In a natural environment, plants remove carbon dioxide from the atmosphere and produce sugar, which is then converted into carbohydrates, proteins, and other plant materials. When these materials are ultimately oxidized (burned), they release the energy of the sunlight which formed them, and also return the carbon dioxide used in their formation back to the atmosphere. Carbon dioxide is an essential part of the cycle.
Plants thrive on levels of carbon dioxide higher than the natural levels provided by the atmosphere--up to five or six times higher. By enriching the carbon dioxide available to plants, growth is accelerated. This fact can be used to abate carbon dioxide emitted by industrial facilities during combustion of fuels.
Aquatic species are ideal candidates for this kind of abatement. Concentrated CO2 captured from an industrial emitter is dissolved in the growth water via a conventional water-gas scrubber or other means. The biomass grows faster, and the CO2 is captured in the biomass for conversion to biofuel. Ultimately the CO2 will be released to the atmosphere when the biofuel is burned, but in the process more energy will be created. In this way, the CO2 per unit of energy will be less than the ratio for the original fuel without abatement. Coal can be made almost as clean as any other fuel in terms of CO2 emissions to the atmosphere.
See typical process diagrams here.