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Of the solar energy impinging on the Ocean, or any other water body, only small amount is available to the plankton, despite this, the absorption of light drives photosynthesis and produces about half of the oxygen on Earth. Developing new methods is a central problem for biological oceanographers and biogeochemists. Photoacoustics is potentially a new approach to study photosynthesis. It is an approach based on measuring the changes in the temperature structure of water due to the heat released from cells associated with the radiation decay of photosynthetic scales. Current estimates are that close to 80% of the absorbed energy. This project is to develop and assess the photoacoustic techniques to probe photosynthesis and cellular physiology. The specific efforts on this project is focused on:
The objective of the proposed study is to describe how light energy that is harvested by the photosynthetic apparatus is used. By doing this we will attempt to calculate an energy budget by quantifying the dissipation of absorbed light as heat and fluorescence, and its partitioning between the two photosystems.
We will combining photoacoustical, fluorescence and oxygen electrode detectors in the same measuring cuvette, and using pulsed laser flashes and alternating series of such flashes with and without a continuous saturating background light, we shall quantify the total energy absorbed by the photosynthetic apparatus and its stepwise dissipation as thermally generated acoustic signals, fluorescence and finally evolved oxygen. We shall also determine how light intensity affects the different energy flow and distribution patterns.
Technology developed: The developed PA system employs a bunk of 7 ultra-bright Light Emitting Diodes (LEDs, Lumileds Co., Star III) to excite photoacoustic signal from phytoplankton or leaves of higher plants (see the right). The use of LEDs instead of previously employed lasers made us possible to construct a compact and user-friendly system that could be potentially used for field studies. The LEDs emit flashes of red light (at wavelength 640 nm with 30 nm bandwidth) with adjustable duration ranging from 1 us to 100 us. The optimal flash duration is selected to match the oscillation semi-period of the piezo-acoustic sensor and was found to be ca. 8 us for the particular detector. A suite of CW blue ultra-bright LEDs (470 nm central wavelength, 30 nm bandwidth) is incorporated into the optical flasher to provide a source of saturating illumination. Excitation light is focused into the sample cuvette using a Fresnel lens (focal length 10 cm, diameter 12 cm), with a spot size of 30 mm. The coupled photoacoustic/fluorescence cell incorporates a custom-built quartz cuvette (10 mm optical path, 50 cm diameter), a high-reflective mirror (with dielectric couting optimized for the red spectral region), and a custom-built microphone detector. The detector is based on a ceramic Lead Zirconite Titanates piezo disk, with the shape, size, and frequency band optimized for a particular application. The signal is accumulated over a preset number of flashes and spectrally analyzed using an incorporated FFT (Fast Fourier Transform) procedure.
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 The pathways of energy dissipation within photosynthetic cells.
 The photoacoustic cell developed by this program funded by the BiNational Science Foundation.
 The photoacoustic signal measured using our photoacoustic cell. The black line is for a reference cuvette filled with black ink. The green line is for a culture of Synechococcus. Red line is the optical signal initiating the thermal wave associated with the cell..
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