On the top middle part of my screen, there's 4 icons related to my turrets.

I understand what the blue icons do (defend, attack all, etc), but the greyed ones at the bottom seem to tell me that my turrets don't work. When I click on them, I can only cycle between what looks like 2 different variants of an offline symbol. The greyed out gun is already the offline symbol for normal guns, whereas the crossed out circle seems to convey the same meaning.
There's no key related to turrets in the keybind menu. There's no explanation in the encyclopedia entry on turrets, or any tutorial on turrets for that matter. I have all my crew working as service. Could someone help me figure this out? I can't tell if they're bugged, I don't have enough energy to power them on, or I'm just missing something.