- "These fluxes suggest an upper limit for biomass productivity of ∼10³ kgC/year from a glycine fermentation metabolism." This is really an upper bound on acce

- "These fluxes suggest an upper limit for biomass productivity of ∼10³ kgC/year from a glycine fermentation metabolism." This is really an upper bound on accessible energy (the "carbon" explanation in the press office release is hand-wavey). The idea is that solar photochemistry in Titan's atmosphere creates chemical energy gradients, in the form of energetic compounds like that glycine; and these energy gradients could migrate through the ice and support a biosphere in the subsurface ocean. The abstract* says that, actually, very little of that energy gradient can get through the ice shell. This situation is different from that of some of the other icy moons: they potentially have much larger geochemical energy sources, from chemical reactions between the water ocean and primordial rock. The problem with Titan is it (probably) has a weird ice shelf on the bottom of its underground ocean, which isolates it from rock. So rock-ocean weathering probably doesn't happen there. * (The paper's not on arXiv or SciHub if you were wondering)