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Earth & Marine Sciences - Turbulent Lives

UMaine researchers study the role of diverse cell shapes in phytoplankton ecology

by Margaret Nagle | Art/Photography by David Townsend


Coscinodiscus: A large, centric diatom. The photographs by University of Maine Professor of Oceanography David Townsend are part of his current research projects focusing on phytoplankton, nutrients and red tides in the Gulf of Maine.

Their shapes are as beautiful as they are mysterious. Spheres covered in round, intricate calcium carbonate plates or appendages like tiny trumpets and flower blossoms. Spiny chains like primitive centipedes. Species that resemble ornate glass pillboxes, toy tops, mesh containers. Geometric shapes and patterned stars.

The marine environment’s version of snowflakes.

A cup of water scooped from the ocean, river or lake can contain millions of the microscopic, single-celled organisms called phytoplankton. Most drift passively in the water. Some have flagella for mobility. Thousands of species exist, with more being discovered every year.

Phytoplankton are some of the most important Protista on Earth. They anchor the bottom of the aqueous food web, providing nutrition for fishes and mammals ranging from zooplankton (one click above phytoplankton) to whales. In their short (days to weeks) life cycles, they remove carbon dioxide from the air and produce oxygen through photosynthesis. As a result, about half of all the photosynthesis on the planet occurs in the oceans.

And because their growth depends on such essential factors as sunlight and nutrients found in colder, deeper depths, phytoplankton are the quintessential harbingers of climate change.

Until recently, most marine research in this area has focused on the size of the single-celled organisms, investigating the difference that dimensions make in the lives of the many different phytoplankton species. But marine scientists at the University of Maine are exploring the role of cell shape in phytoplankton ecology, hoping to better understand how the diversity affects function.

With a more than $520,000, four-year National Science Foundation grant, UMaine biological oceanographers Pete Jumars and Lee Karp-Boss are studying the effects of turbulence on the base of the marine food web. Ultimately, their findings could help address growing concern about how global warming affects phytoplankton.

Climatic warming is expected to decrease turbulent stirring of waters globally, on average, through increased ocean stratification, yet increase turbulence locally and temporarily through more severe weather events, thereby impacting production in the food chain.

The NSF award also has a mathematical geosciences component, with a $471,000 grant to Tulane University computational mathematician Lisa Fauci, who will develop numerical models to complement the UMaine research.

“It is not enough to know how much phytoplankton is there in terms of biomass,” says Karp-Boss, a phytoplankton ecologist who studies the organisms in functional groups characterized by cell size and morphology – the important physical traits that affect many of their functions. In her research, she looks at the effects of turbulent flows on the many species’ forms, such as small versus large cells; single cells versus chains; rigid versus flexible chains; species with spines versus those without.

“It’s important to know – and be able to predict – who is there,” Karp-Boss says, “because the structure and function of aquatic food webs and fluxes of organic carbon from the ocean surface to the deep ocean depend on the taxonomic composition of phytoplankton.”

diatom chain

Diatom chains - Two different diameter chains, along with a Ceratium

The UMaine researchers are trying to close the gap between textbook understanding of turbulent flows and the consequences for suspended organisms and particles. That includes better understanding of how turbulent stirring affects phytoplankton processes.

Phytoplankton are often modeled as spheres, but that is not the prevailing form across the spectrum of cell sizes. Their varied shapes are thought to affect many aspects of phytoplankton ecology and physiology. That’s why a synthesis of the relative effect of shape on all of these functions is essential for realizing the selective pressures on phytoplankton.

The understanding of such a small world that has big implications for life on this planet begins with an investigation of the physical constraints that dictate how the organisms interact with their environment and each other. The question is how the sizes and shapes of phytoplankton affect their performance, and what are the consequences of their structure on the community and next trophic levels.

In their search for answers, Karp-Boss and Jumars are incorporating experimental flow models, the physics of molecular diffusion, fluid dynamics, oceanography, particle mechanics, computer science and mathematical biology. Their goal is to develop new numerical and analog models, incorporating current understanding of small-scale structure and its evolution in natural turbulence, and the little-understood role of vorticity (swirling, as in a vortex). The models will be used to predict and test mechanisms of flow effects on phytoplankton cell motions and their consequences.

Such models may one day help scientists understand how the diverse shapes of one phytoplankton species provide an advantage over another in the varied mixing environments of the seas.

“Different phytoplankton, including diatoms of different sizes, take in nutrients at different rates. The function is determined by size and shape – morphology,” says Jumars, director of the UMaine School of Marine Sciences. “When interacting with turbulence, some are not as beneficial. A fundamental question has to do with the limitations and what we can infer about species that have an advantage under certain conditions.”

Jumars has been studying effects of fluid and particle dynamics on organisms for 40 years; Karp-Boss has been collaborating with him for more than a decade. In 1998, Jumars and Karp-Boss published research on the behaviors of two chain-forming phytoplankton species in a simple shear flow – the first step, they said, toward understanding the behavior of real phytoplankton in natural flows.

The observed diverse behaviors implied that fluxes of nutrients and collision frequencies vary greatly with detailed shapes and mechanical properties of chains and their unit cells. However, their research raised more questions than it answered, they said.

“We know diatoms (one of the three large groups of phytoplankton, along with dinoflagellates and coccolithophorids) need light and nutrients to grow and divide, they need to avoid being preyed upon and, as part of their sexual phase, they have to find a mate,” says Karp-Boss. “We need to understand how they perform these functions in a world that is not intuitive to the one that we live in. What are the constraints of environment in performing these functions?”


Dinoflagellate - With diatom chains on the periphery.

The watery world in which phytoplankton live has been described as being the consistency of honey or molasses for organisms that are so small and slow. It is a world in which the organisms’ movement, ability to flex (in the case of chains), reproduce, eat, escape predation and, ultimately, sink to the sea floor are dependent on the turbulence of their environment.

Most phytoplankton live in a world of low Reynolds numbers, where motion stops as soon as propulsion stops, and the dominant force is friction from the stickiness of water molecules to each other.

Low Reynolds number worlds defy our intuition. For instance, if a semi passes a car in an adjacent lane of the highway, passengers in the smaller vehicle will feel the pull of the wind. But if that truck passes two lanes away, the rush of the wind is indistinguishable. For life at low Reynolds numbers, a diatom or particle experiences substantial fluid motion 100 lanes away, and even more car lengths ahead and behind.

“If you drop a particle and it falls through stagnant water, the object drags substantial volumes of water with it. That’s not what happens at high Reynolds number,” says Jumars. “That’s why it’s important to learn how forces are transmitted through such a continuous medium.”

Jumars and Karp-Boss are testing the hypothesis that diffusing momentum and vorticity on the dissipation scales of turbulence are major contributors to relative motion between water and phytoplankton.

“We know that global warming is going to stabilize the ocean by decreasing the turbulence intensity,” Jumars says. “Understanding what that means to the base of the food web is critical. A signature of climate change is more intense storms at certain places and times. We also have to understand the other extreme.”

Better understanding of biological fluid dynamics will provide insight into the fundamental physics – including motion and behavior of nonspherical shapes – of phytoplankton and other complex particles in turbulent environments.

“The big question is: What processes affect distribution and species composition of phytoplankton?” says Karp-Boss. “We don’t have a good mechanistic understanding of the processes that select for certain species. Hence, our ability to predict who will be there and when is limited.”

The morphological diversity of diatoms in the world is screaming to tell us something, but we don’t yet know what it is, Jumars says.

“We know that their shapes have consequences, but we don’t know how and what,” says Jumars. “They are nature’s art and design, but we don’t understand their function. In design, shape has a function, and that’s our working hypothesis here, too.”

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