Better Living

An article from carl 03|2025

by Karin Hollricher

Take a look outside: every living creature that you can see is the product of a symbiosis. Without such biological alliances, whose deepest form is endosymbiosis, complex life as we know it would not be possible.

The two best know and oldest examples of mutualistic endosymbionts are mitochondria and chloroplasts. These are the descendants of long-since extinct proteo- and cyanobacteria that were incorporated into other organisms about 1.5 billion years ago and, since then, have provided their hosts with energy through cellular respiration (mitochondria) and photosynthesis (chloroplasts). A long time was spent at the microscope trying to prove the existence of an endosymbiont. Only with the advent of genome analyses did a comprehensive description of the abundance of fellow microbial residents become possible.

Insects, the most diverse class of all animals, offer a particularly varied habitat for bacteria: wherever researchers take a closer look, they find new forms of cohabitation. Just last year, a species of bacteria was discovered, which lives in the cells of at least 23 insect species across six orders [1]. The species was appropriately named Symbiodolus clandestinus – clandestine co-resident. 

The extent of symbiotic communities of insects and bacteria ranges from voluntary, informal relationships to complete, mutual dependence. An insect can host one or more types of bacteria, both intracellularly and extracellularly, in specially developed body structures. Endosymbionts can even be passed on to the next generation through egg cells, as is the case with mitochondria and chloroplasts. Symbiosis in aphids is just as pronounced: their endosymbionts, Buchnera bacteria, produce vital amino acids that are not contained in the vegetable nutrition of the aphids. This mutualistic symbiosis has existed for at least 150 million years. Meanwhile, the Buchnera bacteria have lost part of their genome, making them fully dependent on their hosts.

But not all mutualistic symbioses are so old. Just a few hundred thousand years ago, the long-tailed mealybug offered the Tremblaya bacterium a home [2]. This single-cell organism is in turn host to two species of Moranella bacteria. Although this “ménage-à-quatre” is still young by evolutionary standards, the parties involved have already developed a complex metabolic network in which all four are mutually dependent on each other. 

Researchers at the Max Planck Institute (MPI) for Marine Microbiology in Bremen are studying the co-existence of marine organisms and bacteria. For example, the symbiosis department, managed by Nicole Dubilier, has published the world’s first report on an endosymbiont that lives permanently in the endoplasmatic reticulum (ER) [3]. The bacterium Grellia incantans lives in the ER of tricho-plax, a tiny creature with a diameter of 0.5 millimetres and a very simple structure, which lives in bodies of warm salt water and feeds on microscopically small algae. The genome of Grellia reveals that this organism is dependent on the supply of amino acids and nucleotides from the host. The close relationship with a parasitic cousin could be an indication that this Grellia species is currently in a transitional stage between mutualistic and parasitic existence. 

Trichoplax, however, also provide living space for a second lodger, the Ruthmannia bacterium. Whether or not this partnership is mutualistic or not, we don’t yet know. Ruthmannia is happy to eat the lipids that stem from the host’s digestion, but the researchers have not yet found any indication of the bacterium paying a favour in return.

Also at the MPI in Bremen, the working group lead by Marcel Kuypers is searching for still unknown symbiotic communities. The researchers found a new, indisputably mutualistic symbiotic community of diatoms and several species of bacteria [4]. Each of the algae that belong to the diatoms hosts four bacteria. They can reduce molecular nitrogen (N₂) into ammoniac or ammonium and thus make it biologically available to the algae. The marine bacteria are able to fixate a hundred times more nitrogen than they themselves require. According to the researchers, this means they could contribute significantly to nitrogen fixation in the oceans.

These diatom symbionts, however, are not the only nitrogen-fixing marine creatures. There are also N2-fixing bacteria living in the roots of sea grasses, which then deliver nitrogen to their hosts. Sea grasses originate from land plants and returned to the sea around 100 million years ago [5]. They are marine flowering plants that can form huge meadows in shallow water. They are considered to be important for carbon dioxide fixation, but their existence is endangered. That’s why attempts are under way to grow new sea grass meadows – with only 
moderate success. “The cultivation failures may be due to a lack of nitrogen-fixing bacteria in those locations,” Nicole Dubilier suggests.

The list of newly discovered symbiotic communities is almost endless. “These are early days and we have barely scratched the surface of the vast diversity of symbiotic systems that drive the biosphere,” wrote Margaret McFall-Ngai from the CalTech Institute in California [6]. She became famous due to her studies of the symbiotic community of the small Hawaiian squid Euprymna scolopes with bacteria of the species Vibrio fischeri [7], whose light can be switched on and off like a torch.

“The ecological niches filled by invertebrates and plants are so varied that many strategies for living in the microbial world remain to be discovered,” wrote McFall-Ngai. “To address these different systems with the highest possible rigour, strong collaborations between animal and plant biologists and the community of microbiologists will be essential, although this imperative will not be easy, as the fields have been in silos since the 19th century. Despite these cultural challenges, it is a new day for biology, with a vast frontier to explore.” [6] 

[1] J. Wierz et al., 2024, ISME J. 18, doi.org/10.1093/isme-jo/wrae080
[2] A. Garber et al., 2021, Genome Biol. Evol. 13, doi.org/10.1093/gbe/evab123
[3] H. Gruber-Vodicka et al., 2019, Nat. Microbiol. 4, 1465-1474  
[4] B. Tschitschko et al., 2024, Nature 630, 899-904, doi.org/10.1038/s41586-024-07495-w 
[5] W. Mohr et al., 2021, Nature 6090, 105-109 
[6] M. McFall-Ngai 2024, PLoS Biology, doi.org/10.1371/journal.pbio.3002571 
[7] M. McFall-Ngai 2014, PLoS Biology, doi.org/10.1371/journal-pbio.1001783

Image credits: Adobe Stock, Daniel Nimmervoll / Michael G. Hadfield, Wikimedia Commons (CC-BY-SA-4.0) / Bernhard Tschitschko et al. / Wikimedia Commons, Frédéric Ducarme / Daniela Tienken/Soeren Ahmerkamp, © Max-Planck-Institut für Marine Mikrobiologie, Bremen / Margaret McFall-Ngai