It’s hard to study the behavior of plankton under a microscope. How plankton move in the ocean could be considerably different than what they do on a microscope slide, lit by a warm, bright light.
This dilemma led Dr. Houshuo Jiang to build a new type of microscope in his lab at the Woods Hole Oceanographic Institution. And that tool has led to a discovery about eastern oyster larvae that shows a new way ocean acidification could have a significant impact on the species.
But first, the microscope.
If you’re picturing a tall instrument where you look down through a lens at your magnified subject trapped on a piece of glass, this is not that. Jiang’s microscope is called a high-speed microscale imaging system – or HSMIS for short.
It uses a camera with a long lens, pointing horizontally at a clear plastic box. The box is like a miniature fish tank that gets filled with seawater containing the plankton subjects, like larval oysters.
"So you see this is a tank, right?" Jiang asked. "This is not like a small microscope slide, which are very confined space. These are big space. So, like a small ocean here. So, relatively reliable to be able to measure the behavior and the motion of microorganisms in water."
I met up with Jiang at his WHOI lab where he showed me how the HSMIS works. The tank is backlit by a small LED that can be precisely aimed, like a mini spotlight.
"A common microscope, they use a big light," said Jiang. "So you got a lot of heat. And this one, they can use very tiny little light that can light up the space you want to see."
Jiang says the smaller light gives off less heat, meaning it has less of an effect on the subjects.
"This is important to observe microorganisms because they are small," he said. If they are heated the convection will make their motion unreliably measured. Here you can you can measure their motion, their behavior in a relatively bigger tank."
Jiang uses a high speed camera, hooked to a computer, and capable of capturing 2,000 frames per second.
"They’re small but, actually, their motions are extremely fast," he explained. "So, their motion is happening in like a millisecond. So that's their timescale. So it's like you need to have a very fast camera."
Jiang uses HSMIS to study everything from nanoplankton, under 10 microns, to marine copepods and other crustaceans.
He says eastern oyster larvae live in the water column as plankton for about two weeks, after which those that survive settle on the bottom to grow.
Jiang used HSMIS to observe how these tiny oysters feed during the larval phase. He explained they have a velum – which he likened to a hat – covered in hair-like cilium. Larval oysters use the velum to move through the water, and they wave the cilium to bring microscopic algae toward their mouths.
"They generate currents, like water current," said Jiang. "This water current bring the algae particles towards these places. Then they capture them and put them into their mouths."
But that motion isn’t the only thing creating current. Jiang said the oyster’s calcium carbonate shells, although quite small, are still much heavier than the surrounding water. And that means they sink, creating a current as they fall.
"So, essentially, it’s the gravity which helps them to generate the feeding current," he explained.
It’s a technique utilized by larger zooplankton, like copepods. But it’s not something that’s been observed before with larval oysters.
Scientists have known that oyster larvae are susceptible to ocean acidification. It weakens their shells, offering less protection from predators, and requiring they spend more energy to grow a shell. And that’s energy that isn’t going into normal development. Jiang says both of these factors lead to higher mortality rates.
But on top of that, Jiang said oyster larvae with weak, porous shells can’t feed well because they just aren’t heavy enough to generate a strong feeding current.
"And this feeding current, aided by gravity, is utterly important for their life," he said.
Because, Jiang said, if they don’t eat well, they simply die.
Last month Jiang published his findings in the journal Physical Review Fluids.
