From algae to parasites: new 'missing link' species found in Sydney Harbour

It's not often that brown algae is considered exciting or revolutionary, but Associate Professor Dee Carter from the School of Molecular and Microbial Biosciences at the University of Sydney and her former PhD student, Robert Moore, have found a new species of single-celled brown algae that is an evolutionary missing link.

The new marine species, named Chromera velia, was found living in Sydney Harbour and turns out to be the closest living photosynthesising relative to a group of parasites called Apicomplexa. The Apicomplexa are a significant group of parasites, as they cause malaria and other diseases that kill and disable millions of people every year.

What makes this new species special, is that it is the missing link between photosynthesising algae, which use the sun's energy to make food, and the parasitic Apicomplexa, which use the cells of their host organism to obtain food. Chromera velia has a unique genetic feature in the psbA gene, which has been found only in the largest group of Apicomplexa, indicating how closely related the algae species is to the parasites.

"Chromera velia does photosynthesise, but it has lost one of its photosynthetic pigments - the chemicals which allow the sun's energy to be trapped and transformed into sugar in the algae. Many Apicomplexa, such as the species which causes malaria, have an organelle in their cells that is really similar to the chloroplasts in photosynthesising species, such as algae and plants," explained Dee.

Chloroplasts are where photosynthesis goes on in algae and plants, while parasitic Apicomplexa have an unpigmented chloroplast-like organelle called the apicoplast.

"Malaria treatments often target the apicoplast in the invading Apicomplexan cells, so not only is our new species important from an evolutionary perspective, it can also potentially be a surrogate host for developing anti-parasitic drugs," said Dee.

"For researchers working on treatments for diseases caused by Apicomplexa, the parasites are difficult to work on as they need to be grown in living host cells, whereas Chromera velia can be grown simply in the lab. Chromera velia is closely related to the parasites, so it could be a good model to work on in developing treatments."

Finding an evolutionary missing link is a rare and exciting discovery. This newly described species indicates how photosynthesising algae evolved into the fully parasitic Apicomplexa, with their left-over chloroplast remnant indicating their evolutionary past as algae.

"Chromera velia can tell us something about how these parasites, which were in fact once algae themselves, evolved. The Apicomplexans are now the only group of organisms that we can say with certainty transitioned from photosynthesising algae to parasites."

"It will be very interesting to see where Chromera velia fits on the spectrum from symbiont to parasite, since it was found living in corals and is photosynthetic, but has lost one of the normal photosynthetic pigments. It might be at the evolutionary stage of losing photosynthesis on its way to becoming parasitic," said Dee.

Dee's former PhD student, Robert, found the new species while researching algae that inhabit corals and allow them to grow. Their research focuses on understanding the lifecycle and biodiversity of algae living in corals, as they are very important for reef conservation.

"Robert had a few samples from Sydney Harbour and thought he would keep other algae that he came across apart from the ones we were studying. One of these turned out to be Chromera velia! It was thanks to Robert's scientific curiosity and his perseverance growing the algal culture that this discovery was made," said Dee.