Two-tier vessel for photoautotrophic high-density cultures

Two-tier vessels, developed for culturing of microalgae and cyanobacteria at high cell density on a shaken platform, were assembled from a flat lower chamber to be filled with a CO2 buffer and an upper flat sterile chamber for the culture that was separated from the lower chamber by a porous polypropylene membrane. Diffusive gas exchange with the atmosphere was controlled by the O-2 outlet channel. Referred to surface area, rates of CO2 transfer to a shaken weakly alkaline buffer solution across the membrane were higher than those reached on the conventional pathway through the free upper liquid surface. Membrane-mediated CO2 supply enabled rapid growth of Synechocystis sp. PCC 6803 and Synechococcus sp. PCC 7002 up to ultrahigh cell density. The biomass (dry weight) concentration of Synechococcus cultures reached more than 30 g L-1 on a buffered medium with adequate concentrations of mineral nutrients. An increase of 15 to 20 g L-1 was observed during repeated two-day cycles. Separate pathways for CO2 supply and oxygen outlet prevented significant loss of CO2. Convective gas flow through the oxygen outlet channel enabled the estimation of the O-2 generation rate. The permeability of the channel for diffusive O-2/N-2 exchange limited the O-2 concentration to a moderate value. It is concluded that shaken flat cultures using CO2 supply through a porous hydrophobic membrane and diffusive release of O-2 through a separate pathway are promising for research on microalgae and cyanobacteria.