The aquatic ecosystems of Antarctica keep turning up surprises for scientists, who have found flourishing ecosystems comprised mostly of microorganisms. A new paper, published in Science, examined samples drawn from an Antarctic lake and tracked seasonal changes in the viruses that prey on these bacteria. They found large, diverse populations of viruses, some of which have never been seen underwater before, and some that had previously only been found infecting birds, mammals, or plants.

The Antarctic is a great area if you're in want of a virtually untouched ecosystem. Several lakes that are covered with ice for most or all of the year have ecosystems that consist mostly of microorganisms that have adapted to the cold, dark, and nutrient-scarce environment. These microbial ecosystems are balanced by limiting factors, such as the amount of food and predators. In the case of Antarctic lake inhabitants, though, the predators are viruses that have likewise adapted to the conditions.

These systems and their viral predators have previously been difficult to study, due to the obstacles involved in getting the viruses' hosts to grow in lab conditions. To rectify this, a research team collected water samples from Lake Limnopolar on the Byers Peninsula in Antarctica before and after the warm season. The samples showed that Lake Limnopolar harbored an unusually high number of viral genotypes—they ran into the thousands, whereas previously studied ecosystems had only a few hundred.

In the post-summer sample, they noticed an increase in the population of larger varieties of viruses (larger than 50 nanometers) and a decrease in the smaller ones (shorter than 30 nanometers). The researchers also noted that most of the viruses were unfamiliar, as the genome sequences weren't found in current databases—less than 3 percent were at all similar to viral genomes previously found in aquatic environments.

The snag in identifying the viruses turned out to be more fundamental than they expected. Viruses found in aquatic environments usually infect prokaryotes, like bacteria. However, the viruses found in the Antarctic water included a distinct population of small, single-stranded DNA (ssDNA) viruses.

These turned out to be related to known viruses with radically different targets, ones that infect only non-aquatic eukaryotes or larger organisms like mammals, birds, and plants. In fact, some of the Antarctic water viruses have not been found in aquatic environments ever before. The researchers suspect that they must be infecting small eukaryotic hosts in Lake Limnopolar.

The researchers were able to assemble 10 circular ssDNA genomes elements that were previously undescribed. They found these ssDNA viruses dominate during the winter, when there is more ice cover on the lake.

Large-tailed, double-stranded (dsDNA) viruses, which typically attack prokaryotes, dominate during the summer, when there are algae blooms (ironically, a eukaryote) in the lake. This indicates that the inhabitants of the lake's ecosystem wax and wane seasonally, and there are different dominant populations of bacteria depending on the time of year. These warm-season dsDNA viruses were found to help the microorganisms metabolise carbohydrates and amino acids, as well as respirate and effect stress responses, indicating that they actually helped the bacteria survive the summer.

At the outset of the study, the sheer diversity and number of viruses found to be living in the lake were a surprise to the researchers. They not only found an ecosystem that adjusts to the time of year, but one that depends on a viral presence that was expected: an undiscovered family of eukaryote viruses that were slumming with bacteria in the dead of winter. Despite the unexpected findings, the samples put researchers well on their way to understanding the freshwater ecosystems of Antarctica.

Science, 2009. DOI: 10.1126/science.1179287