Sources of organic nitrogen at the serpentinite-hosted Lost City hydrothermal field

Our plain-English summary of Sources of organic nitrogen at the serpentinite-hosted Lost City hydrothermal field, by S.Q. Lang, G.L. Früh-Green, S.M. Bernasconi, et al (Geobiology, 2013).

The study

• Authors: S.Q. Lang, G.L. Früh-Green, S.M. Bernasconi, D.A. Butterfield
• Journal: Geobiology
• Published: 2013;11(2):154-69
• Cited by: 16 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

The reaction of ultramafic rocks with water during serpentinization at moderate temperatures results in alkaline fluids with high concentrations of reduced chemical compounds such as hydrogen and methane. Such environments provide unique habitats for microbial communities capable of utilizing these reduced compounds in present-day and, possibly, early Earth environments. However, these systems present challenges to microbial communities as well, particularly due to high fluid pH and possibly the availability of essential nutrients such as nitrogen. Here we investigate the source and cycling of organic nitrogen at an oceanic serpentinizing environment, the Lost City hydrothermal field (30°N, Mid-Atlantic Ridge). Total hydrolizable amino acid (THAA) concentrations in the fluids range from 736 to 2300 nm and constitute a large fraction of the dissolved organic carbon (2.5-15.1%). The amino acid distributions, and the relative concentrations of these compounds across the hydrothermal field, indicate they most likely derived from chemolithoautotrophic production. Previous studies have identified the presence of numerous nitrogen fixation genes in the fluids and the chimneys. Organic nitrogen in actively venting chimneys has δ(15) N values as low as 0.1‰ which is compatible with biological nitrogen fixation. Total hydrolizable amino acids in the chimneys are enriched in (13) C by 2-7‰ compared to bulk organic matter. The distribution and absolute δ(13) C(THAA) values are compatible with a chemolithoautotrophic source, an attribution also supported by molar organic C/N ratios in most active chimneys (4.1-5.5) which are similar to those expected for microbial communities. In total, these data indicate nitrogen is readily available to microbial communities at Lost City.

© 2013 Blackwell Publishing Ltd.

In plain English

This article is not about asbestos or human health. It was a geochemistry study of the Lost City hydrothermal field, a serpentinite-hosted seafloor vent area on the Mid-Atlantic Ridge where water reacts with underlying ultramafic rocks. The authors analyzed fluids collected in 2003 from seven venting locations, water from the plume above the field, local sea water, and carbonate chimney samples collected in 2003 and 2005. Vent fluids had total hydrolyzable amino acid concentrations of 736 to 2300 nM, higher than the plume sample (629 nM) and local sea water (338 nM). Across different vents, amino acid levels did not significantly track magnesium. They were higher in fluids low in hydrogen (r2 = 0.66, P = 0.002). Most actively venting chimney samples had organic carbon-to-nitrogen ratios of 4.1 to 5.5. One young, hot vent structure was the exception, at 12.9. The authors concluded that the amino acids most likely came from microbes and that microbes there are not nitrogen limited. They also said that a non-biological origin for some amino acids, and nitrogen sources other than nitrogen fixation, could not be excluded.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Field sampling of seafloor hydrothermal fluids and carbonate chimneys, followed by laboratory geochemical analysis (amino acid concentrations, carbon and nitrogen content, stable isotopes). There were no human or animal subjects.
• Who was studied: Hydrothermal fluids from seven venting locations at the Lost City hydrothermal field (30°N, Mid-Atlantic Ridge), collected by the submersible Alvin in 2003, plus one plume sample and one local seawater sample. Carbonate chimney, inactive deposit and fissure samples were collected on cruises in 2003 and 2005.
• Exposure: No asbestos, fiber or human exposure was studied. The subject was organic nitrogen (amino acids) in vent fluids and carbonate chimneys at a serpentinite-hosted hydrothermal field.
• Main results:
  • The local sea water had a total hydrolyzable amino acid concentration of 338 nM and the plume sample 629 nM. The hydrothermal fluids were higher, at 736 to 2300 nM.
  • For samples collected from different vents across the field, amino acid concentrations were not significantly correlated with magnesium (r2 = 0.00; P = 0.86) or with dissolved organic carbon. They were highest in fluids with low hydrogen concentrations (r2 = 0.66, P = 0.002).
  • In the hydrothermal fluid samples, the amino acid mix was dominated by glycine (19 to 26 mol%), alanine (10 to 14 mol%), glx (10 to 16 mol%) and serine (2 to 16 mol%).
  • Non-protein amino acids, an indicator of degradation, made up 1 to 7% in the hydrothermal fluids. This compared with 12% in local sea water and 9% in the plume sample.
  • Most actively venting chimney samples had molar organic carbon-to-nitrogen ratios of 4.1 to 5.5 (average 5.0, n = 9). The exception was a 2-year-old Beehive structure with the hottest fluid flow (91 °C), which had a ratio of 12.9.
  • In all chimney samples analyzed, the amino acids were more enriched in carbon-13 than the bulk acid-insoluble organic carbon.
• The authors' conclusion: The authors concluded that the amino acids were presumably produced by chemolithoautotrophic microbes, that chimney organic matter is consistent with a mainly microbial origin, and that microbial communities at Lost City are not nitrogen limited.
• Limitations the authors state:
  • Low nitrogen isotope values in chimney biomass were consistent with biological nitrogen fixation, but the authors said other nitrogen sources could not be excluded.
  • The authors could not exclude that some of the amino acids were produced non-biologically in the subsurface.
  • The samples from any single vent did not span enough seawater mixing to show whether amino acids follow magnesium at that vent.
  • Some chimney samples classified as inactive may still have had undetected diffuse fluid flow.
  • For two samples, acid treatment shifted the nitrogen isotope values. The authors could not determine whether this was due to acidification losses, inorganic nitrogen, or sample heterogeneity.

Topics

The National Library of Medicine indexes this study under subjects our site covers: Chrysotile.

Medical terms in this study, explained in our Lexicon: Serpentinite, Ultramafic Rocks, Serpentinization, Attribution.

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