Global warming is changing the gut microbes of animals critical to their health and survival Health


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Sasha Greenspan, University of Alabama

(THE TALK) Every day, it seems, scientists are reporting worsening impacts of climate change on animals and plants around the world. Birds that migrate later in the year will not find enough food. Plants flower before their insect pollinators hatch. Prey species have less stamina to evade predators. In short, climatic changes affecting an organism are likely to trigger ripple effects that can disrupt the structure and function of entire ecosystems.

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One component of animal health that largely reflects the environment is the microbiome, the consortium of microbes now known to help digest food, regulate the immune system, and protect against pathogens. The types of bacteria that make up the microbiome are recruited primarily from the environment. Thus, food webs and other animal interactions that affect environmental bacteria have the potential to shape animal microbiomes.

But what happens when climate change disrupts the environment, causing changes in the animals’ microbiome that prevent the microbes from performing the key functions animals need to survive and thrive?

I am an ecologist in Gui Becker’s lab, specializing in tropical research at the interface between emerging amphibian diseases and climate change. Hundreds of amphibians in the global tropics are under increasing pressure from disease and climate change. And there is growing evidence that environmental stressors alter the microbiomes of animals and contribute to the challenges they face.

In a 2020 experiment aimed at finding out how the microbiome of tadpoles is influenced by other animal species in the environment, my colleagues and I studied healthy communities of freshwater bacteria, crustaceans, and insects from wetlands in the Brazilian Atlantic Forest. We focused on their feeding activities – how they filtered water to get their food and broken down dead plant matter.

It is well known that these feeding activities are essential for ecosystem functions such as decomposition. But we found that these food webs also served another purpose: They encouraged the growth of “good” types of bacteria in the environment, such as species that fight pathogenic microbes.

As a result, tadpoles that share the ecosystem with these microorganisms and invertebrates had healthier gut microbiomes. This provided a strong defense against pathogens compared to tadpoles, which did not share their habitat with diverse networks of organisms.

Our latest work took this research a step further by testing how a perturbation such as climate warming could affect these food webs, which help maintain the health of vertebrate microbiomes in the wild.

Mapping species interactions in different ecosystems is difficult under field conditions where the environment is unpredictable, and repeating experiments to confirm the results is challenging.

To address this issue, we used plants from the bromeliad family as mini-ecosystems so my colleagues and I could study the effects of a warming climate on species interactions under the more controlled conditions of a laboratory.

Bromeliads are ideal for experimental work on collaborative interactions as they are natural microcosms and their small dimensions allow us to grow many of them in a small space. Our study sites in the tropical rainforests of Brazil support extremely high densities of ground-to-canopy bromeliads, often reminiscent of a Dr. resemble Seuss.

To replicate natural ecosystems for our experiment, we planted a garden of 60 identical bromeliads outdoors in the shade of a small tropical forest in São Paulo, Brazil. We then allowed invertebrates and microorganisms to colonize the bromeliads naturally for three months. Some of the plants have been exposed to ambient temperatures, others have been heated up to six degrees above ambient using a bespoke outdoor heating system to accommodate predicted global climate change trends.

Nearby, we collected our model host species for the experiment – tadpoles of the tree frog species Ololygon perpusilla, which breed only in the mini-aquariums created from the leaves of bromeliads.

We then brought the bromeliads into the lab from outside, added a tadpole to the tiny pool of water in the center of each plant, and used the same heating system to simulate heating. After a few weeks, we inventoried the bacterial species in the tadpole gut, as well as the bacteria and invertebrate species living in the bromeliads.

The domino effect of climate change

In this study, published in Nature Climate Change, we found that warming effects on ecological community networks—including environmental bacteria, worms, mosquito larvae, and other aquatic invertebrates—impaired the gut flora of tadpoles, resulting in reduced growth, which is an indicator of fitness.

The health of the tadpole gut microbiomes was specifically linked to changes in the community of aquatic bacteria and invertebrates living alongside tadpoles in the bromeliads. That is, warming encouraged the growth and reproduction of certain types of bacteria and invertebrates and inhibited others, and these environmental changes disrupted the tadpole gut microbiome.

The higher temperatures also led to a faster development of filter-eating mosquito larvae. Our results suggest that higher filter feeding rates also altered environmental bacterial species composition in a way that further disrupted the tadpole microbiome.

In fact, tadpole growth—an indicator of the health of the species—was more closely associated with warming-related changes in their gut microbiomes than with direct effects of warming on growth expected in cold-blooded animals such as tadpoles or effects of warming on the tadpoles’ food resources.

Our work shows how global climate change can affect even the smallest levels of biological organization, including the symbiotic bacteria that live in the digestive tract of a tiny species of frog.

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Considering these processes in the context of an entire ecological community helps to broaden our perspective on microbiome health in global change.

Studies examining the effects of warming on vertebrate microbiomes typically focus on direct temperature responses of host flora, rather than locating hosts in the complex and interconnected communities in which they live in the wild.

Our findings support a growing consensus among scientists that while climate warming will push some animals past their thermal thresholds, a far more ubiquitous consequence of warming is that it can trigger an ecological domino effect, disrupting the species interactions that ecosystems need to thrive work correctly.

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