These structures form underneath the surface of the water. They’re called air bladders and maybe they do have air in them, but if the gas is air, where did it come from? Or, if it’s some other gas, where did that gas come from? Do the cells that line these bladders extract gasses from the water? Is it O2 resulting from photosynthesis? PURE Oxygen? How do these little structures form underwater with pockets of gas inside them? Where’s the gas come from?
They’re plants and, as you say, photosynthesis produces excess oxygen, plants also produce carbon dioxide when it’s dark. My WAG is that the surrounding cells expirate into the bladders.
This seems to be a partial answer;
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Carbon monoxide appears to be a minor metabolic product.
In other words air passes from the water into the bladder through the walls of the structure. I’m still not quite clear how these pneumatocysts are inflated against water pressure; maybe it is the mechanical strength of the bladder that prevents collapse. But there must be an active process going on, or the gases would pass out of the cyst just as readily as they pass in, and the cyst would deflate.
could be puffer fish making shade.