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Ocean motion moves microbes to filter light

May 2011
Compiled by BioPhotonics staff

When simulating ocean currents, scientists often add particles to the water so they can visualize its movement. Recently, researchers at MIT used this phenomenon to determine that the microbes found in the ocean also align themselves with the current and, in so doing, help determine how much light penetrates the ocean and how much bounces off as backscatter.

Because many microbes, like phytoplankton, have elongated shapes or live in communities of long chains, scientists have found their orientation to ocean currents could have a substantial effect on ocean light, which influences photosynthesis and phytoplankton growth rates. In addition, it could affect satellite readings of light backscatter used to inform climate models or assess algal blooms.

In a dormant ocean environment, phytoplanktons are oriented randomly, allowing light to filter easily into the ocean. However, with fluid flow, the random arrangement changes, affecting the amount of backscatter.

“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. “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.”

In addition to fluid flow, the researchers studied how microbial size affected light filtration. They observed that very small microbes – less than 1 μm in diameter – did not align with the ocean current because, in an effect called Brownian motion, they were too vigorously kicked around by water molecules. He recreated an ocean environment in microfluidic devices about the size of a stick of gum and used videomicroscopy to trace and record the microbes’ movement in response to food and current.

The researchers plan to test the mechanism in the field in a local environment suitable for experimentation, like a nearby lake.

Their findings were published March 8, 2011, in the Proceedings of the National Academy of Sciences (doi: 10.1073/pnas.1014576108).

The deflection of radiation by scattering processes through angles that exceed 90° with respect to the original direction of motion.
remote sensing
Technique that utilizes electromagnetic energy to detect and quantify information about an object that is not in contact with the sensing apparatus.
algal bloomsAmericasAustralian Research CouncilbackscatterBiophotonicsBioScancarbon fixationclimate modelselongated microbesfluid flowfluid mechanicsHayashi GrantInternational Science and Technology Initiativeslight backscatterlight filtrationlight propagationlight sourcesmarine opticsMassachusettsmicrobesMicroscopyMITNational Science FoundationNewsocean currentsphotosynthesisphytoplanktonremote sensingRoman Stocker

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