초록
<P>The development of an sustainable economy calls for improved energy utilization and storage technologies. Although battery- and carbon-based routes have gained tremendous attention, nitrogen-based routes have rarely been exploited so far. Guanidine (CH<SUB>5</SUB>N<SUB>3</SUB>) which contains 71.1% nitrogen by mass is an exemplary chemical to explore the nitrogen-based routes of energy utilization and storage. Guanidine has a variety of applications including its use as a slow-release fertilizer, a propellant, or as a precursor to pharmaceuticals and antimicrobial polymers. The conventional chemical synthesis of guanidine through the Frank-Caro process is energy-intensive, consumes fossil fuels, and is detrimental to the environment. Herein, a <I>de novo</I> guanidine biosynthesis (GUB) cycle is proposed with CO<SUB>2</SUB> and nitrate/ammonium as the carbon and nitrogen sources, respectively. The ATP and NAD(P)H needed to drive the GUB cycle are generated <I>via</I> photosynthesis in an engineered cyanobacterium <I>Synechocystis</I> sp. PCC 6803 expressing an ethylene-forming enzyme (EFE). Up to 586.5 mg L<SUP>−1</SUP> (9.9 mM) guanidine was produced after seven days of photoautotrophic cultivation, with an average productivity of 83.8 mg L<SUP>−1</SUP> day<SUP>−1</SUP>. In addition, guanidine was directly biosynthesized from CO<SUB>2</SUB>, N<SUB>2</SUB> and H<SUB>2</SUB>O in an engineered N<SUB>2</SUB>-fixing cyanobacterium <I>Anabaena</I> sp. PCC 7120 expressing the EFE. This work demonstrates the first biological conversion of renewable solar energy into chemical energy stored in the nitrogen-rich compound guanidine, which could shed light on harnessing the biological nitrogen metabolism for energy utilization and storage.</P><P>Graphic Abstract</P><P>Direct photosynthesis of the nitrogen-rich compound guanidine from CO<SUB>2</SUB> and N<SUB>2</SUB>.<BR/><IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c9gc01003c'/><BR/></P>