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Recap of SD-CAP Algal Biofuels Symposium 2010: Cyanobacteria – the other Algae by Susan Golden

There is one more talk I want to summarize because I thought it was really good: Susan Golden's talk about cyanobacteria was very educational because she reminds us all that when talking about algae not every algae is eukaryotic.

According to her, cyanobacteria belong to the prokaryotic kingdom. There is a large variety of them out there with very unique metabolic processes and physiologies that can be exploited. Some fix nitrogen, others produce hydrogen, or other secondary products. Many are individualistic, while some form filaments that can be beneficial for harvesting. From a technological perspective, some cyanobacteria are very easily transformable – that foreign pieces of DNA can easily be absorbed into these organisms. From a research perspective, there are 10 genetic model systems providing a wealth of tools and information for further research questions.

There are, however, challenges when it comes to working with cyanobacteria to extract oils for biofuels production. This is so because excess energy is stored in the form of glycogen and not lipids. Membranes of course consist of lipids, but different from other eukaryotic organisms, these are polar lipids! The other hurdle, comes from the fact that genetic models for cyanobacteria do not necessarily have qualities that would make them excellent production systems.

It is this challenge that the Golden Lab has decided to tackle by using a 2-pronged approach. Professor Golden spent most of the time outlining what she meant by this approach giving examples from her own lab. On the one hand, current model organisms are used to identify novel genes and pathways. On the other hand, her lab is working on identifying strains of cyanobacteria that would be suitable to serve as a production strain and use these naturally occurring strains to further evolve them to fully fit industrial needs.

Professor Golden points out that working with cyanobacteria unlike the eukaryotic counterparts is relatively easy because the full complement of molecular biological tools exist. These include: transformation (as mentioned before), conjugation, known regulated promoters, libraries of knockouts and overexpression genes, and all sorts of reporters (fluorescent, bio-luminescent, and chromogenic).

Using these tools, and Solexa sequencing technologies, 73 genomes have been sequenced with genome sizes ranging from 1.6 (marine environments) to 9.0 Mbases (complex habitats). There are some very impressive numbers to note:

  1. Despite the fact that many eukaryotic and prokaryotic genomes are already known, 25% of each cyanobacterial genome is unique.
  2. 40% of all genes are still of unknown function or are hypothetical genes!
Using homology many pathways from other organisms can be transposed onto cyanobacterial metabolic networks. However, this will not get at the functions of unknown genes. To get at these, new technologies and systematic genetic approaches need to be used.

To give one specific example of work going on in her lab, she talked about the work on one by the name of Snechococcus elongates PCC 7942. This strain is an obligate phototroph meaning that it needs light to survive. This strain is known for the ease of transformation. Simply adding DNA to the liquid growth media can be enough for transformation to take place. Its efficient homologous recombination machinery makes gene knock-outs or knock-ins easy to perform similar to techniques available in the more studied baker's yeast. First, a random knock-out library was created using a modified E. coli transposon. To assay the lipid profile, a new high-throughput method to detect lipids was developed because the standard Nile red method absorbs the light spectrum that these cyanobacteria use to perform photosynthesis.

To modify the lipid profile, different saturases were inserted and assayed for any changes in the lipid profile. This approach led to the creation of cyanobacteria that can secrete fatty acids without further modification.

The Golden lab also looked for other traits using similar approaches to screen strains for changes in the carbohydrade profile, as well as looking for better harvesting traits such as better filamentation or settling.

Insights in the model organisms have then been applied to the collection, isolation, identification and characterization of possible production strains that were selected on the basis of tolerance to conditions that are less favorable to grazers such as high temperatures, salt, pH and light tolerance.

In summary, I got the following out of the talk amongst other points: Although much emphasize has been placed on eukaryotic blue-green algae, cyanobacteria should not be discounted in the biofuels and bioproduct discussion. Several traits including better molecular genetics tools seem to make cyanobacteria great production platforms for future products.

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