The association between Sphagnum peat mosses and cyanobacteria represents a fascinating symbiotic relationship. This symbiosis, occurring both as epiphytes and endophytes, is highly influenced by pH levels and plays a vital role in nitrogen fixation. However, the specific molecules exchanged and the underlying physiological processes involved remain largely elusive. Sphagnum-dominated peatlands are crucial for carbon and nitrogen cycling, yet the mechanisms governing these symbiotic interactions remain poorly understood.
This study aims to characterize the symbiotic relationship between Sphagnum moss and Nostoc cyanobacteria, with a specific focus on pH-dependent factors and the unique physiology and metabolic exchange that distinguish this symbiosis from other plant-cyanobacteria associations. The investigation utilizes a multi-omics approach, including metatranscriptomics, metabolomics, and spatial metabolomics, to gain comprehensive insights into the gene expression profiles and metabolic interactions occurring between these two organisms.
By elucidating the gene expression patterns and metabolic exchanges in the Sphagnum moss-Nostoc cyanobacteria symbiosis, this research provides valuable knowledge for understanding the intricate dynamics of this ecological association and its significance in peatland ecosystems.
Case. Characterization of the Symbiotic Relationship between Sphagnum Moss and Nostoc Cyanobacteria Using Plant Metabolomics, Spatial Metabolomics, and Metatranscriptomics. (1)
Study Materials
Cell and Culture Medium
Technical Approach
- The influence of different pH conditions on the Sphagnum moss-Nostoc cyanobacteria symbiotic relationship
- Metabolic cross-feeding study
- Spatial metabolomics research
- Metatranscriptomics research
Research Findings
1. The influence of different pH conditions on the symbiotic relationship between Sphagnum moss and Nostoc cyanobacteria
The authors conducted a study to investigate the effects of different pH conditions on the symbiotic relationship between Sphagnum moss (S. angustifolium) and Nostoc cyanobacteria (N. muscorum UTEX 1037). After preliminary research, the strain N. muscorum UTEX 1037, which exhibited symbiotic capability, was selected for further investigation. Sterile cultures of S. angustifolium and N. muscorum UTEX 1037 were cultivated at pH 3.5, 5.5, and 8.5.
The symbiotic growth of S. angustifolium and N. muscorum UTEX 1037 (cyanobacteria/moss) was evaluated by comparing their growth with individual growth and the sum of individual growth (cyanobacteria + moss). The results showed that symbiosis had significant benefits at low pH (3.5): the change in weight when the species grew together exceeded the sum of individual growth. However, no noticeable growth benefits were observed at pH 5.5 or 8.5.
2. Metabolic cross-feeding study
The observed growth benefits of S. angustifolium and N. muscorum UTEX 1037 under low pH conditions indicate a mutualistic symbiotic relationship, prompting the authors to characterize the metabolic exchanges mediating this interaction. Therefore, the metabolites released by each partner of the symbiotic consortium during growth in fresh liquid medium were evaluated, as well as the changes in the abundance of exometabolites released into the spent medium when cross-fed to the other partner.
The results revealed the identification of 225 exometabolites from the spent medium of individual partners, with N. muscorum UTEX 1037 consuming 53% of the exometabolites released by S. angustifolium, while S. angustifolium consumed 28% of the exometabolites released by N. muscorum UTEX 1037.
To further characterize the metabolite exchange, exometabolites that exhibited significant changes in abundance (p < 0.05) before and after cross-feeding were categorized into six classes: amino acids, carbohydrates, fatty acids and their derivatives, lipids, nucleotides and nucleosides, and organic acids. Within each chemical class, N. muscorum UTEX 1037 consumed a higher percentage of S. angustifolium exometabolites, such as 75% of amino acids, 77% of carbohydrates, and 50% of fatty acids. On the other hand, S. angustifolium consumed 30% of amino acids, 54% of carbohydrates, and 76% of nucleosides from N. muscorum UTEX 1037 exometabolites.
3. Spatial metabolomics research
The authors further investigated the results of the cross-feeding study using matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI). The results revealed that N. muscorum UTEX 1037 and S. angustifolium produced a small amount of xanthosine nucleoside when grown individually. However, when grown in close proximity to each other, N. muscorum UTEX 1037 increased the production of xanthosine nucleoside, while S. angustifolium did not. Cross-feeding experiments demonstrated that S. angustifolium consumed the xanthosine nucleoside provided by N. muscorum UTEX 1037, while N. muscorum UTEX 1037 did not consume the xanthosine nucleoside provided by S. angustifolium. Similar results were observed for adenine nucleoside.
MALDI-MSI imaging revealed that S. angustifolium produced sulfated choline esters regardless of the presence of N. muscorum UTEX 1037. The cross-feeding results showed that 98% of the sulfated choline esters provided by S. angustifolium were consumed by N. muscorum UTEX 1037.
Cross-feeding analysis revealed a pronounced trend of S. angustifolium in producing trehalose, with N. muscorum UTEX 1037 consuming up to 85% of the exometabolites. MALDI-MSI results showed similar ion abundances of trehalose in isolated S. angustifolium and S. angustifolium grown in proximity to N. muscorum UTEX 1037. However, when co-cultivated with S. angustifolium, a significant increase in trehalose abundance was observed in N. muscorum UTEX 1037.
4. Metatranscriptomics study
The transcriptomic analysis of N. muscorum UTEX 1037 revealed a downregulation of genes related to nitrogen fixation, except for glutamine synthetase. When supplemented with exometabolites from S. angustifolium, the expression of genes involved in photosynthesis, specifically in photosystems I and II, was generally reduced in N. muscorum UTEX 1037, despite an increasing trend in the expression of photosystem I genes when co-cultivated with S. angustifolium. Enrichment analysis identified the downregulation of genes involved in S. angustifolium host defense when co-cultivated with N. muscorum UTEX 1037. Furthermore, when S. angustifolium fed on exometabolites from N. muscorum UTEX 1037, the plant defense-related gene phenylalanine ammonia-lyase was induced. However, no significant change in gene expression was observed when co-cultivated with N. muscorum UTEX 1037.
Symbiosis between Sphagnum moss and Nostoc cyanobacteria is pH-driven, and the symbiotic relationship occurs only under low pH conditions. Metatranscriptomic analysis revealed that cross-feeding and mass spectrometry imaging confirmed trehalose as the primary carbohydrate released by Sphagnum moss, which is consumed by Nostoc cyanobacteria along with taurine and sulfonic acid. In exchange, Nostoc cyanobacteria increased the secretion of purines and amino acids. Transcriptomic analysis demonstrated that host defense capacity in Sphagnum moss was downregulated when in symbiosis with Nostoc cyanobacteria, suggesting a modulation of host defense beyond chemical contact alone.
Reference
- Carrell, Alyssa A., et al. "Novel metabolic interactions and environmental conditions mediate the boreal peatmoss-cyanobacteria mutualism." The ISME journal 16.4 (2022): 1074-1085.
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