Thursday, September 26, 2024

OCEANOGRAPHY


‘Invisible forest’ of algae thrives as ocean warms



University of Exeter
CTD rosette – a device equipped with sensors and bottles to collect water samples and measure different properties of the ocean at various depths 

image: 

CTD rosette – a device equipped with sensors and bottles to collect water samples and measure different properties of the ocean at various depths

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Credit: Dr Bob Brewin





An “invisible forest” of phytoplankton is thriving in part of our warming ocean, new research shows.

Phytoplankton are tiny drifting organisms that do about half of the planet’s “primary production” (forming living cells by photosynthesis).

The new study, by the University of Exeter, examined phytoplankton at the ocean surface and the “subsurface” – a distinct layer of water beneath – to see how climate variability is affecting them.

Published in the journal Nature Climate Change, the findings show these two communities are reacting differently.

Over the last decade, the total “biomass” (living material) of subsurface phytoplankton has increased in response to warming.

Meanwhile, surface phytoplankton now has less chlorophyll – making it less green – but in fact total biomass has remained stable.

Based on 33 years of data from the Bermuda Atlantic Time-series Study (BATS) in the Sargasso Sea, the findings also suggest the depth of the “surface mixed-layer” (region of turbulence at the surface of the ocean) has shallowed as the ocean rapidly warmed in the last decade.

“It’s important to understand these trends because phytoplankton are the foundation of the marine food web, and play a key role in removing carbon dioxide from the atmosphere,” said Dr Johannes Viljoen, from the Department of Earth and Environmental Science at Exeter’s Penryn Campus in Cornwall.

“Our findings reveal that deep-living phytoplankton, which thrive in low-light conditions, respond differently to ocean warming and climate variability compared to surface phytoplankton.

“We typically rely on satellite observations to monitor phytoplankton, but the subsurface is hidden from satellite view.

“Our study highlights the limitations of satellite observations, and underscores the urgent need for improved global monitoring of phytoplankton below what satellites can see.”

Co-author Dr Bob Brewin added: “Changes at the base of the food web can have cascading effects on marine life, from tiny zooplankton to large fish and marine mammals.

“So the future of phytoplankton will have major implications for biodiversity, as well as climate change.”

Dr Viljoen added: “Continued monitoring of these deep-living phytoplankton will help scientists better understand ongoing changes in the ocean that might otherwise go unnoticed.”

The research of Dr Viljoen and co-authors Dr Brewin and Dr Xuerong Sun, all from the Centre for Geography and Environmental Science, is supported by a UKRI Future Leader Fellowship awarded to Dr Brewin.

The paper is entitled: “Climate variability shifts the vertical structure of phytoplankton in the Sargasso Sea.”

CTD rosette – a device equipped with sensors and bottles to collect water samples and measure different properties of the ocean at various depths

Credit

Dr Xuerong Sun


Journal

New study: Deep-sea discovery shines light on life in the twilight zone


Unexpected findings expand our understanding of the impacts of climate change, including how and where the ocean stores carbon, said co-author and University of South Florida scholar Tim Conway




Peer-Reviewed Publication

University of South Florida

CMS - iron twilight zone - header.jpeg 

image: 

A conductivity, temperature and depth (CTD) rosette used to sample water from the ocean’s twilight zone during a GEOTRACES expedition in the Pacific Ocean.

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Credit: Alex Fox





TAMPA, Fla. (Sept. 23, 2024) – The ocean’s twilight zone is deep, dark, and — according to new research — iron deficient.

No sunlight reaches this region 200 to 1,000 meters below the sea surface, where levels of iron, a key micronutrient, are so low that the growth of bacteria is restricted. To compensate, these bacteria produce molecules called siderophores, which help the bacteria scavenge trace amounts of iron from the surrounding seawater.

The paper detailing these unexpected findings from the Pacific Ocean will publish on Wednesday, Sept. 25, at 11 a.m. ET (4 p.m. London Time) in Nature, and will be viewable at that time at this link. The study could change the way scientists view microbial processes in the deep ocean and offer new insight into the ocean’s capacity to absorb carbon.

“Understanding the organisms that facilitate carbon uptake in the ocean is important for understanding the impacts of climate change,” said Tim Conway, associate professor of chemical oceanography at the USF College of Marine Science, who co-authored the recent study. “When organic matter from the surface ocean descends to the deep ocean, it acts as a biological pump that removes carbon from the atmosphere and stores it in seawater and sediments. Measuring the rates and processes that influence this pump gives us insight into how and where the ocean stores carbon.”

To conduct the study, researchers collected water samples from the upper 1,000 meters of the water column during an expedition through the eastern Pacific Ocean from Alaska to Tahiti. What they found in the samples surprised them. Not only were concentrations of siderophores high in surface waters where iron is expected to be deficient, but they were also elevated in waters between 200 and 400 meters deep, where nutrient and iron concentrations were thought to have little impact on the growth of bacteria.

“Unlike in surface waters, we did not expect to find siderophores in the ocean’s twilight zone,” said Conway. “Our study shows that iron-deficiency is high for bacteria living in this region throughout much of the east Pacific Ocean, and that the bacteria use siderophores to increase their uptake of iron. This has a knock-on effect on the biological carbon pump, because these bacteria are responsible for the breakdown of organic matter as it sinks through the twilight zone.”

The recent discovery was part of GEOTRACES, an international effort to provide high-quality data for the study of climate-driven changes in ocean biogeochemistry.

The study of siderophores is still in the early stages. Researchers involved in GEOTRACES only recently developed reliable methods to measure these molecules in water samples, and they’re still working to understand where and when microbes use siderophores to acquire iron.

Although the research into siderophores is new, this study demonstrates their clear impact on the movement of nutrients in the ocean’s twilight zone.

“For a full picture of how nutrients shape marine biogeochemical cycles, future studies will need to take these findings into account,” said Daniel Repeta, senior scientist at Woods Hole Oceanographic Institution and co-author of the article. “In other words, experiments near the surface must expand to include the twilight zone.”

Funding for this work was provided by the National Science Foundation and the Simons Foundation. The U.S. portion of GEOTRACES is provided by the National Science Foundation.

Click here for images and a PDF of the journal article

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About the University of South Florida

The University of South Florida, a high-impact research university dedicated to student success and committed to community engagement, generates an annual economic impact of more than $6 billion. With campuses in Tampa, St. Petersburg and Sarasota-Manatee, USF serves approximately 50,000 students who represent nearly 150 different countries. U.S. News & World Report has ranked USF as one of the nation’s top 50 public universities for five consecutive years, and this year USF earned its highest ranking ever among all universities public or private. In 2023, USF became the first public university in Florida in nearly 40 years to be invited to join the Association of American Universities, a prestigious group of the leading universities in the United States and Canada. Through hundreds of millions of dollars in research activity each year and as one of the top universities in the world for securing new patents, USF is a leader in solving global problems and improving lives. USF is a member of the American Athletic Conference. Learn more at www.usf.edu.

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