What do Red Algae and Green Algae Have in Common?

For Debashish Bhattacharya, the knowledge that half the oxygen on Earth is generated by algae through photosynthesis begs an important evolutionary question: How did algae come to be such industrious carbon-dioxide-consuming, sugar-and-oxygen producing factories? Did each of the two major kinds of algae, red and green, evolve their photosynthetic abilities separately? Or did they have a common ancestor?

Red algae are mostly aquatic and include such familiar organisms as sushi wrap and are the sources of agar and carrageenan. Green algae are terrestrial, and are genetically related to all land plants.

Thanks to the recent research of Bhattacharya and his co-authors, published in the journal Current Biology, it appears likely that red and green algae do have a common ancestor, since they share about half the genes in their their genomes. Scientists had long thought this must be so, but had little evidence to back up their hunch, because red algae genomes are usually very large, and their sequencing was such a long, involved, expensive process.

In fact, scientists since Darwin’s time have hoped to construct a great genealogy, or “tree of life,” which could describe the interrelationships of all living things on the planet. Bhattacharya and his co-authors undertook to unite a key limb of that “tree,” the limb that includes all red algae, green algae and land plants.

“What we knew about red algae we learned from sequencing extemophiles – algae that live in extreme environments – because their genomes are relatively small,” said Bhattacharya a professor of ecology, evolution, and natural resources in Rutgers’ School of Environmental and Biological Sciences. “But most red algae are mesophiles, which means they live in relati

vely normal temperatures.”

 But recently, a new generation of gene sequencing machines has become available, and Bhattacharya’s lab has acquired one. This machine allowed him and his co-authors to sequence the entire genome of Calliarthron tuberculosum and a comprehensive list of expressed genes - genes caught in the act of fulfilling their functions - for Porphyridium cruentum, two species of red algae, in just a couple of weeks. They were then able to compare this bonanza of data to all the data available in protein banks, discovering that red and green algae shared half their genomes – and that red algae also shared a considerable part of its genome with other, non-algae organisms.

“The availability of next-generation sequencers has opened the window to generate sequences from all kinds of organisms,” Bhattacharya said. “This has opened our eyes to how complicated the history of genes is in the tree of life. Here, we finally got genomic data for normal red algae, and that’s opened our eyes to, holy smoke, how complicated they are.”

Bhattacharya believes the research has important practical implications. “The ancestor gave rise to the algae that generated all the fuel we currently use in cars and planes, and the photosynthetic organelle they contain is the target for modern biofuel research using fast-growing algae.”

Knowing what red and green algae will help scientists to figure out how they differ, and how those differences may be manipulated to create better algae for biofuel research, according to Bhattacharya. “For example, these algae could be designed to incorporate desirable characteristics from both lineages,” he said.

Bhattacharya’s co-authors are Titas Banerjee, a Rutgers undergraduate at the time she took part in the work; postdoctoral scholar Cheong Xin Chan; Hwan Su Yoon, of the Bigelow Laboratory for Ocean Sciences in Boothbay Harbor, Me.; Patrick Martone, of the University of British Columbia; and Jose Estevez, of the University of Buenos Aires.