14. January 2020 · Press release

Alga of the Year 2020: Chromera velia, autonomous relative of prominent parasites

The alga Chromera velia lives associated with corals in the Australian Great Barrier Reef. It is related to parasites such as the pathogen that causes malaria, however, the alga can live without a host, whereas parasites are dependent upon their hosts for survival. Chromera velia combines the lifestyles of free-living algae and parasites and thus helps to understand the evolution of pathogens. Algal researchers from the German Society for Plant Sciences (DBG) have curated Chromera velia as Alga of the Year 2020.

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Researchers isolated the alga Chromera velia from stony corals they found in Sydney Harbor, off Magnetic Island, and One Tree Island in the Great Barrier Reef. They were in fact looking for other already known algae that live closely with the corals. By coincidence, they discovered an unknown species, which, despite superficial similarity with the sought-after algae, differed from them in many properties. The biologists named the new alga Chromera velia in 2008.

Chromera velia and the corals, relationship status unclear

While the algae that usually live in symbiosis with the corals do this to their mutual benefit, the relationship between Chromera velia and the corals is rather uncertain. Most of the corals in the sea are free of Chromera velia. If young coral larvae are brought together with Chromera velia in the laboratory, the algae colonize the outer tissue of the coral. After a few days however, the coral larvae are free of the alga again. This is different from the algae living in symbiosis with corals, which permanently colonize the corals from the inside via their stomach.

In the laboratory, Chromera velia grows and reproduces easily in artificial seawater, so it is not dependent on the corals. All Chromera velia cultures in the world have been obtained from coral samples, and the alga has never been observed freely in the ocean. Therefore, it is likely that there is a symbiotic relationship between Chromera velia and the corals, but the exact nature of this relationship remains unknown.

Relative of dangerous parasites

Based on its genetic fingerprint and other traits, the newly discovered algae turned out to be related to a group of unicellular organisms called Apicomplexa. This group contains many parasitic pathogens, including the causative agents of malaria and toxoplasmosis. Many people fall ill or die from malaria, which is transmitted by mosquitoes. The toxoplasmosis pathogen can be dangerous to unborn children during pregnancy and is transmitted to humans from cats. Chromera velia was found to be the closest photosynthetic relative of these unicellular parasites, and it is actually a closer relative of Apicomplexa than to the closest related group of algae, the dinoflagellates. The dinoflagellates include, for example, the above-mentioned algae living in symbiosis with corals, or the Alga of the Year 2013, Lingulodinium polyedrum, which causes the milky seas effect.

Puzzle piece between photosynthesis and parasitism

The relationship between Chromera velia and the parasitic Apicomplexa gives indications of the origin of the pathogens. “The parasitic Apicomplexa evolved from free-living algae, although this seems quite unnecessary. Algae perform photosynthesis, they grow from light, CO2, and water. However, parasites depend on nutrients from their hosts for their growth. Chromera velia helps us to understand this switch in lifestyle. It is an alga, but at the same time it has a lot in common with the parasitic Apicomplexa,” explains algal researcher Dr. Ansgar Gruber. He is a member of the Phycology (algal research) Section of the DBG.

Knowing the similarities and differences between Chromera velia, the parasitic Apicomplexa, and their hosts also allows developing new therapies against pathogens. A similarity between Chromera velia and the parasitic Apicomplexa is the presence of a cell component called chloroplast. The alga uses the chloroplast for photosynthesis. In the Apicomplexa, however, this chloroplast is highly reduced. They do not need it for photosynthesis, but for other metabolic pathways. Humans and animals have no chloroplasts. “The reduced chloroplast of the Apicomplexa is an exposed target, a point where they are vulnerable, which we do not have,” says Gruber. The reduced chloroplasts of Apicomplexa are therefore interesting as potential targets for new drugs against malaria and toxoplasmosis; similar to antibiotics, which only work in bacteria, but not in human cells. The chloroplasts of Chromera velia, which is easy to cultivate in the laboratory, are therefore a centre of attention.

Unique genetic code proves common origin

A co-discoverer of the close relationship between Chromera velia and the Apicomplexa is Prof. Miroslav Oborník, who is a researcher at the Institute of Parasitology in the Czech Academy of Sciences in Budweis. Oborník examined the genetic code of Chromera velia. The genetic code defines how genetic information is interpreted during the synthesis of proteins, and in general it is uniform for all organisms. However, Oborník found that the alga Chromera velia uses an alternative genetic code in its chloroplasts. “There is a stop command in a gene that is absolutely necessary for photosynthesis. According to the standard code, this gene could not work at all. But instead of following the stop command, Chromera velia’s protein biosynthesis machinery translates the genetic information of the DNA into a tryptophan amino acid. The same deviation in the genetic code can also be found in the reduced chloroplasts of Apicomplexa. When I noticed this commonality, I was sure that Chromera velia had to be related to the Apicomplexa,” Oborník describes the discovery.

Open questions

Due to the importance of Chromera velia as a relative of devastating parasites, its entire genome was sequenced in 2015. Thanks to the now available data, it could become a model organism for basic research on therapies against malaria and other parasites of the Apicomplexa group. Gruber and Oborník are in contact with researchers from the about 15 groups that work on Chromera velia worldwide. They conduct their research in Budweis, where they are interested in the details of the relationship between Chromera velia and corals, i.e. whether alga or coral benefit from each other or harm each other. “Photosynthesis and parasitism are rarely combined in one organism. Chromera velia could show us why this combination is so unusual,” suspects Gruber. Whether Chromera velia is threatened by the climate change related decline of corals, benefits from weakened corals, or even contributes to coral damage are also exciting questions that still need to be investigated.

The alga Chromera velia under the light microscope. The chloroplasts with the unique genetic code appear golden brown in the light microscope. Image: Miroslav Oborník