NOVELTY - A method for producing at least one lipid product from a gaseous substrate, involves receiving a gaseous substrate in a first bioreactor comprising a culture of at least one microorganism in a liquid nutrient medium, and fermenting the gaseous substrate to produce a product selected from an acid or an acid and an alcohol; and passing at least a portion of the acid product to a second bioreactor comprising a culture of at least one microalgae in a liquid nutrient medium, and fermenting the acid to produce at least one lipid product.    USE - For producing at least one lipid product from a gaseous substrate, where the lipid product is used to produce at least one tertiary product selected from hydrogenation-derived renewable diesel (HDRD), fatty acid methyl esters (FAME), fatty acid ethyl esters (FAEE) and biodiesel (claimed).    ADVANTAGE - The methods improve carbon capture, where CO, CO2 and/or H2, is converted into useful products, lipids. The methods support the production of acetate and ethanol from gaseous substrates comprising CO and optionally H2 containing industrial flue gases such as tail gas from steel production plants. The methods are also applicable to reactions which produce alternative acids; and can be used to reduce the total atmospheric carbon emissions from industrial processes, by capturing CO-containing gases produced as a result of such processes and using them as substrates for the fermentation processes.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for method (m2) for controlling pH in a fermentation process, involving a) passing a gaseous substrate comprising CO, CO2 and/or H2, to a first bioreactor comprising a culture of at least one microorganism in a liquid nutrient medium, and fermenting the gaseous substrate to produce a broth comprising a product selected from acid, or acid and alcohol; b) passing at least a portion of the broth comprising the product to a second bioreactor comprising a culture of at least one microalgae in a liquid nutrient medium, and fermenting the acid to produce at least one lipid product; and c) recycling at least a portion of the broth from the second reactor back to the first bioreactor, where the acid concentration in the recycled broth is substantially reduced.