“Living Sand” organisms have a stable partner and a flexible crew

They are beautiful, they are everywhere, and they are absurdly unknown: you are entirely forgiven if you’ve never heard of foraminifera before. But they play a huge role in the functioning of marine ecosystems, and they are so ubiquitous that there are entire beaches made of their tiny shells – which are actually huge if you consider that “forams” are single-celled organisms. A typical foram is the size of a grain of sand; up to 5 millimeter (0.2”) big. 

Roommates

There is, however, more than one cell inside the tiny shells. Many forams share their homes with two types of roommate: diatomic algae and bacteria. Scientists want to know how these species work together. Not just because they care about the forams, but also because forams tell us a lot about how corals and other living creatures in reefs are affected by their environment. Like corals, they build a skeleton, and cherish several symbioses with algae and bacteria. 

Are the relationships between foraminifera and their symbiotic crew specific, or can they change easily? To answer these questions, the researchers collected 243 specimens of six different species of foraminifera from coral reefs in Indonesia. They analyzed the DNA of the algae and bacteria inside each foram. 

Surroundings

The research, led by dr. Elsa B. Girard of Naturalis Biodiversity Center, found that the relationship between the foraminifera and their algal partners is very specific. For each foram species, a single, dominant type of diatom made up more than 90 percent of the algal community inside the host. This shows that the foraminifera are very selective about their choice of algal roommates. Also, the algal species were rarely found freely living outside the shell, in the surrounding seawater or sand: “If it’s inside the foram, we don’t see it in the water, and the other way around. This suggests that this specific relationship is crucial for the foraminifera", Girard says. 

In contrast to the algae, the bacterial communities inside the foraminifera were highly flexible and diverse. The composition of the bacterial community was strongly influenced by the local environment, such as water depth or the type of seabed. But just like the algae inside the shell, these bacteria were not present in the surroundings. The bacterial community presumably has a purpose in the foram, but many questions remain as to how, what and why. Girard: “We suggest that bacterial partners likely play a role in the adaptation of the foraminifera to thrive in a wider range of habitats, but we still do not know to what extent that is.” 

“Because forams are such important indicators for coral reef health, it really helps us if we understand how they work, and why they are where they are”, Girard emphasizes.

More information

• Article: Specific host-algae relationship, yet flexible bacterial microbiome, in diatom-bearing foraminifera.
• Nature report: Protecting corals with protists.

Text: Naturalis Biodiversity Center
Images: Naturalis Biodiversity Center; Elsa B. Girard