Effects of Polar Light Cycle on Microbial Food Web

Populations of Antarctic lake microbial communities change with the seasons.

Aerial view of Ace Lake in March, as ice begins to reform on the surface. (Courtesy of Anthony Hull)

The Science

Light-harvesting bacteria (phototrophs) in Antarctica’s Ace Lake defy the norm when it comes to nutrient cycling in polar regions. While microbial populations in other Antarctic lakes and the nearby Southern Ocean shift from phototrophs to archaea when sunlight becomes scant, Ace Lake’s bacteria instead go through a boom and bust cycle aligned with light availability.

A long-running time-series study offers a first glimpse into a full season’s cycle of this polar environment. This cycle suggests a reliance on photosynthesis to maintain nutrient cycling that evolved separately from other similar systems, and offers new clues into a unique environment in the natural world.

The Impact

From the polar regions to the deep ocean, cold environments occupy more space on Earth than any other ecosystem. Therefore, understanding the microbial activity that dominates these polar environments greatly contributes to our larger understanding of carbon and other nutrient cycles on a global scale.

Click on the image or go here to watch the video, “How microbes of an Antarctic lake have adapted to the polar light cycle” produced for Microbiome.

Summary

Ace Lake, located in the Vestfold Hills in East Antarctica, was separated from the Southern Ocean roughly 5-7,000 years ago and has been well-studied by many researchers for nearly five decades. In these frigid systems, where organisms higher up in the food web like fish do not exist, nutrient cycling takes place only on a microbial level.

Previous studies focused on the Southern Ocean have found that in winter months, when sunlight is nonexistent, the bacteria are replaced by carbon-fixing archaea. Although Ace Lake was formed from the Southern Ocean long ago, this shift to archaea being the main producer doesn’t occur here. Understanding this fundamental difference offers new insight into the primary producers fueling our planet’s biogeochemical cycles.

Water samples from Ace Lake were collected over a 10-year period and sent to JGI for metagenomic sequencing. A study recently published in Microbiome shows that the way nutrients flow through the ecosystem of Ace Lake is different than nearby Antarctic lakes of similar origin. The study was led by Rick Cavicchioli, professor at the University of New South Wales, Australia, and included researchers at the U.S. Department of Energy (DOE) Joint Genome Institute (JGI), a DOE Office of Science User Facility located at Lawrence Berkeley National Laboratory (Berkeley Lab). The work was enabled by the JGI’s Community Science Program.

For most months each year, Ace Lake is covered in thick ice, with a combination of marine and freshwater below. This stratified mixture holds oxygenated water up top and anoxic water below. Cavicchioli and his team took samples from all three sections of the stratified water and determined that the microbial communities look very different across each section.

The biggest changes occur in the top portions of the lake, they found. Near the lower portion of the top layer, known as the interface, there is a strong chemical and salinity gradient — and green sulfur bacteria, Chlorobium, blooms. The specific mixture of light and salinity in this portion of the lake provides everything the bacteria needs to survive.

In December, January and February, the polar region’s summer months, this study found that the green sulfur bacteria population spiked to 83%, only to dramatically fall to 1% in early spring and rebound by the late spring. Preliminary observations regarding the contributions of algae and viruses to the ecosystem were noted, but require further exploration.

Contacts:

BER Contact
Ramana Madupu, Ph.D.
Program Manager, DOE Joint Genome Institute
Biological Systems Sciences Division
Office of Biological and Environmental Research
Office of Science
US Department of Energy
Ramana.Madupu@science.doe.gov

JGI Contact
Emiley A. Eloe-Fadrosh, Ph.D.
Metagenome Program Head
Environmental Genomics Group Lead
eaeloefadrosh@lbl.gov

PI Contact
Rick Cavicchioli, Ph.D.
The University of New South Wales, Australia, Professor
r.cavicchioli@unsw.edu.au

Funding

This work was supported by the Australian Research Council (DP150100244) and the Australian Antarctic Science program (project 4031). The work conducted by the US Department of Energy Joint Genome Institute, a DOE Office of Science User Facility, is supported by the Office of Science of the US Department of Energy under contract no. DE-AC02-05CH11231. Computational analyses at UNSW Sydney were performed on the computational cluster Katana, supported by the Faculty of Science.

Publications

Panwar Pet al. Influence of the polar light cycle on seasonal dynamics of an Antarctic lake microbial community. Microbiome. 2020 August 9. doi: 10.1186/s40168-020-00889-8