Question

what is source of energy in dark reaction ?

Answer 1

the source of energy regardless of whether you're in a light or dark situation is chemical bonds. How you break those bonds is different, depending on your environment. In your case it's chemosynthesis - 

excerpt: 

Chemosynthesis, process in which some types of microorganisms use energy from chemical reactions to produce organic (carbon-containing) compounds as food for themselves. Chemosynthesis differs from photosynthesis, in which plants and certain microorganisms use light energy from the Sun to produce carbohydrates as food from carbon dioxide (CO2) and water. Organisms that manufacture their own food are called autotrophs. Microbes that use chemicals in the process are called chemoautotrophs and include members of both the bacteria and the archaea. 

Bacteria that use chemosynthesis are important in fixing nitrogen to make the element available in a chemical form that plants can use as part of the nitrogen cycle. Scientists have discovered unusual communities of marine life on the ocean floor that depend on chemosynthetic microorganisms as the first step in a food chain. Other chemosynthetic microbial life has been found in caves and deep inside rocks in Earth’s crust. Different types of chemoautotrophic microbes use hydrogen, hydrogen sulfide (H2S), or methane (CH4) as a chemical energy source