Tiny zooplankton may play giant role in global carbon cycle

A giant larvacean within its mucus "house", which it uses to filter out organic matter. The process helps transfer carbon removed from the air by organisms that photosynthesise to the deep sea, where it can be stored long term.
A giant larvacean within its mucus "house", which it uses to filter out organic matter. The process helps transfer carbon removed from the air by organisms that photosynthesise to the deep sea, where it can be stored long term.PHOTO: NYTIMES

Swimming hundreds of metres beneath the ocean's surface in many parts of the world are prolific architects called giant larvaceans.

These zooplankton are not particularly giants themselves (they resemble tadpoles and are about the size of a little finger), but, every day, they construct one or more spacious "houses" that can exceed 90cm in length.

The houses are transparent mucus structures that encase the creatures inside.

Giant larvaceans beat their tails to pump sea water through these structures, which filter tiny bits of dead or drifting organic matter for the animals to eat.

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When their filters get clogged, the larvaceans abandon ship and construct a new house.

Laden with debris from the water column, old houses rapidly sink to the sea floor.

In a study published in Science Advances, scientists near California's Monterey Bay have found that, through this process, giant larvaceans can filter all of the bay's water from about 90m to 300m deep in less than two weeks, making them the fastest-known zooplankton filter feeders.

In doing so, the creatures help transfer carbon that has been removed from the atmosphere by photosynthesising organisms to the deep sea, where it can be buried and stored long term.

And, given their abundance in other parts of the world, these organisms likely play a crucial role in the global carbon cycle.

When it comes to the flow of carbon in the ocean, "we don't know nearly as much as we should", said Dr Kakani Katija, principal engineer at the Monterey Bay Aquarium Research Institute and the study's lead author.

"If we really want to understand how the system works, we have to look at all the players involved. Giant larvaceans are one important group we need to learn more about."

In the past, other scientists have tried studying giant larvaceans in the laboratory.

But these efforts always failed because the animals' houses were too fragile to be harvested, and collected specimens were never able to build houses outside the ocean.

To study the zooplankton in their natural habitat, Dr Katija and her collaborators developed a new deep-sea imaging instrument, called DeepPIV, which they paired with a remotely operated vehicle.

DeepPIV projects a sheet of laser light that cuts straight through a larvacean's mucus house. A high-definition camera on the remotely operated vehicle can then capture the inner pumping mechanisms illuminated by the laser.

Analysing footage of 10 giant larvaceans, Dr Katija's team estimated filtration rates based on how quickly particles were flowing into the animals' chambers.

On average, larvaceans longer than 1cm filtered 42 litres (about the fuel tank capacity of a small car) of water an hour. The scientists also estimated that the creatures spent about two-thirds of their time filtering sea water.

Combining their filtration measurements with long-term data on the abundance of giant larvaceans in Monterey Bay, the researchers calculated that, at peak population density and maximum filtration rate, the bay's giant larvaceans can sift through all the water in their primary habitat in as little as 13 days.

The filtration rates that Dr Katija's team calculated are higher than those previously inferred from studying smaller larvacean species in the lab, said Professor Kendra Daly, a zooplankton expert at the University of South Florida who was not involved in this study.

This difference, she added, highlights the importance of "using innovative approaches to actually make measurements in the water".

Dr Katija added that the study also demonstrates the usefulness of the new DeepPIV tool. Among other things, she and colleagues want to use the instrument to study how fluid mechanics influences the functioning of other marine organisms and the transport of nutrients through the ocean.

NYTIMES

A version of this article appeared in the print edition of The Straits Times on May 12, 2017, with the headline 'Tiny zooplankton may play giant role in global carbon cycle'. Print Edition | Subscribe