I'm sure this question has been asked before, but I've looked for it in all the sources i know of for this game (admittedly, a pretty short list).  I've only played a couple of games now, but this seems like something I need to have a firm grasp on to get anywhere.

My confusion is around the mechanics for the buildings that consume one resource to produce another, i.e. Reactors.  My understanding of how this should work is that if I have one of these buildings, but do not have a stockpile of the input resource, then I will continually purchase that resource off of the market.  This purchase will be made with cash if available, or with debt if not.  I have noticed in game that these buildings will sometimes show 'Needs H2O' for example and appear to stop producing.

Can someone explain to me the circumstances under which this happens?  Do I need to manually purchase input materials from the market?  Is this a result of running out of cash, and the input material won't be automatically purchased with debt (in contrast to life support materials)?

thanks for any help!