A fermenter can have at least one hollow fluid conduit disposed at least partially within a vessel. An external circumference of the hollow fluid conduit and an interior circumference of the vessel can define a downward flow path through which a multi-phase mixture including a liquid media and compressed gas substrate bubbles flows. An interior circumference of the hollow fluid conduit can defined an upward flow path which is in fluid communication with the downward flow path. The multi-phase liquid can flow through the upward flow path and exit the fermenter. Cooling may be provided in the hollow fluid conduit or the vessel. One or more backpressor generators can be used to maintain a backpressure on the fermenter. One or more fluid movers can be used to variously create an induced and/or forced flow in the downward and upward flow paths.