Guild Wars 2

I believe it’s stated in lore that sylvari do need to eat, although they need to eat less in sunlight. this suggests that they need to eat for energy, not just for nutrients. this, in turn, means they need oxygen consumption like we do.

This doesn’t work well, photosynthesis doesn’t convert sunlight to mass – they still need the same input of C,H,O,N as an animal.

that is the reason why they need a breather. Because there isn’t enough carbon dioxide in water, especially in the present time when it is mostly dominated by the undead which doesn’t use any oxygen in the water.

Its worse than that – if the sylvari are breathing CO2 then they have an anaerobic metabolism like regular plants. That sort of metabolism can’t supply the high level of energy needed for highly mobile organisms – its why anaerobic micro-organisms were outcompeted and largely wiped out when aerobic metabolisms showed sometime within the last 2.5 billion years.

In order for the sylvari to exist at all they would have to pretty much give up nearly all of their plant-like traits – beyond say using cellulose instead of collagen as a tissue binder and a vestgial photosynthetic metabolic paths they would have to be, in effect, animals.

Unless of course a wizard did it.