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By Andrea Wild 10 June 2021 4 min read

In the coastal waters around Tasmania, giant kelp forests have become an icon of marine biodiversity.

Anchored to the sea floor, soaring up to 30 metres to the water’s surface, giant kelp provides habitats for marine animals and safe nurseries for many fish species. But during the past 40 years, Tasmania’s giant kelp forests have declined by more than 90 per cent.

Tasmania is a biodiversity hotspot for marine macroalgae, or seaweeds. They come in reds, greens and browns and Australia has more than 1000 species. Photo: Unsplash

At the extreme end of the algae scale are tiny Synechococcus. This group of single celled cyanobacteria are one of the main primary producers in the ocean, releasing oxygen into the atmosphere and cycling nutrients in the ocean.

Reforesting Tasmania’s seas

Giant kelp is highly sensitive to water temperature. It requires shallow, coastal waters. As the seas surrounding Tasmania warm, giant kelp forests decline. With nowhere further south to retreat, the giant kelp forests are disappearing.

Flasks of algae growing under red light
Super kelp cultures in the lab at IMAS.

Recent work by researchers at the Institute for Marine and Antarctic Studies (IMAS) in Tasmania have revealed that some strains of Tasmanian giant kelp are naturally more tolerant of higher temperatures.

“The IMAS team is now using those strains as the foundation for germination trials at elevated temperatures,” said Dr Anusuya Willis of the Australian National Algae Culture Collection.

Scientists at CSIRO and IMAS are characterising the DNA of those giant kelp samples. They are working with 40 samples collected from different individuals at 6 different locations in Tasmanian coastal waters by IMAS divers, who took cuttings of the spore producing ‘blades’ or leaves.

Although they were collected from towering giants, the kelp samples cultured in the lab are microscopic gametophytes. These microscopic males and females float in tiny 5 mL tubes. Under red light at 4 degrees Celsius, they remain in stasis. At 12 to 15 degrees, they can be cultured to produce more cells for study. Blue light allows sperm from the male to fertilise egg cells from the female gametophyte, forming the sporophytes that grow into mature kelp.

Working with giant kelp cells in the lab is challenging.

“Extracting DNA from kelp is difficult due to its cell walls. They contain compounds such as polyphenols that make it difficult to purify the DNA,” said Anusuya.

The team is looking at single nucleotide polymorphisms (SNiPs), which are sites at which DNA varies across the genome of a species. SNiPs can be used to link particular traits, like temperature tolerance, to individuals.

“Ultimately, we’re aiming to identify what makes the warm-tolerant strains of giant kelp different. We want to help IMAS create a resource like a seed bank to use in reforesting coastal habitats that have lost their giant kelp forests,” said Anusuya.

From giants to microbes

On the other side of the lab, Anusuya is supervising a study of Synechococcus strains that were collected from the Sargasso Sea and freshwater lakes in the Artic and Antarctic. Some have been living in culture for nearly 40 years.

Synechococcus is a group of cyanobacteria, or blue-green algae. They live in both salt and fresh water. Unlike more familiar kinds of blue-green algae, they can be very numerous but don’t bloom in harmful ways or produce toxins.

The name blue-green is also misleading. Synechococcus strains can be different pinks and greens and oranges. Living in the upper 100 metres of water, they all use light in different ways, allowing them to occupy micro niches in nature, living together but not competing.

Under the microscope, the individual cells are so small they look like tiny dots.

“We can’t tell much about Synechococcus just by looking at them. Fortunately, they have a small genome of only a few million base pairs, so we are able to sequence the full genome of each strain,” said Ming Fen Eileen Lee, a research student working at the algae collection.

“We are characterising their genetic diversity related to light and temperature, to find out how Synechococcus strains might be impacted by future warming,” she said.

The team has found different Synechococcus strains have lots of genetic diversity, but don’t seem particularly adapted to the location where they were found.

“Synechococcus strains seem to be generalists. In my temperature experiments, the sample from the Sargasso Sea couldn’t grow below 15 degrees Celsius, but the ones from Arctic and Antarctic have a huge temperature range and actually seem happiest at 20 degrees,” Eileen said.

“This is the algae paradox,” Anusuya explained. “Synechococcus have a lot of diversity but it seems they haven’t lost the ability to live across a broad ecological niche.”

Flasks of algae growing at the Australian National Algae Culture Collection. Being a very diverse group, Synechococcus aren’t classified to species level. Instead, each different strain is given a code.

So much more than seaweed and algal blooms

Algae are a very diverse group that are integral to marine biodiversity and our planet’s wellbeing.

Cyanobacteria are a huge and ancient group, dating back 2.5 billion years. They are the original oxygen producers on the planet. They started oxygenic photosynthesis and are the ancestors of the chloroplasts in plants,” Anusuya said.

“Through photosynthesis, singled-celled algae in the ocean produce half the oxygen we breathe. They are vital to our wellbeing and our planet’s health. This is why we are so interested in Synechococcus,” Anusuya said.

“Giant kelp forests are high value habitat and support many native species, including young fish. Planting thermally tolerant strains to replenish giant kelp forests remains one of our best hopes for the future of these beautiful and important habitats,” she said.

One of the last remaining patches of giant kelp forest on the northeast coast of Tasmania. Photo: Cayne Layton

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