With all the fights that we see in Season 2, there is a substantial amount of Reach fighting. Blue Beetle clashing with Impulse, Batgirl, Black Beetle, et cetera. But- according to a quote from Black Beetle in episode 12 of the second season, he "barely touches the stuff" in regards to oxygen.
So, the question is, what do the Reach breath? Is it the Scarab that regulates the body to consume less oxygen, or an entirely different chemical? Is it just the Reach's physiology?
Throughout the season, we see that Blue Beetle does not have to use a rebreather to breath underwater, in the very first episode. It's a longer swim than just a dip, as Robin had one equipped, but Blue did not. This could point to the fact that the Scarab synthesizes oxygen from any source available- air or water. But- what of the Martian Scarab?
The abilities of Martians are remarkable- including density shifting, shapeshifting, telepathy, telekinesis, a length of invulnerability, flight, camouflage/invisibility, and more, depending on the Martian itself. Green Beetle displays all of these, pointing to the fact that he is a normal Martian, if perhaps for the Scarab attached to his spine. Martians themselves are able to adapt to new environments to breath and function in any environment, seen as Miss Martian is able to shapeshift gills to breath underwater.
The atmosphere of Mars is 95% CO2, 3% Nitrogen, 1.6% Argon, and other trace gases. Meaning that the Martian species is most likely able to breath CO2 most easily, as the other contents of their thin atmosphere are most likely too hard to come by to synthesize effectively.
However, their radiation levels are much higher as their atmosphere does not properly protect it, possibly allowing for their species to evolve as shapeshifters. But that is for another day- I've already gone too far into this.
Back on track- A Martian ability, as we know, is shapeshifting, allowing them to change the composition of their bodies to suit the environment. However, by the establishment of Miss Martian shapeshifting gills onto her body when going underwater, it stands to reason that they do need some kind of chemical like O2 to breath, and therefore cannot go into space without an oxygen mask.
But, Green Beetle is able to fly in space and attack during episode 15. This points to the fact that Scarabs are capable of not only synthesizing O2 from any component, be it air or water, but also storing it so that the host can go into oxygen-depleted environments and still be able to fully function.
That being said, it can be hypothesized that the Scarabs store oxygen, and, in doing so, are able to supply it to the host body as needed for long trips without it. Therefore, the Reach species needs very little of it as the have evolved side by side with the technology that helps them respirate, negating the need for a lot of oxygen and naturally selecting those that require less O2 to function at full capacity than others.
