Which has nothing to do with being indebted. We aren't talking about the feeling of debt or the gratitude for it, but the condition of debt, which comes from unequal distribution of benefits between causally related agents. We are indebted to plants for oxygen becuase we need them much more than they need us for CO2. Animals and plants are even, they are not indebted to each other, we are indebted to plants. For us to be indebted to god, we must first establish that God exists, that some causal/conditional/creational relation exists between plants and God and that this causality is essential to us - only then can the statement "we are indebted to God/should appreciate God for our oxygen" be considered true and acceptable rationally.