Phytoplankton or microalgae are the most common primary producers in the ocean, and they support other higher trophic animals, such as zooplankton, fish, and even whales! They are a large family with multiple species with all kinds of shapes and lifestyles. It is very important to study these tiny creatures to understand their physiological performance, so we can predict their response to different environmental changes, such as ocean acidification and metal pollution. With this knowledge, we can promote further regulation of human activities for a sustainable environment.
How can we study the phytoplankton? One of the most popular methods is to cultivate them in a designed environment in the lab so we can observe and measure their responses. Similar to growing a plant at home, we need to provide light, water, and nutrients to these microalgae. Any unbalanced conditions could cause phytoplankton growth limitations. Incubation experiments allow us to study different phytoplankton responses to various temperatures, light intensities, nutrient limitations, etc. Since they are tiny, we usually sample large quantities of them and measure overall average parameters based on the community. For example, like other plants on land, phytoplankton have photosynthesis, and they contain pigments, like chlorophyll. So, we filter the phytoplankton on a filter and then extract the total chlorophyll-a concentrations from these phytoplankton (Figure 1). In general, if there is a higher Chl-a concentration then it indicates phytoplankton are having a better life in this specific environment because they grow more and larger. With advanced facilities like FRRf (Fast Repetition Rate Fluorometer) and HPLC (High-performance liquid chromatography), we can also measure phytoplankton’s photosynthesis efficiency and community composition.
Oceans are very complex and diverse water bodies, and the phytoplankton community in different locations varied significantly. The equatorial Pacific Ocean is known for its “high nutrient and low chlorophyll” situation, indicating potential metal, such as iron, limitations. To understand more about phytoplankton in the Pacific open ocean, we sail a research vessel and sample the on-site phytoplankton community for short cultivation on the deck. Seawater together with phytoplankton is pumped into the lab using a tow fish pumping system (Figure 2). In voyage SO298, we are specifically interested in phytoplankton responses under additional metal and nutrient supply, and even a small grand of dust could influence our results. Therefore we built a “bubble” using large plastic sheets providing a trace metal clean environment for conducting the experiment. Phytoplankton are kept in a small transparent polycarbonate bottle for 48 hours. In order to make phytoplankton “feel at home”, we store these bottles in a tinted water tank in which the surface seawater is pumped continuously (Figure3, 4). These on-deck incubators provide ambient temperature and light intensity to the phytoplankton. Compared with lab incubation on land, on-deck light incubation experiments provide more close-to-reality information for understanding the open-ocean on-site phytoplankton community in the given condition.
by Jiaying Abby Guo