Extreme polar light environment of the North and South Poles sustains biodiversity

In a recently published research article, Professor of Subarctic Ecology Kari Saikkonen from the University of Turku, Finland, and his colleagues present a new theory on the role of the Earth's polar light environment in sustaining biodiversity on a geological timescale spanning millions of years. The length of the Earth's daylight and night is characterized by year-round equal amount of daylight and night at the Equator, minor seasonal variation moving away from the Equator, and substantial seasonality of day length closer to the Poles. In the far North and far South, inside the Arctic and Antarctic circles, this results in the unique months-long phenomena of the "midnight sun" with 24-hour daylight in the summer and the "Polar Night" of winter, when the sun does not rise above the horizon for months at a time.

"At the centre of our theory is the hypothesis that the extreme light environment of the polar regions creates hybrid zones in both polar regions," says Saikkonen.

Unlike temperature, day length is a stable environmental factor that changes consistently across latitudes but is unaffected by the local or global climate. Thus, many organisms, especially photosynthetic organisms such as plants and many microbes, have adapted to use the seasonal changes in day length to time, for example, their reproduction. Because they use the light as a signal, the light environment in the polar regions increases the likelihood that the flowering of closely related plant species will coincide. This, in turn, creates opportunities for species to hybridize.

Hybridization refers to organisms reproducing with another species or variety. Hybridization can be done intentionally, such as with many agricultural crops to create a particular desired trait, or it can occur naturally when species are near each other and are sufficiently biologically compatible.

"Although hybridization is common in almost all groups of organisms, its role as a force for sustaining biodiversity has not been fully understood. Hybridization may also involve backcrossing, where hybrid individuals mate with individuals of the original species. This allows genes to be transferred from one species to another, while creating new adaptive gene combinations to different environmental conditions," says Saikkonen.

At lower latitudes, the slight change in day length between seasons does not cause overlap in the timing of reproduction among genetically distinct populations, subspecies or varieties belonging to a species complex, nor does it promote hybridization.

"Therefore, species' range shifts across latitudes during the cycles of the Earth's cooler and warmer periods cause recurrent isolation and contact among species. This results in mixing and differentiation of species and creates new biodiversity over long periods of geological time," says Saikkonen.

Microbes play a crucial role in development and sustaining of biodiversity

Microbes have played a key role in the evolution of present biodiversity since the origin of life and continue to have a significant role in maintaining and promoting global biodiversity.

"Microbes are ubiquitous, and mounting evidence continues to reveal that they have high adaptive potential due to their short life cycle. Many microbes are light sensitive and affect the well-being of virtually all plants and animals. Since all plants and animals have a diverse microbiota, they should be treated as a whole," notes Saikkonen.

In the new study, Saikkonen and colleagues hypothesise how photosensitive microbes can help plants adapt to polar regions.

Climate change has a major impact on Earth's polar regions

Climate change and biodiversity loss are among the greatest global threats to ecosystems and ecosystem services in human history. The Earth's polar regions are warming at an unprecedented rate -- up to 2-4 times faster than the Earth's average.

"Climate models predict that Arctic Sea ice will melt by the end of this century. Over the same period, Antarctica's ice-free area will increase from approximately two percent today to almost 25 percent. The melting of the western Antarctic glaciers alone would cause sea levels to rise by five metres, threatening 10 percent of the world's population and many of the world's coastal ocean ecosystems over the next decades or centuries," says Saikkonen.

The researchers challenge the conventional species-focused discussion on biodiversity by focusing not only on species, but also on the genetic diversity of organisms and the significance of essential microbial associates of plants and animals.

"We propose that biodiversity can, in the long term, recover after disturbances and mass extinctions, but ecosystems will restructure as novel species assemblages. This calls for increased attention to the importance of ensuring sufficient genetic, species, and species interaction potential to support future diversification and ecosystem functions and services.

Thus, tackling climate change-driven biodiversity loss is important," stresses Saikkonen.