Phytoplankton in the ocean, through photosynthesis, produce as much plant material, as all the plants on land. Amazingly, they comprise only about 1% of the total biomass of the land plants. One of the reasons for this disparity in biomass is that a peculiar looking, highly active, group of single-celled grazers, referred to as the microzooplankton (Figure 1), eat the phytoplankton almost as fast as it grows.

The biomass that phytoplankton produce as they grow supports the world?s fisheries, contributes to our oxygen-rich atmosphere and drives the drawdown of the greenhouse gas, carbon dioxide, out of the atmosphere, helping to reduce rates of global temperature rise.  In order to understand what controls phytoplankton growth and how much biomass they produce, we need to understand exactly how much the microzooplankton consume.

This is more challenging than it sounds and multiple approaches have been applied to try to obtain this information. That said, biological oceanographers have traditionally used a theoretically simple but practically challenging method called the ?dilution approach?. The unreliability of this approach has driven many researchers to despair over the years but it remains our most useful tool to obtain this important information.

 The major aim of this project was to develop an alternative approach to quantify how much phytoplankton is consumed by microzooplankton. Our new approach relied on the response of microbial grazers to increasing amounts of available food. Generally, they simply eat faster as more food becomes available but they can only do this up to a point before they become saturated and cannot engulf or digest their phytoplankton prey any faster. The new ?saturation approach? adds imitation phytoplankton to a series of bottles of seawater containing the natural mix of phytoplankton and grazers, to create increasing saturation conditions for the grazers (Figure 1). As the grazers become saturated by the imitation prey, this allows the natural phytoplankton to grow faster because fewer of them are eaten. By measuring the rates of growth of the phytoplankton in the experimental bottles over the course of a day (Figure 2), it is possible to deduce the maximum growth rates of the phytoplankton and the rates at which they are getting eaten by the grazers. The new ?saturation approach? provides researchers with an additional, easier to set up and potentially more reliable tool to investigate the relationship between phytoplankton prey and microbial predators in the ocean.

The project provided an excellent learning opportunity for undergraduate scientists. We were able to include four undergraduate student interns in the development and testing of the saturation approach. Over the course of three summers, they were involved in running the experiments that contributed to a published description of the method and its successful application in the coastal waters of the Gulf of Maine and are included as co-authors on the publication (Figure 3). The new approach was also introduced to the scientific community in a number of presentations, including at the Gulf Coast Undergraduate Research Symposium, 2019; and as part of a special session at the Ocean Carbon Biogeochemistry summer workshop in 2018, titled ?The world of microzooplankton: ocean carbon movers and shakers? co-organized by the project lead.

We hope this new experimental tool is widely adopted by aquatic scientists to help determine how rapidly microzooplankton consume and thereby control the vast populations of phytoplankton in the ocean. Certain characteristics of the approach mean that it may also be a useful experimental approach to determine other grazing-related processes, including to quantify how much grazing by microzooplankton on phytoplankton drives the cycling of key nutrients in the oceans and even, how grazing contributes to the exchange of climate-active gases between the ocean and atmosphere.

Last Modified: 05/15/2022
Modified by: Stephen Archer