Peering through a microscope in 2016, Dania Albini gazed at an algae-eating water flea. Its gut appeared full and green with all the ingested teeny-tiny Chlorella vulgaris algae. But she also observed bright green blobs of this phytoplankton in an unexpected place: the herbivore’s brood pouch.
“I was really surprised to see them there,” said Dr. Albini, an aquatic ecologist then at Swansea University in Wales.
As the colonization continued, the algae enveloped the tiny creature’s eggs, killing some eggs and resulting in fewer newborns, according to a study led by Dr. Albini and published Wednesday in Royal Society Open Science. With the algae still alive, the researchers suspect that Chlorella deploy an offense strategy as opposed to a typical defense to protect themselves from herbivory.
“You don’t expect a food to attack a predator in this way,” Dr. Albini said. “You expect it from a parasite, but not food. It’s fascinating.”
Phytoplankton are typically single-celled photosynthetic organisms that form the foundation of aquatic food chains. Among them are microalgae like Chlorella vulgaris that float on surfaces of ponds and lakes, making them easy meals for widespread zooplankton like Daphnia magna. To keep grazers at bay, some microalgae form spines, release toxins or aggregate to a size that’s larger than a predator can swallow.
But sometimes, Chlorella make their way inside a grazer’s body — not in the belly as food, but into the chamber housing the zooplankton’s offspring. Water circulates through this brood chamber and supplies oxygen and nutrients to the young, and seems to pull in some algal cells. While in this chamber, the researchers found during lab experiments mimicking some natural conditions, the algae were alive and able to double in abundance.
When algae managed to colonize a brood chamber, the zooplankton barely produced any viable eggs. Kam Tang, a plankton ecologist also at Swansea and co-author of the study, reckons that the “biological glue” that Chlorella cells produce, helped them stick to each other and possibly to the brood chamber and the eggs, smothering most of the zooplankton’s next generation.
This unexpected occurrence of Chlorella cells inside its herbivores’ reproductive chambers was surprising to Thomas Kiørboe, a marine ecologist at the Technical University of Denmark, who wasn’t involved in the study. “But maybe no one really looked for it previously,” he said.
Why do Chlorella engage in this harmful intrusion? The researchers suggest that this offense strategy might protect algae cells from being grazed upon and trigger a reduction in zooplankton populations in lakes in the long run.
But what remains unknown is whether the live Chlorella inside Daphnia brood chambers actually make their way out into the water or remain trapped?
“There is no reason to assume that this is beneficial for the algae,” said Dieter Ebert, an evolutionary biologist at the University of Basel in Switzerland, who wasn’t involved in the study. “They have no chance to get out.”
Dr. Kiørboe is also skeptical that this is a Chlorella survival strategy. Unless it’s known that the individual Chlorella cells inside brood chambers themselves reap the benefits, “their interpretation can be challenged,” he said.
The researchers plan to do a long-term experiment to see if the algal cells escape when Daphnia die, for instance.
“It’s tricky to study a phenomenon which is out of the ordinary,” Dr. Tang said, “especially when it goes against what a lot of people think.”