NOVELTY - A semi-permeable exterior wall retains microorganism over water-soluble nutrients. A baffle directs flow within vessel in re-circulating pattern when fluid is passed through vessel. A magnet/electromagnet enclosed by semi-permeable exterior wall retains iron compounds within vessel. The magnet/electromagnet spans vessel's width and forms coil. A sieved gate enclosed by semi-permeable exterior wall is movable between blocking position and open position. A self-orienting mechanism orients direction of vessel with respect to current flow when vessel is positioned in current.    USE - Vessel for biomass production system (claimed), used in plankton cultivation, bio-fuel production and nitrogen and phosphorus micronutrient bioremediation.    ADVANTAGE - The biomass can be produced efficiently and with less cost and the harsh environmental conditions can be prevented since the semi-permeable exterior wall retains the microorganism and baffle directs the flow within the vessel in re-circulating pattern when fluid is passed through the vessel.    DETAILED DESCRIPTION - The buoyancy-controlled base includes one or more compartment that is filled with gas that causes the buoyancy-controlled base to rise. INDEPENDENT CLAIMS are included for the following:    (1) method for producing biomass that involves growing microorganisms in vessel comprising semi-permeable membrane in aquatic environment. The microorganism is selected from group consisting of micro-algae, plankton, diatoms, algae, phytoplankton, or zooplankton. The iron compounds are retained within the vessel. The fluid from aquatic environment is allowed to pass freely through semi-permeable membrane. The volume of the vessel is reduced within the semipermeable membrane such that the concentration of the microorganism is increased; and    (2) biomass production system has processing platform that is in fluid communication with interior region of vessel to harvest microorganism from vessel. The processing platform recovers lipids contained within the harvested microorganism and processes the lipids into bio-diesel. The processing platform collects carbohydrates and proteins contained within the harvested microorganism and processes carbohydrates into products for human or animal consumption.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of the vessel.    Collapsible porous material (100)    Magnetic core (102)    Large buoyant top (104)    Buoyancy controlled bottom (106)    Reinforcement beams (108a-108c)