Metabolic carbon partitioning and nitrogen utilization in photosynthetic microorganisms

Abstract

Photosynthetic microorganisms (PSMs) are promising candidates for biofuels because they do not directly compete with the food supply. However, significant improvements are still required to achieve economic feasibility. The high cost of PSM based biofuel production comes from deficiencies in biomass yields, cell recovery, fuel conversion and nutrient inputs. An algal biofuel production and processing pipeline advocated by the National Renewable Energy Laboratory proposes to overcome many of the difficulties associated with the conversion of biomass to fuels. In this approach, acid catalyzed pretreatment is used to release cellular constituents. The oils are then extracted with organic solvents and the remaining carbohydrates used in yeast fermentations to make alcohols. We demonstrate that the biomass residues collected after fermentation can be recycled back to algal cultures as the sole source of nitrogen, eliminating the need for new external supplies of nitrogen for subsequent culturing. The alga Scenedesmus acutus was selected after organism screening for its ability to utilize a diversity of exogenous organic nitrogen sources. The replacement of nitrate with biomass residues (or amino acids/yeast extracts) demonstrated that the protein-rich biomass residuals not only provided an effective nitrogen resource, but also improved overall biomass and lipid yields. Optimizing lipid production through metabolic engineering can improve energy densities in algal feedstocks and further reduce production costs. We studied lipid biosynthesis in an alga that we recently isolated from the Great Salt Lake (Utah), Chaetoceros sp. GSL56, which was shown to produce very high levels of medium chain fatty acids. Fatty acids in the C8-C14 range are industrially useful and are also ideal for biodiesel. Using whole cell transcriptome sequencing, genes encoding the fatty acid synthesis enzymes were identified in this alga and expression of a ketoacyl-ACP synthase in the cyanobacterium Synechococcus sp. PCC7002, a potential biofuel production strain, increased medium chain fatty acid production up to 10 fold. Genetic engineering of PSM cells for high lipid productivity often encounters the problem of carbon partitioning, which diverts carbon away from lipid biosynthesis. In the green alga Chlamydomonas reinhardtii, deletion of starch biosynthesis redirects carbon into lipid biosynthesis, but also attenuates photosynthesis by unknown mechanisms. The comparative analysis of central carbon metabolites, as well as quantifying the changes in major carbon sinks (carbohydrate, lipid and protein), in starchless mutants and control strains revealed the metabolic nodes blocked in carbon rerouting. These results suggest that the fatty acid synthase complex is blocked and should be targeted in future genetic engineering efforts. Photosynthetic studies conducted in starchless mutants revealed that linear electron transport was attenuated because of the absence of an appropriate carbon sink.