October 23, 2012 | 1
At least gazelles can run. But if you’re a tree, a blade of grass, or a hapless kohlrabi, there’s nothing you can do when the choppers, nippers, or clippers of your predator — aka “grazer” — approach. Such is the fate of most photosynthetic organisms, which we landlubbers tend to think of as plants. But not for all.
In the aquatic microbial world, there are photosynthetic organisms that apparently have another option when their grazers approach: flee.
Scientists at the Graduate School of Oceanography at the University of Rhode Island have found — and reported in a recent issue of PLoS ONE — that at least one marine phytoplankton can flee from its predators to low-saline refuges. There, their biology allows them to grow where their predators cannot, and potentially provides an explanation for the tendency of this species — Heterosigma akashiwo — to form harmful algal blooms in estuaries around the world.
Getting eaten is the single largest “mortality factor” for phytoplankton in the ocean (in the jungle of the ocean, it’s hard to imagine that many die of old age). More than 50% of daily primary production — biologist speak for new phytoplankton — is eaten. So anything that affects how much get eaten could have a big effect on how many phytoplankton are swimming at the end of the day.
Some phytoplankton attempt to avoid this fate by changing shapes or actively secreting what biologists politely term “chemical deterrents”. But what if they could just opt for another tried and true method: run away? To the Rhode Island scientists’ knowledge, no one has ever asked that question of phytoplankton.
So they asked that question of Heterosigma akashiwo. The little protist — which belongs to a group defined by its two unequal flagella called the heterokonts — certainly appeared to have the potential to take an active role in its defense. In this diagram, you can see the central nucleus — connected by a microtubule-based apparatus to its two flagella. The bristly tinsel flagellum pulls the cell through the water, while the stern-pointing whiplash flagellum pushes. Surrounding the nucleus are mitochondria (M) and chloroplasts (Gp).
H. akashiwo grows in the waters off the coasts of the temperate zones of the world in all hemispheres. It keeps its nutritional options open by snacking on bacteria or, when bathed in light, firing up the chloroplast solar energy-harvesting system.
H. akashiwo also has an unfortunate tendency to form red tides. “Akashiwo”, in fact, is the Japanese term for the phenomenon. Scientists still don’t know exactly why, but some chemical or physical attribute of H. akashiwo makes their blooms toxic to fish and other marine life. And of course, it possesses some property that makes massive blooms possible in the first place as well.
To test Heterosigma’s ability to escape its predators, scientists Elizabeth Harvey and Susanne Menden-Deuer pitted this creature against a microbe called Favella. Favella is a ciliate, a group of organisms named for their abundance tiny beating hairs. In particular, it belongs to a group called the “tintinnids“.
As you can see in this image of another tintinnid species, the cilia of these creatures crown the cup of their proteinacous shells, also called loricae (sing: lorica). These shells fossilize well, which is how we know the lineage is ancient, stretching back to the Ordovician ~450 million years ago. The cilia draw algae and bacteria to their doom while simultaneously serving as a phytplankton-seeking propulsion system.
Harvey and Menden-Deuer put both these creatures in the same tank together. They found that H. akashiwo exposed to Favella swam much faster than those placed in seawater alone. They also swam upward much faster than H. akashiwo controls to a low-salinity “refuge” at the surface of the experimental tank — a surface layer intended to simulate conditions in river estuaries where a thin film of river freshwater floats atop seawater. Favella cannot survive in low-salinity water. Once H. sigma got there, they decelerated, causing them to accumulate in the refuge.
Adding predators that didn’t feed on H. akashiwo and water filtered from a tank containing predators who did resulted in the same, although weaker, fleeing response.
When the scientists removed the low-salt surface layer but retained the predators, Heterosigma swam faster, but this time downward, since without a low-salt surface layer, the predators congregated at the top. This demonstrated Heterosigma don’t just swim up generically in response to predators, they swim away from them.
As a strategy, running away may offer some advantages over other methods of predator deterrence, like killing or stunning. Fleeing not only 1) leads you to encounter your predator less, but 2) leaves your predator free to eat one of your competitors who can’t run away, increasing nutrient availability to you. Thus, the ability to flee may be one reason these algae are good at forming blooms.
Indeed, when the scientists incorporated the salt tolerance of predator and prey into a computer model simulating a freshwater surface refuge, only by adding in the fleeing behavior of H. akashiwo could they produce blooms in two days; without it, it took six.
Running away worked so well, the scientists concluded, that there seems to be little cost to this defence. Because H. akashiwo is so tolerant of low-salinity water while its predators are not, hanging out in low-salinity water provides the organism a huge competitive advantage. These sort of conditions predominate in places like the Fraser River Estuary in British Columbia where H. akashiwo has bloomed before. Field data, the scientists wrote, seem to indicate that H. akashiwo blooms indeed occur as “surface slicks”, where the double bonus of low-salinity and strong sunshine are present.
Eutrophication has long been blamed as the driving force in harmful algal blooms. And although this is no doubt the case in many and perhaps most instances, this study shows that it’s possible to generate blooms in ways that don’t involve human interference. Though only one species was studied for this paper, it is possible other phytoplankton species may exploit similar tolerance differences, other physical or chemical ocean barriers, and sheer fleeing prowess to their advantage. How many more of these microbial plant-analogues can run? We won’t know unless we keep looking.