Dania Albini peeked through a microscope in 2016 and saw an algae-eating water flea. His intestines appeared full and green with all of the ingested tiny Chlorella vulgaris algae. But she also observed bright green patches of this phytoplankton in an unexpected place: the herbivore’s breeding pouch.
“I was really surprised to see her there,” said Dr. Albini, a water ecologist at Swansea University in Wales.
During colonization, the algae enveloped the tiny creature’s eggs, killing some eggs, and resulting in fewer newborns. This was the result of a study by Dr. Albini, which was published in the Royal Society Open Science on Wednesday. Since the algae are still alive, the researchers suspect that, contrary to a typical defense, Chlorella may employ a strategy of attack to protect itself from herbivores.
Phytoplankton are typically unicellular photosynthetic organisms that form the basis of aquatic food chains. Among them are microalgae like Chlorella vulgaris that swim on the surface of ponds and lakes, making them simple meals for widespread zooplankton like Daphnia magna. To keep the pastures in check, some microalgae spike, release toxins, or aggregate to a size larger than a predator can swallow.
But sometimes chlorella penetrates the body of a grazer – not in the stomach as food, but in the chamber in which the offspring of zooplankton are housed. Water circulates through this brood chamber and supplies the young with oxygen and nutrients. It seems to attract some algal cells. While in this chamber, the researchers conducted laboratory experiments to find that some of the natural conditions were mimicked. The algae were alive and could double in abundance.
When the algae managed to colonize a breeding chamber, the zooplankton barely produced viable eggs. Kam Tang, a planktonic ecologist also in Swansea The study’s co-author believes that the “biological glue” that Chlorella cells produce has helped them adhere to each other and possibly to the brood chamber and eggs, and suffocate most of the next generation of zooplankton.
This unexpected appearance of Chlorella cells in the reproductive chambers of its herbivores surprised Thomas Kiørboe, a marine ecologist at the Technical University of Denmark who was not involved in the study. “But maybe no one really looked for it before,” he said.
Why is chlorella involved in this harmful invasion? The researchers suggest that this attack strategy could protect algal cells from grazing and, in the long term, trigger a reduction in zooplankton populations in lakes.
However, it remains unknown whether the living Chlorella in the Daphnia brood chambers actually get into the water or remain trapped?
“There is no reason to assume that this is beneficial for the algae,” said Dieter Ebert, evolutionary biologist at the University of Basel in Switzerland, who was not involved in the study. “You 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 in the incubation chambers are using the benefits themselves, “their interpretation may be questioned,” he said.
The researchers are planning a long-term experiment to determine, for example, whether the algal cells escape when daphnia die.
“It is difficult to study a phenomenon that is unusual,” said Dr. Tang, “especially if it goes against what many people think.”