Peeking under the microscope in 2016, Dania Albini observed an algae-eating water flea. His intestines looked full and green with all the tiny Chlorella vulgaris algae ingested. But he also observed bright green patches of this phytoplankton in an unexpected place: the herbivore’s brood sac.
“I was really surprised to see them there,” said Dr Albini, an aquatic ecologist at the time at Swansea University in Wales.
As colonization continued, the algae enveloped the creature’s tiny eggs, killing some eggs and causing fewer babies, according to a study led by Dr Albini and published Wednesday in the Royal Society Open Science. With the algae still alive, researchers suspect that chlorella has an offensive strategy instead of a typical defense to protect itself from herbivores.
Phytoplankton are typically unicellular photosynthetic organisms that form the foundation of aquatic food chains. Among these are microalgae such as Chlorella vulgaris which float on the surfaces of ponds and lakes, making them easy meals for widespread zooplankton such as Daphnia magna. To keep grazers at bay, some microalgae form spines, release toxins, or clump to larger sizes than a predator can swallow.
But sometimes, chlorella makes its way inside a grazer’s body – not into the belly as food, but into the chamber that houses the zooplankton’s offspring. Water circulates through this brood chamber and provides oxygen and nutrients to the young and appears to attract some algal cells. While in this chamber, the researchers discovered during laboratory experiments that mimicked some natural conditions, the algae were alive and able to double in abundance.
When the algae managed to colonize a brood chamber, the zooplankton barely produced viable eggs. Kam Tang, plankton ecologist also in Swansea and co-author of the study, believes that the “biological glue” produced by the chlorella cells helped them adhere to each other and possibly to the brood chamber and eggs, choking off most of the next generation of zooplankton.
This unexpected presence of chlorella cells within the reproductive chambers of its herbivores was surprising to Thomas Kiørboe, a marine ecologist at the Technical University of Denmark, who was not involved in the study. “But maybe no one has really looked for it before,” he said.
Why does chlorella engage in this harmful intrusion? The researchers suggest that this offensive strategy could protect algae cells from grazing and trigger a long-term reduction in zooplankton populations in lakes.
But what remains unknown is whether the living chlorella within the brood chambers of Daphnia actually makes its way into the water or gets trapped?
“There is no reason to believe this is beneficial for algae,” said Dieter Ebert, an evolutionary biologist at the University of Basel in Switzerland who was not involved in the study. “They have no chance of getting out.”
Dr. Kiørboe is also skeptical that this is a chlorella survival strategy. Unless it is known that the individual chlorella cells within the brood chambers themselves reap the benefits, “their interpretation can be contested,” he said.
The researchers plan to do a long-term experiment to see if algal cells escape when Daphnia dies, for example.
“It is difficult to study a phenomenon that is out of the ordinary,” said Dr. Tang, “especially when it goes against what many people think.”