Deep sea colonization has been dreamt of for nearly a century now but most approaches, such as the recent movie Underwater rely on maintaining the colony at 1ATM rather than circulating mixed gas at ambient pressure. If you don't need to return to the surface, it seems easier to adapt the colonists to the environment. But in my story I need to address the physiological problems such a life poses, in a science-based way. A [different question] asks about the maximum depth of "comfortable living" which doesn't ask or provide science-based insights into physiological challenges at a specific depth.
This facility is planned for 540 fathoms depth. Assume the human colonists had ample time to habituate (say, 4 weeks to steadily descend from the surface, maintaining proper partial pressure O2 all the way), and that the He-N2-O2 gas mix in the colony has a ppO2 (partial pressure) of 1.31 bar, and Fraction of Oxygen is 1.3%.
The air will be at ambient pressure - approximately 10,000 kPa. This makes construction vastly cheaper and simpler.I understand that water would boil somewhere over 500° F and there would be many other physical challenges, but what if any physiological problems could be expected from extended, or even permanent residence in this colony? Please be specific about the physiological issues, using science-based references. Ambiguous answers are not useful answers, so please do not include broad responses such as "they could survive", or "be comfortable" at this depth.