We'z dont'z need'z ta breathe if we dun't wantz ta. Datz watz da WAAAGH! for If ya needz ta breathe in da middle of yellin' out WAAAGH! You'z a stinkin gretchin git
Orks are also capable of making their own ecosystem from scratch due to their releasing of spores which create more fungi and Orks. I've heard that inside the bigger Roks, fungi has been grown specifically to perform photosynthesis inside the Rok. I've also seen some (canon?) examples of certain Orks performing photosynthesis themselves. So yeah... ... people really underestimate the brilliance that is Ork physiology and biology...
