Secretion of fatty acids by photosynthetic microorganisms

1. A method to produce fatty acids, comprising:

(i) incubating a cell culture of a recombinant cyanobacterium in a culture medium that provides inorganic carbon as substantially the sole carbon source for conversion to fatty acids by the cyanobacterium, wherein the cyanobacterium is modified to contain a nucleic acid molecule comprising at least one recombinant expression system that produces at least one exogenous acyl-acyl carrier protein (acyl-ACP) thioesterase, wherein said acyl-ACP thioesterase liberates a fatty acid chain that contains 6-20 carbons,

wherein the recombinant cyanobacterium secretes the fatty acid formed from inorganic carbon into the culture medium;

and

(ii) recovering the secreted fatty acids from the culture medium.

2. The method of claim 1 , wherein the fatty acids are recovered from the culture by contacting the medium with particulate adsorbents.

3. The method of claim 2 , wherein the particulate adsorbents circulate in the medium.

4. The method of claim 2 , wherein the particulate adsorbents are contained in a chromatographic column.

5. The method of claim 4 , wherein the pH of the medium is lowered during said contacting.

6. The method of claim 5 , wherein said pH lowering comprises adding CO 2 .

7. The method of claim 6 , wherein the medium is recirculated to the culture.

8. The method of claim 2 , wherein the particulate adsorbents are lipophilic.

9. The method of claim 2 , wherein the particulate adsorbents are ion exchange resins.

10. The method of claim 1 , wherein the at least one exogenous acyl-ACP thioesterase is a Fat B thioesterase.

11. The method of claim 10 , wherein the at least one exogenous acyl-ACP thioesterase is a Fat B thioesterase derived from the genus Cuphea.

12. The method of claim 11 , wherein the at least one exogenous acyl-ACP thioesterase is Cuphea hookeriana FatB2 (ChFatB2).

13. The method of claim 1 , further comprising inducing expression of the recombinant expression system that produces at least one exogenous acyl-acyl carrier protein (acyl-ACP) thioesterase.

14. The method of claim 1 , wherein the recombinant cyanobacterium has further been modified to produce an exogenous β-ketoacyl synthase (KAS).

15. The method of claim 14 , wherein the exogenous KAS produces acyl-ACPs having a chain length for which the thioesterase has activity.

16. The method of claim 1 , wherein the recombinant cyanobacterium has further been modified to produce an exogenous acetyl-CoA carboxylase (ACCase).

17. The method of claim 1 , wherein the recombinant cyanobacterium is further modified so that one or more genes encoding acyl-ACP synthetases are inactivated by gene disruption or downregulated by antisense RNA.

18. The method of claim 1 , wherein the recombinant cyanobacterium is further modified so that one or more genes encoding a 1,4-alpha-glucan branching enzyme are inactivated by gene disruption or downregulated by antisense RNA.

19. A method to convert inorganic carbon to fatty acids, comprising:

(i) incubating a cell culture of a recombinant cyanobacterium in a culture medium that contains a carbon source consisting essentially of inorganic carbon, wherein the cyanobacterium is modified to contain a nucleic acid molecule comprising at least one recombinant expression system that produces at least one exogenous acyl-acyl carrier protein (acyl-ACP) thioesterase, wherein said acyl-ACP thioesterase liberates a fatty acid chain that contains 6-20 carbons,

further wherein the recombinant cyanobacterium secretes the fatty acid into the culture;

and

(ii) recovering the secreted fatty acids from the culture medium.

20. The method of claim 19 , wherein the cyanobacterium is further modified to include means for inactivating or downregulating one or more genes encoding acyl-ACP synthetases.

21. The method of claim 19 , wherein the cyanobacterium is further modified to include means for inactivating or downregulating one or more genes encoding a 1,4-alpha-glucan branching enzyme.