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STIMULATING A RESPONSE
There are theories, but no concrete explanations as to how dinoflagellates
sense their fluid environment, such as the turbulence caused by
wind and breaking waves. Scientists also have not identified the
internal pathways or mechanisms in the organisms that trigger physiological
responses to flow, such as bioluminescence and changes in growth
rate, nutrient uptake, and structure.
Flows capable of stimulating a bioluminescent response in dinoflagellates
must be quite strong. Beneath the ocean surface on a windy day the
turbulence usually isn't strong enough to stimulate bioluminescence,
but it does affect the cells in other ways, causing them to reproduce
more slowly and even to change shape.
Dinoflagellates and other microscopic plankton experience their
environment much differently than do larger animals. Because of
their small size, dinoflagellates feel turbulence as laminar shear,
a difference in flow velocity across the cell diameter.
"If you are on a ship in windy conditions, you feel lots of acceleration,"
explained Latz. "For plankton, acceleration isn't as important;
they are so small that they live in a viscous world dominated by
shear."
According to Latz, although the velocity difference across a dinoflagellate
is extremely small, there is sufficient shear for them to sense
and respond to.
Latz is using dinoflagellate bioluminescence as a way of reporting
how cells are affected by flow. Agitating water containing dinoflagellates
results in flashes of light from the cells. The flashes are bright
and nearly instantaneous, allowing Latz to observe exactly where,
when, and to what types of flow the dinoflagellates respond.
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In the lab, Latz and graduate students Andrew
Juhl and Peter von Dassow create carefully defined experimental
flow conditions to test dinoflagellate flow sensitivity. Simple
fluid shear is created using special flow chambers consisting of
two clear, concentric cylinders. The space between the two cylinders
the inner cylinder is held stationary. This causes a linear gradient
of velocity in the gap between the two cylinders, resulting in a
constant shear. This type of flow is called Couette flow and is
used by the scientists to test the response of dinoflagellates to
an exactly defined shear.
At one time it was thought that dinoflagellates
responded only to the rapidly changing, chaotic nature of turbulent
flow. Using Couette flow, Latz has been able to show that smooth
unchanging laminar flows can stimulate bioluminescence too.
Latz and his collaborator Jim Rohr, a physicist
at the Space and Naval Warfare Systems Center in San Diego, have
conducted other studies combining experimental fluid mechanics with
more complex flows. For example, in some studies they send water
filled with dinoflagellates through a clear pipe. By controlling
the flow rate and thus the characteristics of the flow, they test
how the bioluminescent response is affected by the shear stress
(the shearing force of the fluid flow) compared to other flow characteristics,
such as flow acceleration or lent nature of the flow.
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