In a typical terrestrial ecosystem, energy input comes directly from the sun, and it is converted to chemical energy through photosynthesis. interestingly, an unusual ecosystem can exist in the absence of sunlight. what could be the source of energy?

The oxidation of inorganic substances (H2S etc.) serves as the energy source for hydrothermal vents.

Explanation:

Deep hydrothermal vents (the unusual ecosystems) are located in the depths of oceans where sunlight can not penetrate. These ecosystems have the bacteria that oxidize the inorganic substances such as H2S and the released energy is used to fix the CO2 present dissolved in water.  

Here, the inorganic substances such as hydrogen sulfide are released from the volcanic vents present in the ocean floors.  

silt loam

hope this