Climate change and gut microbiota: how are they linked?

What does climate change have to do with the microbiota of animals?

One of the least studied aspects of climate change so far is its correlation with changes in the gut microbiota of animals.

It might seem a very distant connection, on the contrary some studies have shown a two-way relationship between the two: intestinal microorganisms contribute to the tolerance of extreme temperatures, and thermal stress changes the taxonomy and the abundance of bacterial flora species.

These studies were conducted on ectotherms, or “cold-blooded” animals, which include amphibians, fish, invertebrates and reptiles.

These animals, in fact, are the most vulnerable to climate change, since their body temperature depends on that of the external environment, consequently they are highly sensitive to temperature changes.

The microbiota thus becomes an important new factor to be taken into consideration in predicting the response of these animals to global warming, suggesting possible new interventions to maintain its balance and thus protect the endangered species of ectotherms.

Difference in temperature regulation between ectothermic and endothermic animals
Figure 1 – difference in temperature regulation between ectothermic and endothermic animals [credits:socratic.org]

The microbiota regulates the tolerance of ectotherms to thermal stresses

The intestinal bacterial flora is of fundamental importance for numerous host functions, from digestion to immune processes.

Recently, we wanted to observe if and how intestinal microorganisms can also regulate tolerance to acute heat and cold, and consequently to outline their possible role in the response to climate change.

Tadpoles

In a study conducted on American green frog tadpoles, breeding water was sterilized to manipulate the microbiota of the animals, and then assessed the consequences on thermal tolerance.

Two groups of tadpoles were therefore considered, those raised in normal pond water, with a healthy microbiota, and those raised in almost sterile water, which presented fewer intestinal bacterial species.

The two groups were exposed to three different temperatures, 14 °, 22 ° and 28 °. Analyzing the results, it was seen that tadpoles with an altered microbiota were able to tolerate lower temperatures, but were five times more likely to die than those raised in pond water when exposed to high temperatures for long periods.

The causes of this decreased survival to thermal stress probably lie in the fact that tadpoles with reduced microbiota have lower activity of certain mitochondrial enzymes and an altered metabolic rate, both of which are important factors when considering body temperature.

Drosophila subobscura

A further study highlighting a role of the microbiota in regulating heat tolerance considers Drosophila subobscura.

The researchers used unmanipulated and other germ-free Drosophila, exposing them to various stressful temperatures. It has been seen that in mild temperatures the unmanaged midges have a greater tolerance to heat, while in extreme temperatures there is no difference.

Subsequently, the abundance and taxonomy of bacterial species present in unmanipulated Drosophila were compared, some of which are exposed to a high temperature while others are not. These characteristics of the microbiota were found to be altered in flies subjected to transient heat waves.

Therefore: on the one hand, the microbiota modifies the thermal tolerance of the ectotherms, on the other, thermal stresses modify the microbiota.

Figure 2 – measurement of the thermal tolerance of germ-free (red) and non-manipulated (green) Drosophila [credits: pubmed.ncbi.nlm.nih.gov]

The impact of global warming on the intestinal microbiota

Climate change has macroscopic effects on the environment and animal species, which are now clearly visible to everyone, from drought to the increase in species at risk of extinction. But what happens on a microscopic level?

Observing what happens at the level of intestinal microbial communities is important, because the survival of the animal and its possible adaptation to new environmental conditions also depend on these.

Numerous studies reveal the association between the increase in temperatures and the change in the microbiota of various animal species, highlighting the sensitivity of the bacterial flora to the ambient temperature.

In fact, there is a general reduction in the variety of bacterial species and their relative abundance. In particular, it has been seen that in lizards, Drosophila melanogaster, salamanders and tadpoles the phylum of Firmicutes tends to decrease, while that of Proteobacteria to increase.

Changes in the microbiota can have two consequences. On the one hand, the rapid change in intestinal microorganisms could allow the ectotherms to adapt to the new warmer temperatures.

On the other hand, however, this imbalance leads to intestinal dysbiosis, with a negative impact on the absorption of food, on the metabolism and in general on the fitness of the animal, therefore its probability of survival.

Not just ectotherms…

Studies on ectotherms are fundamental because this category is particularly sensitive to changes in external temperature, but alterations in the intestinal microbiota due to thermal stress have also emerged in studies conducted in mammals.

In mice, for example, long-term thermal stresses increase intestinal permeability, damage the mucosa and alter microbial homeostasis, significantly reducing Lactobacilli and Bifidobacteria. Other studies on cows confirm the presence of microbial imbalances and dysfunction of the intestinal mucosa.

Understanding these alterations better is important because the increase in temperatures has a profound impact on farms in tropical and subtropical regions, with serious economic and animal losses. Consequently, in order to reduce the damage, we could act at the level of the microbiota, introducing new specific feeds to restore it.

Conclusions

Due to the expected rise in temperatures in the coming years, one in six animal species is threatened with extinction.

Consequently, it is important to study the mechanisms by which environmental changes impact the physiology of animals, in order to try to conserve biodiversity.

In this, the study of the intestinal microbiota is useful for providing a more complete view on the phenotypic variations of the host, also because it is the ecosystem that changes more rapidly according to the rapid changes in the environment.

Original article “Cambiamento climatico e microbiota intestinale: come sono collegati?” written by Aurora Corbelli

Translation by Giovanna Spinosa

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