Immobilized heterocysts as microbial factories for sustainable nitrogen fixation.

A novel thin-layer biocatalyst for photosynthetic N fixation and H photoproduction was assembled using a Ca-alginate matrix and heterocysts isolated from wild-type Anabaena sp. PCC 7120 filaments. Compared to suspension heterocysts, heterocysts entrapped in Ca-alginate films showed improved stability of the nitrogenase system. While suspension heterocysts lost nitrogenase activity within 24 h, immobilized heterocysts supported nitrogenase activity for up to 125 h. The maximum specific rate of acetylene reduction was the same in both cases (∼0.4 μmol CH mg Chl h), but the catalyst with entrapped heterocysts required a much longer time to achieve the maximum rate (60 h instead of 3 h in suspension). Simultaneously with acetylene reduction, the immobilized heterocysts were able to photoproduce H for 125 h, yielding up to 1.1 mmol H mg Chl. The absence of acetylene increased the H photoproduction rate to a maximum of 25-30 μmol H mg Chl h, and the catalyst was capable of H photoproduction for 190 h, yielding up to 2.5 mmol H mg Chl. The recovery of the catalyst with entrapped heterocysts was achieved through placing the cells in a N atmosphere for 24 h. This engaged a second cycle of H photoproduction, which lasted for another 240 h (10 days), thus yielding ∼3 mmol H mg Chl in total after 454 h. Together, these findings demonstrate great potential for a heterocyst-based thin-layer platform for the sustainable production of chemicals and biofuels.