Our oceans birthed all life on Earth. Some marine creatures are more than 200 million years old and hold bonds that have withstood many changes. A bond between two seperate species is rare. But a bond that spans several million years, is even rarer. 

A group of microalgae classified as dinoflagellates are extremely robust creatures capable of surviving without many nutrients or favorable environmental conditions. The most striking survival ability of dinoflagellates comes from their ability to survive in other animal cells. This symbiotic behavior is common in multicellular creatures and several species share a bond in the natural world. But, these tiny single-cell organisms have engaged in mutually beneficial relationships with corals since primeval times, making it an ancient bond. 

The reason corals do not unleash chemical defences to prevent this is that dinoflagellates are crucial to coral survival. Dinoflagellates are photosynthetic. This means that they can make food using sunlight. They share this with corals and over millions of years, corals have become incredibly dependent on dinoflagellate nutrition, especially in shallow waters with a lot of direct light. By passing on critical nutrients to their hosts, dinoflagellates allow corals to thrive even in barren areas. 

A team of researchers from the Centre for Organismal Studies (COS) of Heidelberg University recently discovered in their study that this is only possible because the dinoflagellate cell can suppress the immune system of their host coral cells and “thereby avoid being spit out” again. 

This amazing immune response to dinoflagellates, according to the team, is an evolutionary marvel. Humans were not aware of the intricate and binding friendship between corals and dinoflagellates. The researchers were able to document the immune response and found that this mechanism, known as vomocytosis, is more widespread than previously assumed.

Before these findings, scientists assumed that corals consume dinoflagellates and absorb ones that were left undigested. Also, it was thought that the dinoflagellates unsuitable for symbiosis were digested to extract nutrition. But this turned out to be incorrect as the unsuitable dinoflagellates were simply ejected or “spit out” as they were not conducive to symbiosis. This shows that corals can actively seek out and filter these microorganisms and pick ones that fit in perfectly. 

The joint research team from the National Institute for Basic Biology (NIBB), Tohoku University in Japan, and James Cook University in Australia also studied the role of dinoflagellates in coral recovery after a bleaching episode. They examined the effect of pre-exposure to heat stress on the capacity of symbiotic algae to infect cnidarian hosts using the Aiptasia (sea-anemone)-zooxanthellae (algae) model system. They discovered that the symbiotic algae lose their capacity to infect the host once they are exposed to heat stress. These results suggest that recovery from bleaching can be limited by the loss of symbiont infectivity following bleaching-inducing heat stress, according to the media release published in University of Heidelberg website.

Symbiotic algae (CS-164) were cultured at 25 ºC or 32 ºC for 3 days and then their infectivity was tested at 25 ºC by counting the number of symbiotic algae in Aiptasia tentacles.

“The infectivity of algae was apparently lost after culturing at 32 ºC.Importantly, culturing Aiptasia, instead of algae, at 32 ºC did not influence infectivity.So our results showed that recovery from bleaching can be limited by the loss of symbiont infectivity following bleaching-inducing heat stress.”, Ms. Kishimoto said.

Also, a positive finding showed that incubation at 32 ºC was non-lethal for the symbiotic algae which corals relied upon. They showed tremendous ability to recover after culturing at optimal growth conditions. Associate Professor Shunichi Takahashi of the National Institute for Basic Biology, who led the research team, said, “Our findings suggest that heat tolerant algal symbionts might give a chance for bleached corals to recover”.

Using Exaiptasia diaphana (Aiptasia), a species of sea anemone, Prof. Guse’s team recently uncovered how immune suppression by the symbionts helps the host cell to recognise suitable microalgae and tolerate them for the long term. The Aiptasia anemone larvae ingest the symbionts from the environment in the same way as coral larvae. Furthermore, their size and transparency make the larvae of this sea anemone perfect for high-resolution imaging and cellular experiments.

The Aiptasia used in the study ingested various microorganisms from the currents using their feeding tentacles. After some time, the unfit dinoflagellates were ejected from the coral. Symbionts that survived vomocytosis were successful at controlling immune responses of the coral. The toll-like receptors (TLRs) of the host cell were manipulated to allow smooth entry into the coral cells. These receptors play a critical role in activating the cell’s own immune system and ensure that unwelcome intruders are detected and removed. In most animals, the toll-like receptors are controlled by the MyD88 gene. 

“We were able to prove that the algae symbionts suppress MyD88 and thus initiate symbiosis. That is how they elude vomocytosis,” explained Prof. Guse in the media release. 

At the same time, the findings of the Heidelberg researchers indicate that vomocytosis involves a mechanism that is more widespread than assumed. Until now, it was believed that the expulsion of harmful intruders was self-initiated to evade the in part highly specialised immune responses of the potential host cell. The study of the Aiptasia model, however, suggests that this process can also be triggered by the host cell. The researchers therefore assume that vomocytosis is an evolutionarily ancient immune mechanism that corals or cnidarians like Aiptasia use to select appropriate symbionts. Prof. Guse: “This suggests that vomocytosis is an important process that led in the first place to the emergence of the intracellular lifestyle of the coral symbionts.”