Step 1: Concept
Detritus food chain (decomposer food chain).
Step 2: Meaning
The detritus food chain begins with dead organic matter (detritus) and is driven by decomposers that break down this material.
Step 3: Analysis
Fungi and bacteria are the primary decomposers that initiate and drive the detritus food chain. Bryophytes, herbivores, and algae are part of the grazing (producer/consumer) food chain, not the detritus chain.
Step 4: Conclusion
The detritus food chain mainly consists of fungi and bacteria (decomposers).
Final Answer: (A)