Microbes help their hosts survive high temperatures

• Symbiotic microorganisms (or symbionts) could play a major role in the adaptation of certain organisms – such as insects – to the global rise in temperatures, according to our partner The Conversation.

• This long unknown part of biodiversity is now arousing the interest of scientists who seek to predict the responses of organisms and their symbionts to their environment, and more particularly to global warming.

• This analysis was conducted by Kévin Tougeron and Corentin Iltis, researchers in ecology.

Planetary biodiversity is bearing the full brunt of the effects of an increase in the average global temperature and the frequency of heat waves as manifestations of global warming: population declines of iconic organisms, disruption of biological cycles, or migrations of invasive species have become commonplace.

However, there is a part of biodiversity that remains hidden from the spotlight: symbiotic microorganisms (symbionts) living in the body of their host and providing them with various benefits.

These symbionts could play a major role in the adaptation of certain organisms, such as insects, to the global rise in temperatures, with repercussions on agriculture for insect pests (aphids, bedbugs) and pollinators (bees, bumblebees), but also on human health for those vectors of diseases (mosquitoes, tsetse flies).

Almost all animals are involved in microbial symbioses, to varying degrees.

In humans as in bees, the bacterial community that colonizes the intestine plays a major role in the digestive, immune, or even neurological functions of their host.

This long unknown part of biodiversity is now arousing the interest of scientists, who seek to predict the responses of organisms and their symbionts to their environment, and more particularly to global warming.

A fragile cohabitation: what doesn't kill you can make you... weaker

The high degree of codependency between the partners of the symbiosis makes this interaction very vulnerable to any environmental modification.

For example, warming is a major cause of the bleaching of coral reefs, occurring when the animal loses its microscopic ally: an algae housed in the cells of the coral and providing it, by photosynthesis, with the nutrients necessary for its survival (sugars, acids amines).

The disappearance of the algae resulting from the increase in water temperature in turn causes the death of the coral.

So while the animal derives vital benefits from its close dependence on its symbiotic partner, it also inherits its weaknesses.

In terrestrial ecosystems, many studies explore the effect of symbionts on aphids, faithful laboratory soldiers for some, painful pests in gardens and crops for others.

Like corals, aphids harbor an “obligatory” symbiont called

: a bacterium that provides essential nutrients that the insect cannot find on its own in its sap-based diet.

In exchange, the aphid offers it lodging, cover and transport, since the bacteria multiplies inside the cells of the insect, before being transmitted to the offspring.

The bacterium

undergoes thermal variations linked to climate change, just like its “cold-blooded” host, which is unable to maintain a stable body temperature.

A rise in external temperature thus induces a decline in this bacterial population, from 25°C.

And since the aphid cannot live without the bacteria, the real weak link in the interaction, the system collapses long before reaching temperatures that could directly affect the insect.

​Renting and shared accommodation: the more the merrier, the more we survive

Certain species of aphids do not stop there and can be associated with "facultative" bacteria, which provide them with varied but not always vital advantages depending on the environmental context.

For example, certain facultative bacteria such as

increase the resistance of the aphid to heat by preventing the collapse of the symbiosis.

The mechanism actually involves a threesome: the optional bacterium

does not directly protect the aphid from high temperatures, but limits the decline of populations of the obligate bacterium

by releasing certain molecules, which also benefits the aphid.

'bug.

In contrast,

confers no benefit (and may even prove harmful) if the aphid remains exposed to tolerable temperatures (generally below 25°C).

A lessor (the insect), a lessee (the obligatory bacteria), roommates (the optional bacteria) and global warming likely to end the rental contract (the benefits that each party derives from the association).

Against this backdrop, the obligate bacterial partner appears as the Achilles' heel of the interaction.

Then comes the interest of associating with optional bacteria acting as life insurance when environmental conditions become unfavorable.

​Facultative Bacteria: Perfect Roommates or Squatters?

All this is a little too good to be true, according to certain scientific studies: far from a sentimental romance, the bond between insect and symbionts is very pragmatic, each pulls the sheet on his side to make the most of it.

Certainly, these symbioses have been maintained during evolution because they provide benefits to both parties.

However, two rules prevail in nature: nothing is simple, and nothing is free.

Nothing is simple, because aphids can harbor several species of facultative bacteria simultaneously.

In addition to the function of heat tolerance, some protect their host against natural enemies (parasites, predators), and others allow it to colonize new species of plants.

Nothing is free, because harboring a symbiont remains an infection, and bacterial proliferation is costly for the health of the insect.

These costs are outweighed by the benefits conferred by the bacteria, and both are dependent on the environment.

The whole question is therefore to know how global warming can modify this cost/benefit balance which will decide the fate of each partner.

Among the winners, heat-protective bacteria could spread through aphid populations.

On the other hand, other bacteria could lose out if the beneficial functions they are normally supposed to provide fail.

In a very pragmatic nature, if the aphid no longer derives direct benefit from its association with an optional bacterium, it could well “terminate the lease”.

​Beyond aphids

We bet that few would mourn the disappearance of a notorious pest or an obscure bacterium with an unpronounceable name.

But herbivores are an essential link in terrestrial ecosystems, on which the survival of many species that consume them depends.

Above all, the knowledge gained from research on aphids makes it possible to understand how a complex system made up of a host and a microbial assembly can respond to an environmental disturbance.

Discoveries about these systems can help to better understand how much more complex associations cope with current global changes and can adapt to them.

Our “INSECTS” file

This statement could apply to the extremely diverse microbial communities that take up residence in the digestive system of many mammals, including the human species.

An equation with a dizzying number of unknowns, but ever more exciting research prospects.

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This analysis was written by Kévin Tougeron, researcher in ecology at the University of Picardie Jules Verne, and Corentin Iltis, researcher in ecology at the Catholic University of Louvain (Belgium).

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