The invisible hitchhikers alerting scientists to changes in the East Australian Ocean

Thursday, 10 December 2020

For the first time, tiny marine organisms – known as microbes – have been recorded riding the iconic East Australian Current (EAC) from the Great Barrier Reef right down to Tasmania, alerting researchers to significant oceanic changes impacting the marine food chain.

A radar image of an east coast low

Microbes – which take the form of bacteria, fungi, algae and plankton – constitute almost 98 per cent of the ocean’s biomass (100,000 microbes per ml) and control the supply of key nutrients required by all life. Through their various biological processes including photosynthesis, microbes produce half the oxygen we breathe and ultimately maintain the ocean’s health.

As a result of shifting currents in the Australian ocean, new research published in Global Change Biology reveals the consequences of the changed microbial makeup on the environment.

“With changes in the climate across the Pacific, the EAC is becoming stronger and travelling further south than ever before,” said senior author and University of Newcastle research fellow, Dr Mark Brown.

“The EAC plays host to millions of microbes and other tropical organisms hitching a ride down the coast. These including both tiny plants capable of photosynthesis, known as phytoplankton, as well as microbes that consume the carbon these plants produce”.

“More strength and intensity in the EAC means more north-based flora and fauna infiltrating temperate seas, impacting the environment they’re displacing. This has a ricochet effect on the broader ecosystem,” Dr Brown said.

Over 60 years, the EAC has experienced a 350km southerly expansion of its warm water flows into the Tasman Sea.

Previous research has indicated these changes have led to sea surface temperatures warming at nearly four times the global ocean average and increased marine heatwaves, making our iconic eastern coastline a ‘climate change hotspot’.

Space invaders

Lead author, Dr Lauren Messer from QUT (Queensland University of Technology), said microbes from northern waters could cause significant changes in southern marine ecosystems.

“Warmer waters contain fewer nutrients than colder waters, so the microorganisms travelling down from the north are well-adapted to a nutrient-poor environment and have traits such as smaller cells with a larger surface area to volume ratio,” Dr Messer said.

“When they arrive in the south, they can out-compete the existing microbes that are more suited to the southern nutrient-rich environment. One important change that occurs is that the size of the organisms carrying out most of the photosynthesis becomes smaller, making them less likely to be eaten by resident filter-feeding organisms like zooplankton” she explained, “hence leading to a cascading effect up the food chain”.

With one of the most diverse marine environments on the planet, home to critical marine ecosystems such as the magnificent giant kelp forests, Australia’s Tasman Sea is already under increasing pressure from warming waters and marine heatwaves.

“The potential knock-on-effect these invading microbes are having on larger species up the food chain is just another threat to the myriad of plants and animals who call the temperate Tasman Sea their home,” she explained.

Intrepid data collection

The team used advanced genetic sequencing to investigate the makeup of the EAC. Spending time aboard CSIRO’s former research vessel Southern Surveyor and drifting with the current over a number of days, they collected samples which were refined for DNA extraction through a microscale water filter.

By sequencing the DNA to determine the makeup of the water, the researchers were able to map microbial movement and generate a genetic database of the organisms in the EAC.

“Next, working with the Integrated Marine Observing System (IMOS), we compared our EAC DNA data with DNA data collected at their National Research Stations, which take samples from multiple depths every month at several stations around Australia,” Dr Brown said.

“This allowed us to map the movement of specific EAC microbes into southerly regions, and clearly shows a change in the abundance of northern-based organisms.”

A marine canary in the coalmine

In 2018, Dr Mark Brown and colleagues established an unprecedented microbial baseline in the oceans around Australia against which to measure the effects of climate change and human activities.

Having now painted a picture of the EAC environment, they hope to continue their work to map other important ocean currents and identify the impact on larger species.

“This is the first-time microbes have been used as indicators of ocean currents, and we hope it will become a permanent feature in EAC research,” Dr Brown said.

“Whilst we can’t easily implement strategies to manage microbes because of their small size and enormous abundance, it’s important when looking at future predictions of climate change and the ecosystem to understand the impact of microbes as part of a bigger picture.”

The team included researchers from The University of Newcastle, QUT, UTS Sydney, Macquarie University and CSIRO.

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