venus was habitable recently, then it's easier to have shared life. But if that hypothesis is wrong, and it wasn't habitable for the last several billion years, Venus has been more isolated from Earth than any of those. Even the Europan oceans could potentially share DNA with Earth through impacts on Earth sending debris all the way to Europa. This probably was only be possible for Venus in the very early solar system. The Venusian surface might also have been too hostile for Earth life already by the time Earth was habitable.
Here the situation is similar to the studies of risk for Mars sample return. Often new planetary protection studies bring up the possibility of new risks not considered in previous studies. The 2009 Mars sample return study by the US National Research Council brought up the new possibility that Mars life forms might be smaller than previously thought and added a new recommendation to contain ultramicrobacteria at 0.2 microns across. The 2012 Mars sample return study by the European Space Foundation added another new recommendation, this time to contain Gene Transfer Agents only 0.01 microns across if possible - it was published just after the discovery of easy transmission of GTA's. between unrelated species of microbes in sea water.
Both studies of Mars sample return mention XNA but they do not go into it in any depth, particularly, they don't mention the researches into safety considerations for XNA in Earth laboratories. Also neither study considered the possibility that the life forms to be contained are smaller than the smallest known Earth microbes. This seems at least possible since, though 0.2 microns seems to be the smallest organism that could contain all the cell machinery of modern life, early cells on Earth must have been smaller than the ultramicrobacteria of the order of tens of nanometers across. Also, we have no way to be sure of the size of XNA lifeforms.
The Venus planetary protection study "Assessment of Planetary Protection for Venus Missions" didn't consider GTA's. or XNA. It is rather short. This is all that it says on protection of Earth from Venus life
"The cloud layers in the atmosphere of Venus provide an environment in which the temperature and pressure are similar to surface conditions on Earth. However, the chemical environment in the clouds, and specifically in the cloud droplets, is extremely hostile. The droplets are composed of concentrated (82 to 98 percent) sulfuric acid formed by condensation from the vapor phase. As a result, free water is not available, and organic compounds would rapidly be destroyed by dehydration and oxidation. Therefore any terrestrial organism having survived the trip to Venus on a spacecraft would be quickly destroyed. It is not possible to demonstrate conclusively that a spacecraft returning to Earth after collecting samples of Venus's surface and atmosphere will not come into contact with hypothetical aerial life forms and inadvertently carry them back to Earth; however, this has to be considered an extremely unlikely scenario. At any rate, any life forms that had adapted to living in the extremely acidic environment of Venus's cloud layer would not be able to survive in the environmental conditions found on Earth."
But as we've seen, doubts were raised about their conclusions about the possibility of Earth originated acidophiles to survive in the Venus atmosphere. Also the study was not based on experimentation and we have limited knowledge of the Venus upper atmosphere. We don't know enough yet to make an accurate simulation of it in a laboratory on Earth for testing.
Schultz Makuch is quoted by Space.com as saying:
"As the task force explained, there shouldn't be any significant interaction between putative Venusian cloud microbes and Earth organisms. However, there is some uncertainty because most Earth microbes are still unknown and there are some known organisms that come close to living in Venus-like conditions.We do not know and thus cannot estimate capabilities of any alien organism. Perhaps, if they originated in an earlier Venus ocean they may have still retained the capability to quickly adapt to their earlier environment. Thus, they might be capable of competing in selected, rare niches on Earth, such as volcanic vents... The chances of an indigenous microbial community floating around in the Venusian atmosphere are not remote but are significant in my view"
On the other hand Jim Rummel and David Grinspoon in the same article are quoted as saying they are satisfied with the report.
The clouds may well turn out to be so utterly hostile to Earth life that there is no chance it could survive there. It may well have no Venusian life in it either. But I'm not sure we can conclude this for certain yet, when faced with a diversity of views like this amongst experts. I think it is possible that a new study, taking account of these ideas, would change the provisional classification of the Venus atmosphere for both forward and backward contamination.