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Kelle Freel and Mike Rappé sampling seawater in Kāneʻohe Bay. (Photo credit: A. Murat Eren)

Within the community of microscopic plankton are the ocean’s smallest predators; small but significant in the critical ecosystem. Those predators are the focus of a new project led by researchers at the University of Hawaiʻi at Mānoa. Kyle Edwards and Grieg Steward of the Department of Oceanography and Mike Rappé of the Hawaiʻi Institute of Marine Biology were awarded $2 million from the National Science Foundation program “Biodiversity on a Changing Planet.” The program funds research on challenges in biodiversity science within the context of environmental change.

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Research technician Kelsey McBeain at work with cultures. (Photo credit: Petra Byl)

“Here in Hawaiʻi, we are surrounded by ocean waters that are beautifully clear and blue because the nutrients that support photosynthesis are scarce,” said Steward. “And we expect more of the ocean to look like this in the future. By studying these predator-prey interactions in detail, we aim to predict how the composition of the complex plankton communities will respond to changing ocean conditions.”

The five-year project will focus on examining microbial food webs that are the foundation of open ocean ecosystems. The most abundant prey in the ocean are minute bacteria, and their primary predators are tiny, single-celled flagellates. The researchers will study which predator species eat which prey species, and how efficiently energy and nutrients are transferred up the food chain.

“A fascinating feature of the open ocean food web is that so many of the primary producers are also prolific predators,” said Edwards. “These hybrid plankton—called mixotrophs because they mix two modes of nutrition—gain energy from sunlight like a plant, even as they consume prey like an animal. Models suggest that mixotrophs help sustain higher ocean productivity and that their importance may increase in a warmer future ocean, but we really need a better foundation of data from lab and field experiments to make more confident, detailed projections.”

Tricky observation conditions

Studying open ocean plankton in the lab can be daunting, but the researchers will capitalize on their decade of prior effort to coax many new species of predator and prey to grow under controlled conditions.

“This research is possible because of two unique culture collections created by the Steward and Rappé labs,” said Edwards. “These collections allow us to do lab experiments with globally important microorganisms that are far away from most oceanographic institutions, but relatively accessible to us, here in the center of the Pacific.”

For more information, see the School of Ocean and Earth Science and Technology’s website.