Study examines the role of deep-sea microbial predators at hydrothermal vents
Researchers emphasize the need for baseline information of microbial food webs
IMAGE: A VIEW OF THE APOLLO VENT FIELD AT THE NORTHERN GORDA RIDGE, WHERE SAMPLES WERE COLLECTED BY THE ROV HERCULES FOR STUDYING MICROBIAL PREDATORS view more
CREDIT: IMAGE CREDIT: OET/NAUTILUS LIVE
The hydrothermal vent fluids from the Gorda Ridge spreading center in the northeast Pacific Ocean create a biological hub of activity in the deep sea. There, in the dark ocean, a unique food web thrives not on photosynthesis but rather on chemical energy from the venting fluids. Among the creatures having a field day feasting at the Gorda Ridge vents is a diverse assortment of microbial eukaryotes, or protists, that graze on chemosynthetic bacteria and archaea.
This protistan grazing, which is a key mechanism for carbon transport and recycling in microbial food webs, exerts a higher predation pressure at hydrothermal vent sites than in the surrounding deep-sea environment, a new paper finds.
"Our findings provide a first estimate of protistan grazing pressure within hydrothermal vent food webs, highlighting the important role that diverse deep-sea protistan communities play in deep-sea carbon cycling," according to the paper, Protistan grazing impacts microbial communities and carbon cycling ad deep-sea hydrothermal vents published in the Proceedings of the National Academy of Sciences (PNAS).
Protists serve as a link between primary producers and higher trophic levels, and their grazing is a key mechanism for carbon transport and recycling in microbial food webs, the paper states.
The research found that protists consume 28-62% of the daily stock of bacteria and archaea biomass within discharging hydrothermal vent fluids from the Gorda Ridge, which is located about 200 kilometers off the coast of southern Oregon. In addition, researchers estimate that protistan grazing could account for consuming or transferring up to 22% or carbon that is fixed by the chemosynthetic population in the discharging vent fluids. Though the fate of all of that carbon is unclear, "protistan grazing will release a portion of the organic carbon into the microbial loop as a result of excretion, egestion, and sloppy feeding," and some of the carbon will be taken up by larger organisms that consume protistan cells, the paper states.
After collecting vent fluid samples from the Sea Cliff and Apollo hydrothermal vent fields in the Gorda Ridge, researchers conducted grazing experiments, which presented some technical challenges that needed to be overcome. For instance, "prepping a quality meal for these protists is very difficult," said lead author Sarah Hu, a postdoctoral investigator in the Marine Chemistry and Geochemistry Department at the Woods Hole Oceanographic Institution (WHOI).
"Being able to do this research at a deep-sea vent site was really exciting because the food web there is so fascinating, and it's powered by what's happening at this discharging vent fluid," said Hu, who was onboard the E/V Nautilus during the May-June 2019 cruise. "There is this whole microbial system and community that's operating there below the euphotic zone outside of the reach of sunlight. I was excited to expand what we know about the microbial communities at these vents."
Hu and co-author Julie Huber said that quantitative measurements are important to understand how food webs operate at pristine and undisturbed vent sites.
"The ocean provides us with a number of ecosystem services that many people are familiar with, such as seafood and carbon sinks. Yet, when we think about microbial ecosystem services, especially in the deep sea, we just don't have that much data about how those food webs work," said Huber, associate scientist in WHOI's Marine Chemistry and Geochemistry Department.
Obtaining baseline measurements "is increasingly important as these habitats are being looked at for deep-sea mining or carbon sequestration. How might that impact how much carbon is produced, exported, or recycled?" she said.
"We need to understand these habitats and the ecosystems they support," Huber said. "This research is connecting some new dots that we weren't able to connect before."
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The research was supported by NASA, the National Oceanic and Atmospheric Administration, Ocean Exploration Trust, the National Science Foundation, and WHOI.
About Woods Hole Oceanographic Institution
The Woods Hole Oceanographic Institution (WHOI) is a private, non-profit organization on Cape Cod, Massachusetts, dedicated to marine research, engineering, and higher education. Established in 1930, its primary mission is to understand the ocean and its interaction with the Earth as a whole, and to communicate an understanding of the ocean's role in the changing global environment. WHOI's pioneering discoveries stem from an ideal combination of science and engineering--one that has made it one of the most trusted and technically advanced leaders in basic and applied ocean research and exploration anywhere. WHOI is known for its multidisciplinary approach, superior ship operations, and unparalleled deep-sea robotics capabilities. We play a leading role in ocean observation and operate the most extensive suite of data-gathering platforms in the world. Top scientists, engineers, and students collaborate on more than 800 concurrent projects worldwide--both above and below the waves--pushing the boundaries of knowledge and possibility. For more information, please visit http://www.
Authors :
Sarah K. Hu1*, Erica L. Herrera1, Amy R. Smith1, Maria G. Pachiadaki2, Virginia P. Edgcomb3, Sean P. Sylva1, Eric W. Chan4, Jeffrey S. Seewald1, Christopher R. German3, and Julie A. Huber1
Affiliations :
1 Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA, USA
2 Department of Biology, Woods Hole Oceanographic Institution, Woods Hole MA, USA
3 Department of Geology & Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA, USA
4 School of Earth, Environment & Marine Sciences, UT-RGV, Edinburg, TX, USA
*corresponding author
Scientists get to the bottom of deep Pacific ventilation
The team's findings, with important implications for ocean biogeochemistry and climate science, have been published by Nature Communications in a paper by Associate Professor Mark Holzer from UNSW Science's School of Mathematics & Statistics, with co-authors Tim DeVries (UCSB) and Casimir de Lavergne (LOCEAN).
"The deep North Pacific is a vast reservoir of remineralized nutrients and respired carbon that have accumulated over centuries," says A/Prof. Holzer. "When these deep waters are returned to the surface, their nutrients support biological production and their dissolved CO2 can be released into the atmosphere. As such, the deep Pacific plays a key role in the earth's climate system."
But what are the pathways of the ocean circulation that supply newly ventilated surface water to the deep Pacific? And how and where does this old water eventually return to the surface? To date, there were two competing theories for the role that the overturning circulation plays in this.
One theory - the 'standard conveyor' - envisions broad overturning with Antarctic Bottom Water upwelling to around 1.5 km depth before flowing back south to the Southern Ocean. The other theory - the 'shadowed conveyor' - argues that the overturning is compressed to lie below about 2.5 km with a largely stagnant "shadow zone" above it.
"Our work reconciles these two theories: the shadowed conveyor correctly captures vertically compressed overturning beneath a shadow zone, while the standard view must be broadly interpreted in terms of water paths diffusing through the shadow zone. Because the shadow zone is largely shielded from the overturning circulation the question becomes how exactly does water get into and out of it," A/Prof. Holzer says.
Using novel mathematical analyses applied to a state-of-the-art ocean circulation model that optimally fits the circulation to observed tracer distributions and surface forcings, the authors were able to quantify in detail the pathways and timescales with which the shadow zone exchanges water with the surface ocean.
"Our analyses allowed us to come up with a new schematic of the large-scale deep circulation in the Pacific. We find that diffusive transport both along and across density surfaces plays a leading role in ventilating the shadow zone."
Contrary to the widely held view that Pacific deep waters exclusively follow density surfaces to upwell in the Southern Ocean, the authors found that only about half of the water in the shadow zone follows this route, with the other half returning to the surface in low latitudes and in the subarctic Pacific, helping to explain the high biological production there.
The scientists say this new understanding of the deep Pacific circulation and transport pathways will help interpret observed tracer distributions and biogeochemical processes.
"An exciting direction for future research is to understand how the shadow zone, already low in oxygen and sensitive to increased oxygen demand, shapes the response of the ocean's biological pump to climate change," A/Prof. Holzer says.
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