Várzea Ecosystem and its Microbiota

Introduction

The Várzea forests are located along the Amazon River. Due to their location, high annual rainfall occurs and the white-water rivers surrounding can rise 10-15 meters higher than normal and create flooded habitats. There are 5 várzea ecoregions including the Iquitos várzea, Purus várzea, Monte Alegre várzea, Gurupá várzea, and Marajó várzea. These forests can be split into 2 categories of low várzea and high várzea. The low category is determined by an average water height of more than 3m more than 50 days a year, while the high is an average water height of less than 3m less than 50 days a year.

Picture of the Várzea forest during flood season. [1].

The vegetation of these forests can range in species and traits, and can be attributed to the type of water that floods them throughout the year. Igapó forests are different from várzea forests, as these get flooded instead by black-water rivers, resulting in different nutrients and effects carried into the ecosystem. These effects affect the taxonomic diversity in the ecosystem, as well as the microbial diversity.

Section 1 Genetics

When looking at the genetic makeup of the várzea forests, the vegetation of plants and trees in these flooded areas have relatively low taxonomic diversity. This low diversity is due to the long degree of flooded periods in the forests resulting in only species surviving that have the ability to survive and reproduce in hypoxic and anoxic conditions. When looking specifically between the várzea and igapó forests of the amazon, the reason for less taxonomic diversity and speciation of this vegetation can likely be attributed to less time for evolution of species due to the Meta river flooding the várzea forests, which sediments are technically younger than the other rivers sediments that flood the igapó systems.

Picture of Varzea. [2].

While the taxonomic diversity of vegetation is low in the várzea forests, the functional diversity is large. This is seen comparing the várzea and igapo systems once again, and targeting the vegetation in the area. The trait variation was often higher in the várzea forests when compared to the igapó at both the genus and family levels. When looking at both sectors, the leaf area, specific leaf area, and leaf nutrient concentration of many tree species from both forests, the white water tree species had on average 1.2-3 times higher values. So while there is low taxonomic diversity due to the flooding of the várzea forest, the functional diversity of the species that do reside there is high as it is necessary for the species that can survive to find their niche in the stemming further from factors past the flooding of the forest in the environment, creating a high variability of traits.

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Section 2 Microbiome

In general, a good way to identify, classify, and quantify microbes is using eukaryotic 18s rRNA gene sequencing to examine. In the várzea regions, however, this was difficult to do in the plants and soil with many organisms having different responses to the flooded plane. Specifically, looking at the interactions between plants and fungi, the mycorrhizae, can be changed. Potentially the different soil fertility causes negative or positive effects turning the mutualistic relationship into a competition for the nutrients. The soil in the várzea regions is highly fertile, which in this case can change the fungi-plant interactions and possibly make the fungi itself harder to determine using the traditional 18s rRNA gene sequencing. Of the fungi functional guilds observed in the várzea region, there was an overlap of guilds among the different levels of várzea and flood levels(high várzea and mid várzea). All fungal functional guilds were present in these 2 separate levels, with ectomycorrhizal fungi more prominent in the lesser flooded areas and lichenized and parasitic fungi more prevalent at the higher flooded levels.

Picture of ectomycorrhiza. [3].

Similarly how the flood levels in the várzea can create an effect on fungi, it can do the same for protists. Looking at the Iquitos várzea region, the Jurúa river is present. Among the different flood ranges here, different bacterial community composition occurs. This difference in composition can possibly be attributed to different tolerances towards stress due to water on bacteria. For example, nitrogen-fixing bacteria are collected and seen more often in the várzea forests that flood seasonally rather than a non flooded forest. So, the bacterial community composition can vary due to how bacteria tolerates the flooding in the várzea systems, and nodulation could be an example of better toleration of it, creating a favored community composition favored towards bacteria with this ability in várzea regions.

Conclusion

The várzea forests are regions with unique environmental factors of seasonal and prolonged flooding from white-water rivers. This factor creates low taxonomic diversity of vegetation in the ecosystems with hypoxic/anoxic conditions. However, the functional diversity of the vegetation is vast. The genus and family levels of vegetation species display that when compared with Igapó forests, várzea forests on average have larger phenotypic traits when looking at leaf size and nutrient absorption. Of the species that are able to live in these rough environments, this high functional diversity stems from the need to fill a niche in the ecosystem to survive and reproduce.

Microbes in varzeá forests are difficult to spot using eukaryotic 18s rRNA gene sequencing. This could be attributed to the variety of reactions different plant-microbe interactions have in the flooded environment. The mycorrhizae interaction between fungi and plants is evidence of this, where the change in soil fertility could possibly change the positive interactions to negative ones, and change the fungal interactions, possibly causing them to be harder to be identifed, classifed, and quantifed. Looking at high várzea versus mid várzea levels, the fungal guilds that were spotted were found in both, with only certain fungi more or less prevalent in each due to differences in flood levels. The effects on microbes due to the flooded environment is seen in bacteria as well, with the bacterial community composition being altered when compared to a non-flooded environment in the Iquitos várzea. This can stem from the different tolerances of bacteria and the possibility of nodulating creating an advantage for bacteria in flooded environments.

The várzea systems are unique to the Amazon and while intriguing, are not fully studied. There is much work to be done to learn more about the ecosystem and the relationships between organisms in the flooded várzea forests, however what we know now is exciting information.

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References

Edited by [Dylan Carlquist], student of Joan Slonczewski for BIOL 116 Information in Living Systems, 2022, Kenyon College.