NOVELTY - Gas-fed fermentation system for stimulating biological growth, comprises at least one hollow fluid conduit disposed within an interior space formed by a vessel; at least one gas distributor disposed in the downward flow path to introduce one or more gases in at least a portion of the downward flow path; at least one first fluid mover to provide a downward fluid flow in the downward flow path and an upward fluid flow in the upward flow path; and a thermal transfer system thermally coupled to at least one of the downward flow path or the upward flow path.    USE - The gas-fed fermentation system, method, or apparatus is useful for stimulating biological growth (claimed).    ADVANTAGE - Method and apparatus are provided for efficient mass transfer of gaseous substrates for microbial fermentation. A scalable fermentor design is provided for allowing high flux gas-phase to liquid-phase mass transfer in addition to efficient heat exchange and waste gas removal. Systems and methods for fermentation provide the public with new methods for the optimal production of a variety of products.    DETAILED DESCRIPTION - Gas-fed fermentation system for stimulating biological growth, comprises at least one hollow fluid conduit disposed within an interior space formed by a vessel, at least one hollow fluid conduit permitting a fluid flow to it, at least one fluid conduit including at least one inlet and at least one outlet; where the inlet of at least one hollow fluid conduit is in fluid communication with the interior space of the vessel to provide a downward flow path bordered by an external perimeter of at least one hollow fluid conduit and by an internal perimeter of the vessel and an upward flow path bordered by an internal perimeter of at least one hollow fluid conduit; at least one gas distributor disposed in the downward flow path to introduce one or more gases in at least a portion of the downward flow path; at least one first fluid mover to provide a downward fluid flow in the downward flow path and an upward fluid flow in the upward flow path; and a thermal transfer system thermally coupled to at least one of the downward flow path or the upward flow path. INDEPENDENT CLAIMS are:    (1) a method for stimulating biological growth comprising dispersing a gas in a liquid media including at least water and one or more nutrients to form a multi-phase mixture comprising gas bubbles dispersed in the liquid media; flowing at a first velocity and a first pressure the multi-phase mixture in one or more downward flow paths, one or more downward flow paths formed between an exterior perimeter of a number of hollow fluid conduits and an interior perimeter of a vessel at least partially surrounding the number of hollow fluid conduits; increasing the pressure of the multi-phase mixture from the first pressure to a second pressure to produce compressed gas bubbles in the multi-phase mixture while contacting the multi-phase mixture with a number of microorganisms existent in the downward flow path to provide a first biomass; and flowing at a second velocity the multi-phase mixture in a number of upward flow paths, each of the respective number of upward flow paths formed by an interior perimeter of each of the number of hollow fluid conduits and maintaining the pressure at or above a third pressure to maintain compressed gas bubbles in the multi-phase mixture while contacting the multiphase mixture with a number microorganisms existent in the upward flow path to provide a second biomass; and    (2) an apparatus to stimulate biological growth, comprising a number of hollow fluid conduits disposed at least partially within a vertically arranged vessel to provide a number of downward flow paths bordered by an exterior perimeter of each of the number hollow fluid conduits and an interior perimeter of the vertically arranged vessel; where the vertically arranged vessel includes at least one fluid inlet connection fluidly coupled to at least a portion of the number of downward flow paths; where each of the number of hollow fluid conduits provides a respective number of upward flow paths, each of the upward flow paths bordered by an interior perimeter of each of the respective hollow fluid conduits, each of the number of upward flow paths fluidly coupled to at least connection to remove a multi-phase fluid; at least one gas distributor disposed in at least a portion of the number of downward flow paths; and a number of first structures disposed within at least a portion of at least one of: the number of downward flow paths or the upward flow paths, each of the number of first structures to promote biological growth.