What can we learn from microbial jungles carried by sea turtles?

You probably already know that microbes are present in and on humans and that they are helping us with digesting complex nutrients, and fighting off other microbes when necessary. The diverse abilities of microorganisms are exploited for food and medicine production (think of fermented foods and antibiotics), and scientists are also investigating the possibility of microorganisms to improve the health of their hosts through probiotics.

So, microbes are extremely important for us humans, but what about other animals on earth? What about the myriad of creatures having relationships with microbes which humans might have never seen before?

What have microbes got to do with (wild) animals?

The way we study microorganisms changed from mostly labor-intensive isolation and culturing to being able to describe microbial communities based solely on their DNA and/or RNA in an environment that interests us. The approach, based on analyzing microbial nucleic acids, enabled scientists to bypass “the great plate count anomaly”, which describes the difference between the number of microorganisms that we can observe in an original sample, and the number of microbes that we can cultivate. The “plate/culture-free” approach means scientists can gather knowledge about the 99% of microbes that cannot be cultured with the knowledge we have today.

Studying microbes with methods based on DNA and RNA analyses made it possible for more researchers to investigate the previously unseen microbial worlds associated with humans, animals, and plants. Most of the research focuses on human and mammalian microbial communities, however other hosts and environments are getting their fair share of research time as well!

Why should we investigate microbes of wild animals? As the earth is continuously burdened with human activities, wild habitats are disappearing, the environment is changing at great speeds, while scientists observe losses in biodiversity on a daily basis. The knowledge about microbial communities in the wild can add to preservation efforts already in motion, to keep the earth’s biosphere diverse and healthy in the present and the incoming future.

Sea turtle. Image source: Photo by Giorgia Doglioni on Unsplash — *Sea turtle. Image source:* *Photo by Giorgia Doglioni on Unsplash*

Atlas would be proud of sea turtles

Luckily, the curiosity and enthusiasm of scientists brought us this far, and there are plenty of recent studies specifically on endangered wild animals and their microbes. One of the protagonists of such studies are the sea turtles: ancient (they appeared 110 million years ago) and large marine reptiles (the extinct Archelon was around 4 meters in length and 2 tonnes in weight). There are currently 7 species of sea turtles roaming the oceans, many of them endangered due to bycatch, diseases, and habitat disruption. Sea turtle rescue centers around the world work tirelessly on their protection, rescue, and recovery.

The sea turtles are not just ancient giants, but main characters in mythologies trying to explain how the world works. The legends of turtles carrying the world on their back go as far as Hindu, Chinese, and Native American mythologies (similar to Atlas holding the skies in Greek mythology). We can even find their motifs today, in modern culture, within the storylines designed for online games; in World of Warcraft there is an elusive Wandering Island that is actually a huge sea turtle providing a home for panda warriors.

Today we know that turtles do not hold the entire world on their backs. However, such myths are true in a microbial sense! Sea turtles are home to microbial jungles on their shells (along with some larger macro-organisms). The travelers on the turtle shells visible to the human eye are barnacles (Chelonibia testudinaria species grows only on sea turtles), seashells, red and green algae. But if we look closer (and with greater magnification) we will find bacteria, cyanobacteria, diatoms, and other microscopic hitch-hikers.

Loggerhead sea turtle at the Aquarium Pula rescue center: Video by Klara Filek

Who is out there? And what can they tell us?

Bacteria (most famous of all microorganisms) are the first to colonize any surfaces they can find in the sea including sea turtles! Marine bacteria (belonging to groups Proteobacteria and Bacteroidota) can be commonly found in marine biofilms, making the area welcoming for the next microbial hitch-hiker. Cyanobacteria (usually called blue-green algae) also inhabit sea turtles, using the sun as an energy source through photosynthesis. After the small and mighty have prepared the surfaces by excreting sticky extracellular complex sugars, other, larger microorganisms can start building their own colonies.

The following colonizers on sea turtles are often diatoms. Diatoms are eukaryotic microalgae that are known for their “glass houses” (frustules) made out of silica in an organic matrix. Diatom frustules have intricate patterns that are used for their identification. The great thing about diatoms is that even if the cells die, their frustules remain intact and can be detected and described even after the diatom cell is long gone. The perseverance of silicate frustules enables scientists to take a glimpse into the past, determining which diatoms were present in an environment even though there might be some other microorganisms currently inhabiting that area.

*Colorized scanning electron microscopy image of the loggerhead sea turtle shell (colored orange) with diatoms (blue, green, and pink) and bacteria. Image source: colorized by Klara Filek at Bosak lab.*

Diatoms on turtles not only stick to the turtle and build diverse micro-jungles on the turtle's back, there are also many new species of diatoms being discovered. Some of these cannot be found anywhere else yet and might be specific to the sea turtle. Achnanthes elongata is one of the newly discovered turtle diatoms, and even though it was not found anywhere else yet, it seems to be happily growing in the laboratory once the cells are isolated and provided with nutrients. Why is A. elongata found on the sea turtles and nowhere else, even though it seems to grow without the turtle pretty well, remains a mystery (for now).

Research so far has shown that sea turtles are a source of many new species of microorganisms (mostly diatoms so far), but in addition to discovering new microorganisms, we also learned something new about the sea turtles themselves. Microbes living on sea turtles can differ depending on the geographical origin of the turtle, which means scientists might be able to tell where the turtle has been or how it behaved based on the microorganisms that travel with it.

Achnanthes elongata diatom cells in culture. Video by Klara Filek in Bosak lab

What about other microbial jungles?

Sea turtles are just one group of animals that offer a peculiar habitat for microorganisms. If we investigate further into microbes associated with wild animals who knows what we can find. Today's technology gives us the ability to prospect environments for microorganisms and their metabolites. Different ways microbes deal with problems in their microsphere (nutrient acquisition, competition, cooperation) might reveal solutions humans could implement for global issues in medicine, agriculture, and climate change.

On the other hand, we often hear about preventing the loss of biodiversity on earth, which mostly includes big and charismatic flora and fauna. But we tend to forget that biodiversity includes microorganisms of all shapes, sizes, and evolutionary histories, especially the ones we yet have to discover and admire!