Colombia is renowned for its exceptional ecological wealth, consistently ranking among the most biodiverse countries in the world. Within its borders lies the Biogeographic Chocó, a region of critical ecological importance along the Pacific coast that extends into neighbouring Panama and Ecuador. This area is distinguished by its extraordinary rainfall—among the highest globally, reaching up to 12,000 millimetres per year—which nurtures dense rainforests harbouring a remarkable array of endemic species. These include rare plants, amphibians, birds, and invertebrates that thrive in its unique climatic and geographical conditions.
Among the myriad species inhabiting the Chocó are wild relatives of Theobroma cacao, commonly known as the cocoa/ cacao tree, the source of chocolate’s essential ingredient. These wild relatives include Theobroma glaucum (glaucous cacao), Theobroma simiarum (monkey cacao), Herrania cf. purpurea (purple herrania), and Theobroma cf. hylaeum (hylaeum cacao). They are not only crucial for maintaining biodiversity but also hold potential solutions to some modern agricultural challenges, such as disease resistance and adaptability to changing climates. These plants may offer genetic traits that can improve cultivated cacao, which is economically and culturally significant worldwide.
With increasing environmental pressures from climate change and deforestation, understanding the relationships between these wild cacao species and their surrounding ecosystems becomes imperative. Exploring the microbial diversity in the soil where these wild relatives grow can uncover biological interactions that aid in biocontrol, improve soil health, and enhance the plants’ resistance to stressors, including heavy metal accumulation like cadmium. A recent study has yielded interesting findings.
Cadmium Concerns in Cacao Cultivation
Cadmium is a toxic heavy metal that poses significant concerns in agriculture, particularly in cacao cultivation. It can accumulate in the soil and be absorbed by cacao plants, leading to contamination of cocoa beans and, consequently, chocolate products. This contamination presents health risks to consumers, including kidney damage and bone demineralisation. Moreover, high levels of cadmium in cacao beans can affect their marketability, as strict international regulations limit cadmium content in food products. Understanding how cadmium interacts with cacao plants and their associated soil environments is crucial for developing strategies to mitigate its impact.
Soil Sampling Study Methodology
In March and April 2021, researchers conducted a soil sampling study to capture the microbial diversity associated with wild cacao relatives. They collected 25 soil samples from previously geo-referenced trees in the village of La Victoria, located in the Department of Chocó, Colombia. The targeted species were Theobroma glaucum (glaucous cacao), Theobroma cacao (cocoa tree), Theobroma simiarum (monkey cacao), Herrania cf. purpurea (purple herrania), and Theobroma cf. hylaeum (hylaeum cacao). These trees were situated in two distinct areas of La Victoria: Baudó and Atrato.
For each tree, the researchers established a circular plot with a one-metre radius around the base. Soil samples were collected from eight equidistant points within this plot to ensure a representative sample of the surrounding area. Before sampling, surface litter and organic layers were carefully removed to access the non-rhizosphere soil from the upper soil horizon, between 0.00 and 0.30 metres deep. Approximately 250 grams of soil from each of the eight points were combined into a single homogenised composite sample for each tree, capturing the variability of microbial communities around each tree.
The samples were stored in sterile, airtight plastic bags to prevent contamination, and in the laboratory, they were immediately frozen at –20 °C until DNA extraction. The study employed extracellular DNA metabarcoding as the primary method for investigating the soil samples. This technique involves extracting DNA directly from environmental samples to identify a wide range of microbial species present, without the need for culturing them in the lab. It is highly effective for analysing complex microbial communities and provides insights into the biodiversity of soil microorganisms.
In addition to microbial analysis, subsamples of 500 grams from each composite sample were sent for physicochemical testing. This analysis included assessments of various soil properties, such as pH levels, electrical conductivity, cation exchange capacity, organic carbon content, and cadmium concentration. These measurements are crucial for understanding soil health and its potential impact on cacao plants, particularly concerning heavy metal accumulation.
Microbial Diversity Findings
The microbial community analysis highlighted the diversity of bacteria and fungi present in the soil. The dominant bacterial phylum identified was Acidobacteriota, known for its role in nutrient cycling and adaptation to various environments. Other significant bacterial phyla included Proteobacteria and Verrucomicrobia, both critical for maintaining soil health and supporting plant growth.
The fungal communities were primarily composed of Ascomycota, Mortierellomycota, and Basidiomycota. These fungi play various roles in the ecosystem, from decomposing organic matter to forming symbiotic relationships with plants. Some fungi, such as those from the genus Mortierella, are known to promote plant growth and enhance nutrient uptake.
However, the study also identified potentially harmful fungal species, including Fusarium and Colletotrichum. These pathogens could adversely affect cacao health, causing diseases that impact yield and quality.
Soil Physicochemical Properties
The analysis revealed differences in soil properties between the two sampled locations, Baudó and Atrato. Variations were observed in several soil characteristics, including pH levels, magnesium saturation, aluminium saturation, and cadmium concentration. The soil acidity or alkalinity (pH levels) can influence microbial communities and plant nutrient availability. Differences in magnesium content affect soil fertility and plant health, as magnesium is a vital nutrient for plants. High levels of aluminium can be toxic to plants, impacting growth and productivity.
Notably, variations in cadmium levels were linked to specific species of Theobroma, particularly Theobroma glaucum (glaucous cacao). This species showed significant correlations with cadmium content in the soil, suggesting it may be more affected by cadmium accumulation compared to other cacao relatives studied. Understanding these differences is essential for developing strategies to mitigate cadmium uptake in cacao plants.
Implications for Cacao Cultivation, Conservation, and Future Research
This study highlights the intricate relationships between wild cacao relatives, soil properties, and microbial communities, which collectively influence plant health, nutrient uptake, and resistance to stressors such as heavy metal accumulation and pathogens. Beneficial microbes can enhance plant resilience, while pathogenic organisms pose risks that require management. Understanding these interactions is essential for developing sustainable agricultural practices.
There is an urgent need for conservation strategies in the Chocó region to prevent biodiversity loss, particularly of wild cacao relatives. Protecting these species is crucial not only for maintaining ecological balance but also for safeguarding genetic resources that could enhance cacao cultivation globally. Future research could focus on exploring (genetic) cadmium tolerance dynamics among cacao plants and their associated microbial communities, as well as investigating how beneficial soil microorganisms can improve plant resilience and reduce cadmium accumulation.
Additionally, comprehensive biodiversity assessments will deepen our understanding of soil organisms and their functions. Utilising beneficial microbes—such as introducing specific microbial inoculants—could improve nutrient uptake, enhance disease resistance, and mitigate heavy metal accumulation in cacao plants for long-term sustainability of cacao production.
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