Tenure Tracked Assistant Professor, Qian Li at School of Oceanography of Shanghai Jiao Tong University, has just published her latest discoveries on predatory phytoplankton in The ISME Journal. This paper, titled ‘Broad phylogenetic and functional diversity among mixotrophic consumers of Prochlorococcus’,was group efforts with coauthors Dr. Christopher Schvarcz, Professors Grieg Steward and Kyle Edwards from University of Hawai’i at Manoa.
Micrographs illustrating the predatory nature of diverse small phytoplankton isolated from the open ocean. Green beads (arrowheads) mimic bacteria; seen inside phytoplankton after eating beads. (Photo credit: Qian Li)
Phytoplankton are often viewed as the plants of the sea. They use light from the sun to photosynthesize and grow, and they are the primary food source that sustains all life in the ocean. Although this view is generally true, life among the plankton can be considerably more complicated. It turns out that not all of the diverse phytoplankton are just photosynthesizing; in many cases they are also voracious predators! Because these phytoplankton mix two different feeding modes (photosynthesis and predation), they are referred to as mixotrophs (from mix plus the Greek trophikós pertaining to food).
“Oceanographers have known about mixotrophs for a long time, but interest in them has really been growing over the past decade as we learn just how prevalent and important they are in the ocean’s ecosystem” said Qian Li, the lead author the study.
These authors have spent years isolating and taming these mysterious marine microbes to grow in the lab then investigating their feeding habits on Prochlorococcus. Prochlorococcus is a photosynthetic bacterium and one of the single biggest contributors to photosynthesis in the open ocean, so figuring out who eats these tiny cells is fundamental to understanding the ocean ecosystem.
Qian Li prepares phytoplankton cultures for an experiment to test their feeding behavior. (Photo credit: Christopher Schvarcz)
Mixotrophs appear to be major consumers of bacteria, and that would include photosynthetic bacteria like Prochlorococcus, but until now there has been no direct observations that any mixotroph eats this key player of the marine food web, and no data on the rates at which they do so. That situation has dramatically changed with this new study that compared the feeding rates of dozens of unique phytoplankton isolates and quantified how feeding rates are affected by food availability. These data help fill a large gap in our knowledge about the how different mixotrophs fit into the marine food web.
The authors concluded from their work that mixotrophs—those tiny predatory plants of the sea—are very diverse, abundant, and widespread, and collectively they are eating a lot of primary production. “We hope this work will give people a new perspective on these things we call ‘phytoplankton’”, said Li, “A lot of beautiful complexity is hidden in that one word, and the more we look the more interesting things we find!”