NOVELTY - Method of producing liquid fuel comprises: (a) cultivating aquatic plants (12) in an aquatic medium (11) exposed to light in a photosynthetic culture chamber (10); (b) transferring organic material of the aquatic plants to an anaerobic digester; (c) fermenting the organic material in the digester to produce a biogas comprising methane and carbon dioxide; (d) converting at least a portion of the biogas to synthesis gas comprising carbon monoxide and hydrogen; and (e) contacting at least a portion of the synthesis gas with a catalyst to produce a liquid fuel.    USE - The method is useful for producing: a liquid fuel, which is alcohol or diesel fuel; and biogas including methane and carbon dioxide (all claimed).    ADVANTAGE - The carbon dioxide reforming leads to higher yield of liquid fuel. The method is simple to transfer the aquatic medium containing aquatic plants to the digester without separating the aquatic plants from the aquatic medium. The toxic oxygen in the fluids to the methanogens can be consumed in the digester vessel containing microorganisms that are tolerant of oxygen.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for:    (1) a method of producing biogas, comprising cultivating aquatic plants in an aquatic medium exposed to light in a photosynthetic culture chamber, transferring organic material of the aquatic plants to an anaerobic digester, fermenting the organic material in the digester to produce a liquid effluent and a biogas comprising methane and transferring carbon dioxide to the aquatic medium in the photosynthetic culture chamber by (i) transferring at least a portion of the liquid effluent with dissolved carbon dioxide of at least 3 mM from the anaerobic digester to the photosynthetic culture chamber when contacted with the aquatic medium in the photosynthetic culture chamber, (ii) contacting at least a portion of the biogas with an aqueous volume outside of the anaerobic digester and the photosynthetic culture chamber to dissolve carbon dioxide from the biogas in the aqueous volume and generate a carbon dioxide depleted biogas, and mixing the aqueous volume with the aquatic medium in the photosynthetic culture chamber, or (iii) contacting at least a portion of the biogas with the aquatic medium in the photosynthetic culture chamber to dissolve carbon dioxide from the biogas in the aquatic medium and generate a carbon dioxide depleted biogas;    (2) a system for producing liquid fuel, comprising (a1) a photosynthetic culture chamber for culturing aquatic plants in an aquatic medium exposed to light, (a2) an anaerobic digester for producing biogas comprising methane and carbon dioxide from organic material of the aquatic plants in an anaerobic digestion mixture, (a3) a device (comprising a steam reformer, a carbon dioxide reformer and/or a partial oxidation unit) for producing synthesis gas comprising carbon monoxide and hydrogen from biogas comprising methane, and (a4) a catalyst for converting synthesis gas to a liquid fuel, where (a1) is functionally coupled to (a2), which is functionally coupled to (a3) and is functionally coupled to (a4);    (3) a system for producing biogas, comprising (a1) and (a2) as above per se, and (a5) a conduit functionally coupled to (a1) and (a2) and adapted for transferring liquid effluent from (a2) to the aquatic medium in (a1) without equilibration with air before the liquid effluent is mixed with the aquatic medium in (a1), or adapted to transfer at least a portion of the biogas from (a2) to (a1) to contact the biogas with the aquatic medium in (a1) to dissolve carbon dioxide from the biogas in the aquatic medium and generate a carbon dioxide depleted biogas; and    (4) a system for producing biogas, comprising (a1), (a2) and (a5) as above per se, and (a6) a chamber separate from (a2) and (a1) for holding an aqueous volume.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic drawing of a system for producing biogas or liquid fuel.    Photosynthetic culture chamber (10)    Aquatic medium (11)    Aquatic plants (12)    Sparger (13)    Carbon dioxide rich combustion exhaust gas (14)