Happy Plankton?

The overall goal of our project is to try to understand the forcing factors that control phytoplankton abundances, community composition, and community shifts in the Ross Sea. The hypothesis is that light, iron, and carbon dioxide (CO2) interactively control phytoplankton dynamics in this region of the Southern Ocean. To test our hypotheses we run a suite of measurements to determine both the boundary conditions (e.g., temperature and salinity) and the macronutrient levels. We also measure rate processes and conduct experiments to test more specific hypotheses. As part of my responsibilities onboard I run and maintain instruments to measure variable fluorescence.

Fluorescence in the ocean is mostly produced by phytoplankton. These organisms have organelles called chloroplasts with pigments to absorb light and drive photosynthesis. The extra energy that is absorbed and cannot be converted into biochemical energy is re-emitted in small part as heat but mostly as light at a longer wavelength and lower energy, which is called fluorescence. To quantify phytoplankton fluorescence all that is needed is a blue light and a detector to measure the re-emitted light. Such a measurement is usually considered a good proxy for the chlorophyll concentration in the water as a measure of phytoplankton biomass.

To determine variable fluorescence, we use three different instruments. One is a Pulse Amplitude Modulated Fluorometer (PAM) produced by Waltz. This instrument measures the variable fluorescence and the fluorescence quantum yields of photosystem II (PSII), telling us the percentage of light absorbed by the phytoplankton that is converted into biochemical energy and used to assimilate carbon and nutrients, grow, and replicate. The amount of light used, or photosynthetic quantum yield, is a good indicator of the "health" and "happiness" of phytoplankton. If the organisms have been exposed to high irradiance for periods of time, parts of their photosynthetic apparatus can be damaged and, as we do, they can get sunburned; this will result in a lower quantum yield. Similarly, if the organisms find themselves in low nutrient water they will soon starve and generate a lower photosynthetic quantum yield. This is particularly true when iron is in very low concentration, because this element it is essential in many components of the photosynthetic apparatus.

Our other two instruments are Fast Repetition Rate Fluorometers (FRRF). One produced by Chelsea is submersible, and mounted on the ship profiler. The other is a bench-top prototype designed and built by Zbigniew Kolber and Denis Klimov at MBARI. The FRRF, in addition to the photosynthetic Quantum Yield (Fv/Fm), can also estimate optical absorption cross-section and electron flow between the PSII and the plastoquinone pool (PQ). This prototype is also set up to make Rapid Light Curves (RLC) to estimate Electron Transport Rate (ETR) and measure photosynthetic bacteria (Fv/Fm). All of these parameters give detailed information on the "health and happiness" of the cells and their capability to photosynthesize. Part of my interest is seeking out the photo-physiological differences between phytoplankton taxa living in this area and using variable fluorescence. Hopefully this information will help us understand the distribution and growth of particular species in the ocean.