Adding
particles to liquids to make currents visible is a common practice in
the study of fluid mechanics, one that was adopted and perfected by
artist Paul Matisse in sculptures he calls Kalliroscopes. Matisse’s
glass-enclosed liquid sculptures contain an object whose movement
through the liquid creates whorls that can be seen only because
elongated particles trailing the object align with the direction of the
current; light reflects off the particles, making the current visible to
the viewer.
Researchers
at MIT recently demonstrated that this same phenomenon is responsible
for the swirling patterns scientists typically see when they agitate a
flask containing microbes in water; many microbes are themselves
elongated particles that make the whorls visible. More importantly, they
say this phenomenon occurs in the ocean when elongated microbes caught
in a current align horizontally with the ocean surface, affecting how
much light goes into the ocean and how much bounces off as backscatter.
Because many ocean microbes, like large phytoplankton, have either an
elongated shape or live in communities of long chains, this orientation
to ocean currents could have a substantial effect on ocean light — which
in turn influences photosynthesis and phytoplankton growth rates — as
well as on satellite readings of light backscatter used to inform
climate models or assess algal blooms.
In
a quiescent ocean, phytoplankton are randomly oriented and light
filters through easily. This random arrangement is usually assumed in
models of light propagation in the ocean and in satellite readings. But
fluid flow can change things.
“Even
small shear rates can increase backscattering from blooms of large
phytoplankton by more than 30 percent,” said Roman Stocker, Professor of
Civil and Environmental Engineering at MIT and lead author on a paper about this work.
“This implies that fluid flow, which is typically neglected in models
of marine optics, may exert an important control on light propagation,
influencing the rates of carbon fixation and how we estimate these rates
via remote sensing.”
Another
consideration is microbial size. Very small microbes (less than 1
micrometer in diameter) don’t align with the ocean current no matter
what their shape. “These very small things don’t align because they are
too vigorously kicked around by water molecules in an effect called
Brownian motion,” said Stocker, who studies the biomechanics of the
movements of ocean microbes, often in his own micro-version of a
Kalliroscope called microfluidics. He recreates an ocean environment in
microfluidic devices about the size of a stick of gum and uses
videomicroscopy to trace and record the microbes’ movements in response
to food and current.
In
this case, however, the research methodology was observation, followed
by mathematical modeling (much of which was handled by graduate student
Marcos, who created a model that coupled fluid mechanics with optics),
and subsequent experimentation carried out by graduate students Mitul
Luhar and William Durham using a tabletop-sized device.
But
the impetus for the research was an observance of swirling microbes in a
flask of water and a question posed by Justin Seymour, a former
postdoctoral fellow at MIT. “Justin walked up to me with a flask of
microbes in water, shook it, and asked me what the swirls were,” said
Stocker. “Now we know.”
In
addition to Seymour, who is now a research fellow at the University of
Technology Sydney, other co-authors on the paper are Marcos, Luhar and
Durham; Professor James Mitchell of Flinders University in Adelaide,
Australia; and Professor Andreas Macke of the Leibniz Institute for
Tropospheric Research in Germany.
Next
steps: The researchers plan to test this mechanism in the field in a
local environment suitable for experimentation, most likely a nearby
lake.
Funding:
Funding was provided by grants from the National Science Foundation and
the Australian Research Council and by a Hayashi Grant from MIT’s
International Science and Technology Initiatives Program.
Source: “Microbial alignment in flow changes ocean light climate,”
by Marcos, Justin Seymour, Mitul Luhar, William Durham, James Mitchell,
Andreas Macke and Roman Stocker, in PNAS Early Edition online Feb. 21,
2011.
