ABSTRACT Anoxic subsurface sediments contain communities of heterotrophic microorganisms that metabolize organic carbon at extraordinarily low rates. In order to assess the mechanisms by which subsurface microorganisms access detrital sedimentary organic matter, we measured kinetics of a range of extracellular peptidases in anoxic sediments of the White Oak River Estuary, NC. Nine distinct peptidase substrates were enzymatically hydrolyzed at all depths. Potential peptidase activities ( V max ) decreased with increasing sediment depth, although V max expressed on a per-cell basis was approximately the same at all depths. Half-saturation constants ( K m ) decreased with depth, indicating peptidases that functioned more efficiently at low substrate concentrations. Potential activities of extracellular peptidases acting on molecules that are enriched in degraded organic matter ( d -phenylalanine and l -ornithine) increased relative to enzymes that act on l -phenylalanine, further suggesting microbial community adaptation to access degraded organic matter. Nineteen classes of predicted, exported peptidases were identified in genomic data from the same site, of which genes for class C25 (gingipain-like) peptidases represented more than 40% at each depth. Methionine aminopeptidases, zinc carboxypeptidases, and class S24-like peptidases, which are involved in single-stranded-DNA repair, were also abundant. These results suggest a subsurface heterotrophic microbial community that primarily accesses low-quality detrital organic matter via a diverse suite of well-adapted extracellular enzymes. IMPORTANCE Burial of organic carbon in marine and estuarine sediments represents a long-term sink for atmospheric carbon dioxide. Globally, ∼40% of organic carbon burial occurs in anoxic estuaries and deltaic systems. However, the ultimate controls on the amount of organic matter that is buried in sediments, versus oxidized into CO 2 , are poorly constrained. In this study, we used a combination of enzyme assays and metagenomic analysis to identify how subsurface microbial communities catalyze the first step of proteinaceous organic carbon degradation. Our results show that microbial communities in deeper sediments are adapted to access molecules characteristic of degraded organic matter, suggesting that those heterotrophs are adapted to life in the subsurface.
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This content will become publicly available on August 18, 2024
Complex organic matter degradation by secondary consumers in chemolithoautotrophy-based subsurface geothermal ecosystems
Microbial communities in terrestrial geothermal systems often contain chemolithoautotrophs with well-characterized distributions and metabolic capabilities. However, the extent to which organic matter produced by these chemolithoautotrophs supports heterotrophs remains largely unknown. Here we compared the abundance and activity of peptidases and carbohydrate active enzymes (CAZymes) that are predicted to be extracellular identified in metagenomic assemblies from 63 springs in the Central American and the Andean convergent margin (Argentinian backarc of the Central Volcanic Zone), as well as the plume-influenced spreading center in Iceland. All assemblies contain two orders of magnitude more peptidases than CAZymes, suggesting that the microorganisms more often use proteins for their carbon and/or nitrogen acquisition instead of complex sugars. The CAZy families in highest abundance are GH23 and CBM50, and the most abundant peptidase families are M23 and C26, all four of which degrade peptidoglycan found in bacterial cells. This implies that the heterotrophic community relies on autochthonous dead cell biomass, rather than allochthonous plant matter, for organic material. Enzymes involved in the degradation of cyanobacterial- and algal-derived compounds are in lower abundance at every site, with volcanic sites having more enzymes degrading cyanobacterial compounds and non-volcanic sites having more enzymes degrading algal compounds. Activity assays showed that many of these enzyme classes are active in these samples. High temperature sites (> 80°C) had similar extracellular carbon-degrading enzymes regardless of their province, suggesting a less well-developed population of secondary consumers at these sites, possibly connected with the limited extent of the subsurface biosphere in these high temperature sites. We conclude that in < 80°C springs, chemolithoautotrophic production supports heterotrophs capable of degrading a wide range of organic compounds that do not vary by geological province, even though the taxonomic and respiratory repertoire of chemolithoautotrophs and heterotrophs differ greatly across these regions.
more » « less- Award ID(s):
- 2121637
- NSF-PAR ID:
- 10484769
- Editor(s):
- Yin, Yanbin
- Publisher / Repository:
- PLOS One
- Date Published:
- Journal Name:
- PLOS ONE
- Volume:
- 18
- Issue:
- 8
- ISSN:
- 1932-6203
- Page Range / eLocation ID:
- e0281277
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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We grew wetland plants under ambient and nutrient‐enriched conditions to generate plant litter of differing nutrient content. In two experiments, we investigated: (1) the influence of algal photosynthesis on fungal and bacterial production and the activities of four extracellular enzymes throughout a 54‐day period of microbial colonisation and growth; and (2) the influence of litter stoichiometry on these relationships.
Ambient and nutrient‐enriched standing‐dead plant litter was collected and then submerged in wetland pools to allow for natural microbial colonisation and growth. Litter samples were periodically retrieved and transported to the laboratory for experiments manipulating photosynthesis using the photosystem II inhibitor DCMU (which effectively prevents algal photosynthetic activity). Algal (14C‐bicarbonate), bacterial (3H‐leucine), and fungal (14C‐acetate) production, and β‐glucosidase, β‐xylosidase, leucine aminopeptidase, and phosphatase activities (MUF‐ or AMC‐labelled fluorogenic substrates) were measured under conditions of active and inhibited algal photosynthesis.
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Microbial communities on high‐nutrient litter occasionally showed increased extracellular enzyme activity, fungal growth rates, and bacterial production compared to communities on non‐enriched litter, but algal and fungal production were not affected. Litter stoichiometry had no effects on fungal, bacterial, or enzymatic responses to photosynthesis, but the mean enzyme vector analysis angle (a measure of P‐ versus N‐acquiring enzyme activity) was positively correlated to litter N:P, suggesting that elevated litter N:P led to an increase in the relative activity of P‐acquiring enzymes.
These results supported the hypothesis that algal photosynthesis strongly influences heterotrophic microbial activity on macrophyte leaf litter, especially that of fungi, throughout microbial community development. However, the strength of this photosynthetic stimulation does not generally depend on small differences in litter nutrient content.
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Abstract As tall shrubs increase in extent and abundance in response to a changing climate, they have the potential to substantially alter Arctic and boreal ecosystem nutrient cycling and carbon (C) balance. Siberian alder (
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