In detritus food chain, initial energy comes from:
The immediate resource entering a detritus food chain is dead organic matter: litter, wood, carcasses, feces, or dissolved organic carbon. Microorganisms secrete enzymes, assimilate soluble products, and condition the material for detritivores; predators then consume microbial grazers and detritivores. Sunlight usually supplied the original energy when producers formed that biomass, but it is not the direct input at the chain’s first consumer step. This distinction separates detrital from grazing pathways while recognizing their common ultimate origin. Detrital and grazing channels continually
Ref: Fundamentals of Ecology, Odum & Barrett, 5th Ed., Ch. 3