Photoacoustics as a diagnostic tool for probing the physiological status of phytoplankton

In nature, conditions for the growth of phytoplankton rarely support their maximal potential doubling rates. The main rate-limiting factor determining the doubling rates of phytoplankters is photosynthesis. This key process is in turn controlled, and frequently limited, by the available light, by lack of essential nutrients, or by the presence of noxious pollutants. Any decrease in the rates of photosynthesis is associated with a reduction in the efficiency of light utilization in that process. We present here the decreased photosynthetic efficiency of phytoplankton upon depletion of nitrogen, phosphorus, and iron as measured by photoacoustics. Such effects have been documented extensively by other methods, such as fluorescence yield, growth rate, carbon assimilation, and oxygen evolution. However, photoacoustics measures directly the energy efficiency of the photosynthetic process. We determined that efficiency by this method in three phytoplankton species belonging to different taxa. Our results illustrate the power of photoacoustics as a tool in aquatic ecology and in the physiological research of phytoplankton.