Some tiny forms of life double-dip to sustain themselves: They’re both photosynthetic and predatorial. But as the planet warms, such “mixotrophic microbes” are apt to shift away from being sunlight driven to being more predatory, researchers have found. And because photosynthesis consumes carbon dioxide (CO2) and respiration expels the greenhouse gas, that transition has important implications for the climate. Furthermore, the early-warning signs that signal an impending shift from a carbon sink state to a carbon source state are muted in the presence of high levels of nutrients, the team reported in Functional Ecology.
Mixotrophy isn’t all that common among macroscopic organisms, but the Venus flytrap is one well-known example, said Dan Wieczynski, a biologist at Duke University in Durham, N.C. “It’s sort of like a predator and a plant at the same time.” However, such double-dipping is believed to be downright common among the smallest forms of life, Wieczynski said. “Mixotrophy is probably the dominant strategy in microbes.”
Tiny but Mighty
The collective amount of all the CO2 that mixotrophic microbes are capable of sequestering or releasing is apt to be substantial, Wieczynski said. That’s because the combined biomass of Earth’s microbes, including bacteria and plantlike organisms known as protists, exceeds that of animals by roughly a factor of 40, according to recent estimates. “They outweigh animals by a country mile,” Wieczynski said.
To better understand how changes in ambient temperature and nutrient concentrations affect the dynamics of mixotroph-dominated environments—that is, whether they’re more likely to practice photosynthesis or predation—Wieczynski and his colleagues focused on photosynthetic protozoa. These tiny life-forms photosynthesize but can also feed on bacteria. “People have known about these organisms for centuries, but they’ve more or less been considered curiosities,” said Aditee Mitra, a marine ecologist at Cardiff University in the United Kingdom not involved in the research.
Sun Bathing or Hunting?
Wieczynski and his collaborators investigated how protozoa variably engaged in photosynthesis or predation at temperatures ranging from 19°C to 23°C and under different nutrient concentrations. The team used differential equations to model the temperature- and nutrient-dependent changes in mixotroph and prey biomass due to photosynthesis and predation. Wieczynski and his collaborators furthermore quantified how shifts in the balance of photosynthesis and predation translated into overall changes in CO2 flux from the mixotroph system.
“We were interested in understanding how warming and nutrient loads, particularly high nutrient loads, would affect mixotrophic systems,” Wieczynski said.
The researchers found that at low temperatures, their model consisted of very low densities of prey and that the protozoa therefore relied primarily on photosynthesis: Only 17% of biomass production was derived from predation at 19.8°C, for example. As a result, the system was absorbing far more carbon than it was producing at lower temperatures, the team concluded.
At high temperatures, the researchers’ model was dominated by predation; at 22.2°C less than 1% of biomass production derived from photosynthesis. Because respiration was far more prevalent than photosynthesis, the system was producing much more carbon than it was absorbing at higher temperatures, the researchers surmised.
But things got interesting at intermediate temperatures. Between 20.7°C and 21.1°C, Wieczynski and his collaborators found that the mixotrophs tended to fluctuate in abundance and, furthermore, to cycle rapidly between engaging in photosynthesis and predation. All that flip-flopping meant that the system variably functioned as a carbon sink and a carbon source. “You get these wildly fluctuating dynamics,” Wieczynski said.
In nature, such fluctuations could be viewed as early-warning signals that mixotrophic microbes are beginning to shift away from a carbon-absorbing photosynthetic state and move toward a carbon-emitting predatory state, the team proposed. But the temperature window over which that flip-flopping persisted—essentially the amount of warning time—tended to shrink as nutrient concentrations increased, Wieczynski and his collaborators found. “That early-warning signal shrinks and eventually disappears,” Wieczynski said. That’s a worrying trend, the researchers explained, given that nutrient levels have been increasing in the environment, in part because of fertilizer runoff from agriculture.
A Vicious Cycle
These new findings are cause for concern given global temperature trends, the researchers suggested. The planet has already warmed by nearly 1°C from the 20th century average, and up to 3°C of additional warming is predicted by the end of the century. “Warming may cause mixotrophs to pump more carbon dioxide into the atmosphere, which, in turn, causes more warming,” Wieczynski said. “Because mixotrophs are so abundant globally, such a positive feedback loop may not just be a local phenomenon restricted to a small pond or a tiny region of the ocean, but instead could affect carbon cycling and climate on a global scale.”
Mixotrophic microbes no doubt play a role in the planet’s carbon budget, Mitra said, but more sophisticated modeling is necessary to better understand the impact of these tiny organisms. The model that Wieczynski and his colleagues used is overly simplistic, she said, and some of their conclusions are at odds with the findings of other studies.
For instance, Mitra and other researchers recently found that communities of mixotrophic microbes don’t rapidly toggle between photosynthesis- and predation-dominated states. “There’s no flip-flopping,” Mitra said. It’s clear that more work is needed, she said. To that end, Mitra and her colleagues have begun compiling a database of mixotrophic plankton to better understand how these organisms grow, photosynthesize, and ingest prey.
—Katherine Kornei (@KatherineKornei), Contributing Writer