Small creatures, great deeds: the role of leaf cutters in the greenhouse effect of neotropics

What being is often associated in human culture with diligence, endurance, collectivity and even responsibility? If you thought about bees, then you are right, but today is not about them. Today we will talk about ants, and not about "simple", but about leaf cutters. Why did we suddenly decide to talk about some insects? Is not something more important. Do not rush to conclusions, because these little workaholics, who work diligently for the benefit of the colony (without intravenous injection of coffee) with little or no rest, play an important role in shaping the whole ecosystem. That's what it means - small, but remote.

More specifically, the leaf cutters, as scientists have found, produce N ₂ O (nitrogen oxide), the volume of which is three orders of magnitude higher than that produced by the wastewater treatment tank. Thus leaf cutters play one of the central roles in the mechanism of the greenhouse effect neotropics. How did the scientists take measurements, what exactly did they find out, and what do they propose to do with leaf-cutting ants? We will find the answers in the report of the research group. Go.

A bit of myrmecology

Ants are considered one of the most developed insect families. Their colonies are one big living mechanism that works practically without failures. At the moment there are, for a second, about 14,000 species of ants, most of which inhabit tropical regions.



The heroes of today's research are not the whole family, but certain species of ants: Atta and Acromyrmex, which are better known under the common name of leaf cutters. You must have seen them at least in some documentary about South America. The name of this type speaks quite directly about their main features, at least those that are immediately visible to the observer. Leaf cutters literally cut off pieces of leaves with their powerful upper jaws (mandibles / mandibles) and transfer them to their colony. But they do not feed directly on the foliage, they need it for agriculture. Yes, these little laborers are great farmers.

In the depths of their colony, leaf shears break fungal gardens that feed, that is, fertilize with chewed foliage. Larger working ants collect the foliage, and smaller ones chew it and make compost.


A small movie about leaf cutters.

Also worth noting is an unusual feature of one of the species studied, Acromyrmex. These ants are “equipped” with a gland that produces the secret necessary for the existence of the Streptomyces bacterium. This bacterium is needed by ants as a “vaccine” against the harmful effects of Escovopsis fungi, which are some form of parasite weeds on a mushroom farm in an anthill.


Incredibly colorful Streptomyces coelicolor.

Peculiarities of leaf cutting are considered in this study. More specifically, the researchers analyzed the leaf cutter waste to identify previously unknown ways of processing N, which leads to the release of a large amount of N ₂ O into the atmosphere, which is part of greenhouse gases. Waste was analyzed in terms of chemical and microbial composition.

Preparation for observations

Samples for the study were collected in the tropical forests of Costa Rica (the exact location is called the Piro Biological Station ). The main feature of this region is forest confusion: one part (primary) is a forest untouched by man, and the second (secondary) - newly grown thickets on a pastureland 50 years ago. Height above sea level varies from 0 to 100 meters. The average annual temperature is about 26 ° C, and the average precipitation reaches 3400 mm. Weather conditions vary from dry season (January - March) to rainy with a peak in September and October.


Image number 1

Samples were taken from 22 colonies of ants of the species Atta colombica, which were located on an area of ​​4 km ² , affecting both the primary zone and the secondary zone of the forest. Scientists recorded the coordinates of all collection points for further analysis of the distribution of gases in the area around the colonies. Were also carried out measurements of the area of ​​colonies and "landfills" of waste, in which several pipes (22 cm in diameter) were placed to a depth of 3 cm and covered with ventilated PVC lids on top. This was done 10 minutes before taking images. Subsequently, the depth was measured, at which, after a certain test period, the pipes turned out to be. Thus it was possible to understand the rate of waste generation.

Every 10 minutes, samples were collected for half an hour and placed in 10 ml pre-evacuated flasks.

The samples were subjected to gas analysis for the presence of N ₂ O, CO ₂ and CH ₄ . For this, a gas chromatograph with a methanator, an electron capture detector, and a flame ionization detector was used.

Samples (waste and soil) were kept at 48 ° C for 3 days and analyzed to determine the potential activity of the denitrification enzyme.

Denitrification * - reduction of nitrates to nitrites, and then to gaseous oxides and molecular nitrogen.

Samples from 10 fence zones were combined into one, which was divided into aliquots. Next, the attraction for 90 minutes was incubated with or without adding a sample from the surface of the garbage heaps.

Aliquot * - accurately measured multiple fraction of the sample.

To determine the microbial biomass, samples of 8 grams were withdrawn 1 hour before and 3 days after the fumigation process with chloroform. During the analysis, special attention was paid to the presence of organic carbon and nitrogen in the samples, in which the Shimadzu TOC-V CPN / TNM-1 analyzer helped.



Observation results


Image number 2

Emissions of N ₂ O from the ant col ant Atta varombica varied greatly: from 180 mg N ₂ ON m ^-2 h ^-1 to 89000 mg N ₂ ON m ^-2 h ^-1 ( 2a ). These measurements are perfectly correlated with observations in a real ecosystem, that is, in nature. Thus, the average value of the produced gas is 11200 mg N ₂ ON m ^-2 h ^-1 .

m ^-2 h ^-1 * - per square meter per hour.

Graph 2b shows a comparison of N ₂ O production of garbage heaps of leaf cutters and anthropogenic structures (designed or maintained by humans): anaerobic lagoons, sewage treatment plants, etc. And as this comparison shows, leaf cutters with the task of developing N ₂ O do much better.

The volume of gas produced by trash heaps is very different from what scientists observed in the primary and secondary forest. When the emissions of garbage heaps reached their peak in June 2017 at the beginning of the rainy season, the volume of N ₂ O from the neighboring colony did not noticeably differ from the surrounding forests (primary and secondary) and was about 67 mg (images 1b and 2a ). The amount of N ₂ O produced by the forest itself surrounding the ant colonies was 11 mg N ₂ ON m ^-2 h ^-1 , the average annual rate was only 2 mg in general.

The researchers note that the most important microbial processes in the production of nitrous oxide are nitrification and denitrification, which are very sensitive to the partial pressure of oxygen and the flow of the substrate. Consequently, the structure of the ant garbage heaps must have certain features that increase the production of N ₂ O.


Image number 3

And such a feature was discovered, namely an increased N concentration and a reduced C: N (carbon and nitrogen) ratio in the garbage entering the landfill - 13.5 + 1 ( 3f , 3e ). It is worth comparing this with the C: N ratio in the tissues of green foliage (21 + 5) and fallen foliage (32–42).

And here the usual logic helps to get a little closer to the clue: the foliage covers the surface of the ground in the forest quite evenly, while the ants pull it into one small area, thereby increasing the concentration of organic waste. Analysis of the data showed that the concentration of nitrogen in garbage heaps is 5 times higher than in the litter of the surrounding forest. At the same time, an incredibly high concentration of NH ₄ (500 times higher) and a very low concentration of NO3 ^- ( 3g , 3h ) together with signs of anoxia are the factors of denitrification, not nitrification, as the main mechanism for the production of N ₂ O.

Anoxia, that is, the absence of oxygen, is also confirmed by a high concentration of methane (CH ₄ ) - from 0.01 to 250 mg CH ₄ -C m-1 h-1. Considering that methanogenesis at pH 5.5 is exactly less effective than denitrification, as in trash heaps. This suggests that the redox potentials of garbage heaps can be greatly reduced. By comparing the potential activity of the enzyme denitrification in garbage heaps and in the surrounding forest floor, scientists found that this indicator is 25 times higher in heaps ( 3d ).

Thus, it can be concluded that garbage heaps of leaf cutters are a very well-coordinated "mechanism" of production of CO ₂ , CH ₄ and nitrous oxide, due to the presence of certain factors: a large amount of microbial biomass, availability of the substrate, ideal abiotic conditions, etc. The only thing that is not completely clear is the difference in the amount of N ₂ O produced between different garbage heaps. In other words, the above factors for different landfills are almost identical. Scientists believe that the difference in the N ₂ O index may depend on more internal factors, so to speak, on the individuality of the structure of the garbage heaps: microclimate, influence of other living organisms, food quality, changes in microbial biomass, condition of the colony (individuals), etc.


Image number 4

Next, the researchers collected together the following indicators: the amount of N ₂ O produced by leaf cutters, the area of ​​garbage heaps, the density of garbage heaps both in the primary and secondary forests, and in the border zones. The combination of these data makes it possible to understand how globally the production of N ₂ O with leaf cutters affects the neotropics atmosphere and ecosystem.

And now the fun part. Per square hectare per year, garbage heaps of leaf cutters with an average density of 1.2 per hectare produce about 0.01–0.35 kg N ₂ ON. And this is 200% of the production of N ₂ O with the litter of the forest itself (primary).

The researchers note that in nature periodically there are jumps in the production of N ₂ O associated with abrupt changes in the ecosystem (weather conditions, for example). However, in the garbage heaps of leaf cutters, the temperature remains fairly stable throughout the year. And very frequent rains additionally supply the wet organic mass enriched with nitrogen into the garbage heaps, which contributes to the development of a large amount of N ₂ O.

Also important is the "migration" of leaf cutting colonies throughout the forest. Moving on to a new place, the ants leave both the colony and the landfill, which decomposes within 1 year. However, even after this, an increased concentration of nitrogen remains in the soil. And this suggests that even abandoned garbage heaps continue to generate N ₂ O, without the participation of their former ant owners.

For more detailed acquaintance with the nuances of the study I strongly recommend to look into the report of the research group and additional materials to it.

Epilogue

The main conclusion of this study is the fact that the life of the leaf cutter plays a significant role in the formation of the ecosystem of the whole region. Leaf cutter dumps are unique structures with unusual chemical characteristics that together contribute to the massive production of N ₂ O.

With their work, scientists once again prove how strong the influence of small organisms on the environment is. Everything in nature is interconnected. But sometimes we do not fully understand how and where this connection is manifested and what it affects. Well, thanks to this research, we know a little more about our large and mysterious world.



Thank you for your attention, stay curious and have a great weekend, guys.

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