The open ocean is the world’s wettest desert, containing vanishingly small concentrations of nutrients. Yet phytoplankton, a type of water-dwelling microscopic plant, have adapted to grow and survive in this near barren environment. These microorganisms play a key role in the biological pump, an important process that drives the ocean’s carbon cycle and absorbs carbon dioxide from the atmosphere.
Research into phytoplankton’s adaptations to different nutrient levels around the world tells us a lot about the entire marine food web’s response to the changing ocean as climate change continues. Over the years, scientists have made efforts to determine which nutrients set the maximum limit of phytoplankton growth. Unfortunately, experimentally identifying which specific nutrients (usually nitrogen (N), phosphorus (P), or iron (Fe)) limit phytoplankton growth in the ocean is a strenuous process. Through bottle experiments involving incubating phytoplankton in differing nutrient conditions (ex: with added Fe, N, and P), scientists have been able to identify the limiting nutrients in the ocean for different organisms. Still, scientific models have difficulty systematically determining nutrient stress across the global ocean, forcing scientists to continue relying on laborious bottle experiments.
In an effort to create more reliable computational models, recent research by Ustick, et al. (2021) aimed to corroborate results from models of nutrient limitations in different oceans with results from previous bottle experiments. These bottle experiments – the current standard for confirming nutrient limitation – focused on nutrient limitations of Prochlorococcus, a type of cyanobacteria. Prochlorococcus is one of the most abundant phytoplankton in ocean waters and is better equipped to absorb nutrients than most other phytoplankton due to its small size. Therefore, if even Prochlorococcus were experiencing a particular nutrient stress, then we can hypothesize that the entire phytoplankton community might experience the same stress. While it is also possible this could not be the case, as different phytoplankton have different nutrient requirements, understanding the map of nutrient limitation for one major phytoplankton player is a great start and lays groundwork for further study of other species.
Ustick et al. (2021) compared bottle experiment results with compared with two indirect, but larger-scale ways of assessing Prochlorococcus nutrient limitation from the surfaces of the Atlantic, Pacific, and the Indian Ocean. Limiting nutrients for Prochlorococcus were inferred from DNA sequencing samples by looking at the prevalence of genes associated with stress for nitrogen, phosphorus, and iron. The presence of stress genes in different oceans was compared to bottle experiments, as well as the limiting nutrient predicted by a global climate model. It turned out that all three independent methods tended to agree on which regions were primarily iron, phosphorus, or nitrogen stressed.
The biogeochemical cycles in the ocean are affected by which organisms are absorbing what available nutrients. Phytoplankton makes up the base of aquatic food webs, so scientists can use them to represent the states of specific ocean ecosystems. Therefore, understanding how phytoplankton might respond to changing environments is important for understanding the impact on the whole food web, from microorganisms to large fish and marine mammals. Accurately identifying areas of nutrient stress is a key factor in determining ocean physiology and identifying whether different species are adapting to the ocean’s changing environment.
For more information, read the full paper by Ustick et al. (2021) published in Science.