Single-celled algae are far more complex than previously assumed
Single-celled algae generate approximately 50% of the world's oxygen production. Despite the huge prevalence of these algae and in the world's oceans, knowledge about their life cycle is often rather sparse. However, researchers have now shown that the reproductive capacity of single-celled algae depends on their growth conditions, and that the microscopic algae are perhaps far more complex than previously assumed.
A research study has just been published in the prestigious scientific journal, Science Advances. In the study, researchers from Aarhus University and the University of Copenhagen show that two of the more common algae species, Teleaulax amphioxeia and Plagioselmis prolonga are in fact same species, but they change the amount of their DNA depending on the nutrition available and the season.
Algae typically reproduce asexually through cell division. It was actually something of a coincidence that the researchers discovered that certain species of single-celled algae can vary the amount of their DNA depending on their growth conditions and their environment.
Lumi Haraguchi, a PhD student from Aarhus University had forgotten about an algae culture in the laboratory. She was very surprised when she later found the sample again and, out of pure curiosity, studied the algae that had grown. The algae had apparently changed species!
Lumi Haraguchi assumed that there must be some type of contamination in the sample, so she repeated the experiment under more controlled conditions. With the same result. She then collaborated with research groups at the Museum of Natural History and the Department of Biology at the University of Copenhagen, who helped with molecular analyses. Together, they were able to demonstrate that the two seemingly different species were actually the same, but that they were morphologically in two very different forms.
What Lumi had found was algae of the same species, but at different stages in their life cycle. One of them had a single set of chromosomes, the so-called haploid, which we know from the eggs and spermia of higher organisms, while the other was an organism with two sets of chromosomes, diploid, found in animals and plants.
Shortage of resources is a key factor
The researchers continued their analyses in Roskilde Fjord, and they were able to demonstrate that the algae changed between the two forms, depending on whether there was a shortage or abundance of resources. Transferred to humans and other multicellular organisms, this would correspond to dropping half of our DNA if food was hard to find, and doubling it when there was enough food again.
The study has revealed that single-celled algae can have a far more complex life cycle than previously assumed, and that they can change their reproductive strategy from being asexual in the diploid stage of their life cycle, when there are abundant nutrients, to being sexual in the haploid stage, when there is a shortage of nutrients. The haploid stages can then - perhaps when the nutritional conditions become better again - multiply sexually and become the diploid stage! Which then continue to multiply asexually. The researchers have never seen this before. When single-celled algae are exposed to a lack of nutrition, they typically form single-celled resting stages, which can survive for years in plankton or benthic sediments, and which do not germinate until the conditions again become favourable.
The molecular biology studies were conducted with Nina Lundholm from the Museum of Natural History, University of Copenhagen. Enumeration and isolation of species in algae cultures from Roskilde Fjord by Lumi Haraguchi from Aarhus University.