NOVELTY - Extracting fuel gases from a underwater plume emitted from an underwater hydrothermal vent, comprises (a) receiving, at a first inlet of a first underwater heat exchanger in a water body, a first fluid containing water and at least one gas for forming a clathrate hydrate, (b) receiving an ambient second fluid at a second inlet of the first underwater heat exchanger, and (c) discharging a slurry containing the clathrate hydrate at a first outlet of the first underwater heat exchanger.    USE - The method is useful for extracting fuel gases including methane, hydrogen and carbon dioxide from a underwater plume emitted from an underwater hydrothermal vent, generating methane hydrate using microorganisms such as methanobacteria (all claimed) for powering a generator located on or near platform, and extracting minerals such as copper, iron, zinc, sulfur and manganese in sea water.    ADVANTAGE - The method reduces the emission of greenhouse gases in the environment during extraction of fuel gases e.g. methane from the underwater plume.    DETAILED DESCRIPTION - Extracting fuel gases from a underwater plume emitted from an underwater hydrothermal vent, comprises (a) receiving, at a first inlet of a first underwater heat exchanger in a water body, a first fluid containing water and at least one gas for forming a clathrate hydrate, (b) receiving an ambient second fluid at a second inlet of the first underwater heat exchanger, where the ambient second fluid has sufficiently lower temperature than the temperature of the first fluid to cause a transfer of sufficient heat between the inlets to form a clathrate hydrate in the first fluid, and (c) discharging a slurry containing the clathrate hydrate at a first outlet of the first underwater heat exchanger. INDEPENDENT CLAIMS are included for:    (1) generating methane hydrate using microorganisms comprising (a) as above per se, (a1) directing ambient water into a second inlet of the underwater heat exchanger, where ambient water has a temperature sufficiently lower than the temperature of the first fluid to cause the water to cool the first fluid sufficiently to cause the formation of at least one of methane hydrate and hydrogen-methane hydrate, and (a2) conveying at least one of methane hydrate and hydrogen-methane hydrate through a duct connected to a first outlet of the first heat exchanger, at least partially due to increased buoyancy of the hydrates to the surface of the water body;    (2) a system-I for harvesting gases from the underwater vent comprising an underwater plume collector positioned above the hydrothermal vent in a water body bed for collecting plumes emitted by the hydrothermal vent, and a first underwater heat exchanger in fluid communication with the plume collector positioned in general vicinity of the water body bed, where the first underwater heat exchanger comprises a first inlet to receive a first fluid containing the plumes, a second inlet to receive ambient water, a first outlet for ejecting a slurry containing the clathrate hydrate, and a second outlet for ejecting the water; and    (3) a system-II for generating clathrate hydrate using microorganisms comprising a hydrogen-sulfide bioreactor to receive at least hydrogen sulfide from a hydrothermal vent and containing at least sulfur reducing bacteria for consuming hydrogen sulfide and releasing hydrogen and sulfur, a carbon dioxide bioreactor to receive at least carbon dioxide from at least the hydrogen sulfide bioreactor and containing at least methanobacteria for consuming carbon dioxide and releasing methane, an underwater heat exchanger in fluid communication with the carbon dioxide bioreactor for receiving a first fluid containing at least hydrogen, methane and brine.