Secretion of fatty acids by photosynthetic microorganisms

Abstract

Recombinant photosynthetic microorganisms that convert inorganic carbon to secreted fatty acids are described. Methods to recover the secreted fatty acids from the culture medium without the need for cell harvesting are also described.

1 . A cell culture of a recombinant photosynthetic microorganism, said microorganism modified to contain a nucleic acid molecule comprising at least one recombinant expression system that produces at least one exogenous acyl-ACP thioesterase,
 wherein said acyl-ACP thioesterase preferentially liberates a fatty acid chain that contains 6-20 carbons, and   wherein the culture medium provides inorganic carbon as substantially the sole carbon source and   wherein said microorganism secretes the fatty acid liberated by the acyl-ACP thioesterase into the culture medium.   
     
     
         2 . The culture of  claim 1 , wherein the at least one exogenous acyl-ACP thioesterase is a Fat B thioesterase. 
     
     
         3 . The culture of  claim 1 , wherein the at least one exogenous acyl-ACP thioesterase is a Fat B thioesterase derived from the genus  Cuphea.    
     
     
         4 . The culture of  claim 1 , wherein the at least one exogenous acyl-ACP thioesterase is ChFatB2. 
     
     
         5 . The culture of  claim 1 , wherein the recombinant photosynthetic microorganism has further been modified to produce an exogenous β-ketoacyl synthase (KAS). 
     
     
         6 . The culture of  claim 5 , wherein the exogenous KAS preferentially produces acyl-ACPs having the chain length for which the thioesterase has preferred activity. 
     
     
         7 . The culture of  claim 1 , wherein the recombinant photosynthetic microorganism is further modified so that one or more genes encoding beta-oxidation pathway enzymes are inactivated or downregulated, or said enzymes are inhibited. 
     
     
         8 . The culture of  claim 1 , wherein the recombinant photosynthetic microorganism is further modified so that one or more genes encoding acyl-ACP synthetases are inactivated or downregulated, or said synthetases are inhibited. 
     
     
         9 . The culture of  claim 1 , wherein the recombinant photosynthetic microorganism is further modified so that one or more genes encoding an enzyme involved in carbohydrate biosynthesis are inactivated or downregulated, or said enzymes are inhibited. 
     
     
         10 . The culture of  claim 9 , wherein the enzyme involved in carbohydrate biosynthesis is a branching enzyme. 
     
     
         11 . A method to convert inorganic carbon to fatty acids, said method comprising:
 incubating the culture of  claim 1  such that the recombinant photosynthetic microorganism therein secretes the fatty acid into the culture medium; and   recovering the secreted fatty acids from the culture medium.   
     
     
         12 . The method of  claim 11 , wherein the fatty acids are recovered from the culture by contacting the medium with particulate adsorbents. 
     
     
         13 . The method of  claim 12 , wherein the particulate adsorbents circulate in the medium. 
     
     
         14 . The method of  claim 12 , wherein the particulate absorbents are contained in a fixed bed column. 
     
     
         15 . The method of  claim 14 , wherein the pH of the medium is lowered during said contacting. 
     
     
         16 . The method of  claim 15 , wherein said pH lowering process comprises adding CO 2 . 
     
     
         17 . The method of  claim 16 , wherein the medium is recirculated to the culture. 
     
     
         18 . The method of  claim 12 , wherein the particulate adsorbents are lipophilic. 
     
     
         19 . The method of  claim 12 , wherein the particulate adsorbents are ion exchange resins. 
     
     
         20 . A composition comprising a fatty acid produced by the culture of  claim 1 . 
     
     
         21 . The composition of  claim 20 , wherein the composition is used to produce another compound. 
     
     
         22 . The composition of  claim 20 , wherein the composition is a biocrude. 
     
     
         23 . A composition comprising a derivative of a fatty acid produced by the culture of  claim 1 . 
     
     
         24 . The composition of  claim 23 , wherein the composition is a finished fuel or fuel additive. 
     
     
         25 . The composition of  claim 23 , wherein the composition is a biological substitute for a petrochemical product. 
     
     
         26 . The composition of  claim 23 , wherein the derivative is an alcohol, an alkane, or an alkene.