It doesn’t even eat sugar. How picky is this fungus?

Why do people like to eat candy? Because candy is sweet. Then why do people think candy tastes sweet?

In fact, taste is not a property of food itself, but is completely determined by the human tongue . Some people find the same vegetable very bitter, while others do not. Generally speaking, the reason why people find something delicious is because the human body has historically needed it. Of course, now "sugar excess" has become a new problem, but this does not affect the importance of sugar itself to human physiology.

The bottom line is that sugar is particularly easy to convert into energy . All living things need energy to survive, but the energy they need is relatively mild. Too much energy will destroy the structure of the organism - so people cannot survive by burning fire or connecting electrical wires.

Sugar is the best source of energy: together with oxygen, it undergoes a series of complex chemical reactions under the catalysis of dozens of enzymes, and gently releases the energy inside little by little for human use. In emergency situations, the human body will also use other chemicals to generate energy, but normally sugar has always been the mainstream. This is true not only for humans, but also for almost all creatures on earth.

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New findings from the study:

A microbe that doesn't eat sugar

However, there are always exceptions in the biological world. Nature Communications recently reported a microorganism called Helicopterus. It does not eat sugar, and even if you soak it in sugar water, it will act as if it does not see it. Studies have shown that it has completely lost the ability to metabolize sugar in its body, and it has basically lost all the enzymes that other organisms use to react to sugar.

So where does its energy come from? It comes from a very obscure channel, arginine hydrolysis. This channel only requires three enzymes, but the energy efficiency is very different . Theoretically, one molecule of glucose can produce up to 38 portions of energy in the body, and in practice it usually produces 30 to 32 portions. But one molecule of arginine can only produce a little more than one portion of energy. It is really hard for it to survive with such a tiny amount of energy.

So why is this bacterium so energy-efficient? Probably because it throws away everything else in its body. Not only does it throw away most of the dozens of enzymes used to metabolize glucose, it also throws away most of the other important components.

Most of the important information of organisms is stored in genes. The human body has more than 20,000 genes responsible for encoding enzymes and other proteins, and more than 10,000 genes responsible for various regulatory functions, which add up to less than 40,000. However , this bacterium only has 359 genes, only one percent of humans.

How does it survive?

How does a creature that is so simplified survive? The answer is that it does not live independently, but lives inside a deep-sea octocoral.

A deep-sea coral and its symbiotic relationship with an ophiuroid. The bacteria that scientists studied live in similar corals. Image source: wiki

Most of the tropical shallow sea corals we are familiar with rely on photosynthesis of symbiotic algae in their bodies to obtain energy. However, there is almost no sunlight in the deep sea, and the corals living there are basically predators, living on other tiny animals and organic debris. They also have other organisms in their bodies. We still lack understanding of these organisms. The bacteria discovered this time is a case in point.

Why do bacteria live in people's bodies? One obvious benefit is that they can use ready-made nutrients instead of making them themselves. The simplest independent organisms discovered by humans have 1,100 genes. After all, any organism needs to carry out some basic metabolism and produce a batch of basic body parts as long as it is alive. But if these parts can be obtained from the outside, then there is no need to make them yourself.

For independent organisms, it is extremely difficult to find these parts by chance in the vast ocean, and it is completely impossible to count on them. However, if you live directly in other organisms, most of the parts will be directly around you.

How do corals allow it to live in their bodies?

But wait, isn't that stealing? Yes, many parasitic microbes do steal. But in this case, the deep-sea coral and its bacteria seem to be getting along. The coral shows no signs of trying to drive it away. Usually, this means the bacteria do something good for the coral in return, and the relationship becomes mutually beneficial. This relationship is no longer parasitic, but symbiotic.

So what good does this bacterium do for the coral in return? The researchers are not entirely sure, but the paper proposes two possibilities: one is defense, and the other is waste disposal.

"Defense" means that this fungus can drive away other parasites. Scientists have discovered a very active DNA cutting system in the fungus, called CRISPR. CRISPR is a very important gene editing tool in contemporary biotechnology. However, its original function is to destroy the genes of invaders. Perhaps this fungus can use CRISPR to eliminate other harmful parasites, and in return, the coral will be responsible for providing some nutrition to the fungus. This is a bit like raising a cat to catch mice. Although cats also need people to feed them, it is insignificant compared to the harm caused by mice.

"Waste disposal" refers to the energy source of the bacteria mentioned earlier, which is arginine. Arginine hydrolysis is a very inefficient energy source, far less than eating sugar. It seems difficult to understand why this energy choice is made, but what if it is intentional? What if it is in the interest of the coral? All organisms have the need to break down amino acids. It is entirely possible that the corals hand this task over to the bacteria, and the energy released in the process is considered to be given to the bacteria. The bacteria, on their part, concentrate on breaking down arginine and do not compete with the coral for precious sugar. In this way, it becomes a win-win situation again.

Of course, the two possibilities mentioned above may not be contradictory.

Bacteria are the true masters of biochemistry on Earth, inventing countless unimaginable biochemical metabolic pathways. In contrast, the biochemical metabolism of large animals and plants is almost the same. For this reason, bacteria play an extremely important role in contemporary biological research. This strange metabolism of bacteria probably cannot directly help people control sugar, after all, sugar metabolism is too important to be discarded - but it may deepen human understanding of biochemical processes, and may even provide a powerful research tool for biological research.

Fungi may not have brains, but the many tricks they have developed through billions of years of trial and error are not necessarily inferior to humans' imagination over decades.

References

[1]https://www.nature.com/articles/s41467-024-53855-5

Planning and production

Author: Fang Gang, a popular science author

Reviewer: Tao Ning, Associate Professor, Institute of Biophysics, Chinese Academy of Sciences

Planning丨Xu Lai

Editor: Yinuo

Proofread by Lin Lin

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