Plants – even microscopic ones – come in all shapes and sizes. As important players in the Earth’s carbon cycle, marine researchers are particularly interested in what determines the size of phytoplankton, photosynthesizing microorganisms that live near the ocean’s surface. When these microscopic plant-like organisms sink to deeper ocean waters, they sequester the carbon they’ve used for photosynthesis, preventing the carbon from returning to the atmosphere as carbon dioxide. Phytoplankton size can determine whether they sink or stay near the surface to absorb more carbon dioxide, impacting the efficiency with which that carbon sinks to the deep ocean. With an important role in the ocean’s carbon cycle, the factors that determine size distribution in phytoplankton communities is an area of ongoing research.
Prevailing theories of phytoplankton ecology suggest the size distribution of phytoplankton in the ocean is determined largely by two factors: water turbulence and nutrient availability. Such theories predict that water turbulence prevents larger cells from sinking to a depth where they no longer receive sunlight to photosynthesize. Larger phytoplankton also grow faster than their smaller counterparts in the presence of ample nutrients. Therefore, larger species are predicted to comprise most phytoplankton in regions with high turbulence and high nutrients, such as the Southern Ocean by Antarctica. In this paper, Behrenfeld et al. suggest a different way to view phytoplankton abundances. Instead of competition amongst phytoplankton for nutrients as the main determinant of phytoplankton size, Behrenfeld et al. investigate the role predator and prey interactions play in structuring these communities, and how well phytoplankton respond to sudden changes in their environment.
Though phytoplankton are eaten by animals as large as whales, their more common predators are zooplankton, microscopic animals similar to krill that are the intermediate step in the food chain between phytoplankton and larger fish. To understand how zooplankton might determine phytoplankton size, Behrenfeld et al. used a theoretical model to study a food web with phytoplankton (prey) and zooplankton (predators) of different sizes. In this model, smaller zooplankton were limited to eating smaller phytoplankton, but larger zooplankton could eat small to large phytoplankton. Using these assumptions, the authors were able to calculate how many phytoplankton there should be across different sizes. They compared their calculations to global observations of phytoplankton size distributions. While models based on competition for nutrients underestimate the relative proportion of small phytoplankton, Behrenfeld et al.’s model did not.
Though Behrenfeld et al.’s model accurately predicted phytoplankton size under normal conditions, their model did not consider seasonal changes in phytoplankton communities called ‘blooms.’ Blooms occur when changes in ocean mixing provide additional nutrients to the surface, ‘fertilizing’ phytoplankton and allowing them to grow to such a large population size that they can be seen from space. To explain seasonal blooms, the authors adjusted their model to incorporate how quickly different microorganisms respond to rapid changes in nutrient availability. While phytoplankton can respond more or less immediately to nutrient fertilization, predators have a lag time before they start eating more. The authors claim that small predators can respond to changes in food availability quickly while it takes longer for larger predators to respond to their environments. The slow response of large zooplankton gives an advantage to large phytoplankton, who don’t face increased grazing pressure until long after smaller phytoplankton do. By adapting each microorganism’s response time to nutrient changes, Behrenfeld et al.’s model was able to account for seasonal phytoplankton blooms.
Prevailing theories claim competition amongst phytoplankton of different sizes plays a primary role in structuring phytoplankton communities. Veering from traditional models, this study instead suggests zooplankton predation plays an important role in determining the phytoplankton landscape. Their model sufficiently explained real-world observations of phytoplankton communities in global observations, encouraging future research on phytoplankton community assembly to pay increased attention to predation and not just competition for nutrients.
For more information, read the full paper by Behrenfeld et al. (2021) published in ISME Communications.