So, after some minimal deliberation I decided the most productive way to move forward concerning cell no.10 would be to see if I can resurrect the warm loving microbes.  

I would have re-innoculated the substrate when I replaced the top half of the soil substrate anyhow, so they should be present.  The fish tank heater was ill-equipped to maintain 10degC, which seemed like the area that the current culture liked, but it tops out at about 30degC.  Which sounds perfect for the warmer cultures.

I'll let you know how far the output falls, with any luck though it will begin rising again shortly thereafter.  

And if all else fails, it'll get the hose.  We'll start fresh with completely new substrate, and get a chance to inspect the electrodes for signs of corrosion.  

In natural soil similar shifts happen when the seasons change, microbes that are dormant in the summer due to heat become active in the cooler fall weather.  I know that this is a gross over-simplification of these incredibly complex bio-chemical processes, but also something that seems easily testable in these circumstances.

So, we shall see what happens when the warm water circulator gets up to temperature and the fuel cell starts warming up.