Thinking about phytoplankton

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Published on: October 16, 2012
H. akashiwo on the right, with predator Favella sp.

I’m aware phytoplankton are not usual subjects for plant science research, but these small algae are quite plant-like, in general – although they don’t have organs to complicate things. Like plants, they photosynthesize and are able to respond to their environment. Importantly, unlike plants, phytoplankton are mobile, hence the name which in Greek means ‘drifting plants’. Being extremely tiny ‘plants’, phytoplankton present an excellent opportunity for plant scientists to consider synthetic biology, which seems more feasible on a cell-scale rather than an entire plant. The super-theme of the FP7 2013 funding call was ‘The Oceans of Tomorrow,’ and while that call closes in a few short months, synthetic biology, water security and bio-sensors are important research themes which are here to stay.

Highlighted article: Elizabeth L. Harvey and Susanne Menden-Deuer (2012) Predator-Induced Fleeing Behaviors in Phytoplankton: A New Mechanism for Harmful Algal Bloom Formation? PLoS ONE 7(9): e46438. doi:10.1371/journal.pone.0046438

This research focuses on toxic phytoplankton Heterosigma akashiwo, a known cause of harmful algal blooms (HABs; for a fairly recent review of HABs and their effects on human health, see Backer and McGillicuddy Jr., 2006).

click on figure to go to the source

Harvey and Menden-Deuer put H. akashiwo into large tanks with Favella sp., one of its predators, and observed their interactions. They already knew that H. akashiwo is able to move into low salinity waters where some predators and other prey species are not able to survive (Bearon et al., 2006), so they provided a low-salinity ‘refuge’ surface layer in the tank. In the presence of Favella sp., H. akashiwo movement toward the refuge started as soon as Favella was introduced into the system and was complete in 6 hours. This was an obvious difference from the lazy movement observed in the control tank, which contained no predator (Left; Figure 2 in the paper). Interestingly, the presence of predator species that do not graze on H. akashiwo, and (separately) a cell-free filtrate from Favella sp. had much smaller effects – both caused a slight increase swimming speed and migration to the refuge. So neither physical nor chemical cues on their own cause the decisive fleeing action of H. akashiwo away from its predator.

When Harvey and Menden-Deuer built the data from this experiment into a model to predict HAB formation, the model successfully predicted field observations of H. akashimo blooms. Whether or not this ability to escape is widespread in phytoplankton species is unknown, but as Susanne Menden-Deuer said, ‘If it is common among phytoplankton, then it would be a very important process.’

Although some zooplankton species are known to flee predators, in some cases even leaping out of the water to escape, this piece of research was the first time fleeing mechanisms were observed in phytoplankton – the first recorded observation of actively motile ‘plants’. The mechanism underlying the fleeing action remains a mystery after this research, but its discovery and potential manipulation may be an exciting future branch of plant science.

Image credit: University of Rhode Island; PLOS ONE

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