NOVELTY - A system (S1) for producing at least one energy source comprises: an anaerobic digester, where the digester comprises at least one flexible first container supported by a water body; and at least one feedstock material, where the feedstock material comprises at least one of a plant material and an animal material.    USE - For producing at least one energy source; and for sequestering a quantity of carbon dioxide. The energy source is methane (all claimed), or hydrogen.    ADVANTAGE - The system provides renewable energy source; provides cost effective and off-shore energy production technologies; produces energy sources from organic material via anaerobic digestion; produces renewable natural gas from plankton and liquid carbon dioxide for sequestering in the sea floor (thereby reduces greenhouse gases); recycles nutrients for production of more plankton; delivers greater than 99% pure methane; and achieves economic energy source production with an inexpensive but extremely large volume.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for following:    (1) an aquatic method for producing at least one energy source, involving anaerobic digestion of at least one feedstock material by at least one bacterium; and the feedstock is digested within at least one flexible container supported by a water body, thereby producing at least one energy source; and optionally, substantially purifying the energy source;    (2) an off-shore method for producing at least one energy source;    (3) sequestering (M1) a quantity of carbon dioxide, involving providing a quantity of liquid phase carbon dioxide; optionally, substantially purifying the liquid phase carbon dioxide; collecting the liquid phase carbon dioxide; and placing the collected liquid phase carbon dioxide in at least one storage container, or transporting the collected liquid to a position on or near the floor of a water body;    (4) sequestering (M2) carbon dioxide, involving anaerobic digestion of at least one feedstock material by at least one bacterium; the feedstock material is digested within at least one flexible container supported by a water body, thereby producing a quantity of carbon dioxide; substantially purifying the carbon dioxide by separating a liquid phase carbon dioxide from at least one other liquid; and sequestering the substantially purified carbon dioxide by collecting the liquid phase carbon dioxide in at least one container or transporting the liquid phase carbon dioxide to a position on a floor of the water body;    (5) an aquatic method for sequestering carbon dioxide according to (M1) or (M2); and    (6) increasing (M3) the density of solid aquatic plant material in a liquid or slurry, involving transporting the liquid into or through a porous container.