system
Given the following post, respond as if you're commenting on Reddit:
human
[Avengers] How was Ironman able to survive underwater in the suit but not in space?

If he could breath underwater, why couldn't he breath when he brought the missile through the portal?
gpt
His suit ran out of power which caused the life support system to fail.

When Fury calls him, he tells Jarvis to save what power he has left 'for the turn' (in other words, turning the missile upwards towards the portal). He then rides the missile in to the portal.

Although, you would think, he would have a few minutes of air in the helmet, the loss of power may also have affected his heart. Though the suit isn't powered by the arc reactor in his chest, he may have risked his own life further by diverting power from it to ensure the missile safely made it through the portal.