Toward on-chip directed evolution of unicellular organisms for efficient hydrogen production*

To provide an energy resource alternative to fossil fuels, photosynthetic organisms must increase their energy conversion efficiency. The green algae \textit{C. reinhardtii} stores light energy in hydrogen gas at 0.1\% efficiency, less than the 10\% required to compete with established fuels. This work combines hydrogen sensing in liquid culture with micro habitat patch (MHP) chips for directing hydrogen-producing organisms to evolve improved energy conversion efficiency. A MHP chip contains 87 1 mm $\times$ 1 mm $\times$ 100~$\mu\mathrm{m}$ interconnected chambers. By measuring hydrogen output from different chambers, we will select less productive patches to annihilate. We microfabricated chips from poly(dimethylsiloxane). Color changes in fluorescence micrographs confirm that 254~nm radiation kills algae in MHPs, liberating nutrients and space for exploitation by adjacent populations. We demonstrated colorimetric detection of hydrogen gas production at a rate of $10^{-8}~\mathrm{mol~H_2~mL^{-1}~s^{-1}}$ using tungsten film on sub-mL liquid cultures of \textit{C. reinhardtii} during 2-hrs. of fermentation in darkness.