Environmental and microbial factors shape dissolved organic matter across multiple ecosystems – Nature

Executive Summary

This report details an investigation into the composition of Dissolved Organic Matter (DOM), a critical component of the global carbon cycle, across a continuum of four major aquatic ecosystems: glaciers, mountain rivers, coastal zones, and the open ocean. The study’s findings are crucial for understanding carbon dynamics in the context of climate change and for advancing several Sustainable Development Goals (SDGs), particularly SDG 13 (Climate Action), SDG 14 (Life Below Water), and SDG 6 (Clean Water and Sanitation). Using ultrahigh-resolution mass spectrometry, the analysis revealed a dual mechanism governing DOM composition. A trend toward increasing homogenization and recalcitrance was observed along the aquatic continuum, driven primarily by physicochemical processes and terrestrial inputs. This “universal” DOM, common to all ecosystems, increased from 65% in glaciers to 97% in the open ocean. Conversely, unique, ecosystem-specific “non-universal” DOM declined significantly, shaped by biological transformations from microbial communities. These findings underscore the distinct roles of physical and biological drivers in carbon turnover and provide a molecular-level basis for predicting how aquatic ecosystems will respond to environmental changes, thereby informing strategies for sustainable management.

Introduction: DOM, Carbon Cycling, and Sustainable Development

The Role of Dissolved Organic Matter in Global Ecosystems

Dissolved Organic Matter (DOM) represents one of the largest active carbon pools in aquatic systems, playing a fundamental role in the global carbon cycle. It functions as a primary energy source for microbial communities and facilitates the transfer of carbon from terrestrial to marine environments. The molecular composition of DOM provides critical insights into biogeochemical processes. As DOM travels from land to sea, it undergoes continuous transformation, linking local ecosystem dynamics to the global climate system. A comprehensive understanding of the drivers shaping DOM composition is essential for predicting the future of carbon cycling and its impact on planetary health.

Aligning DOM Research with Sustainable Development Goals (SDGs)

This research directly informs several key United Nations Sustainable Development Goals by clarifying the mechanisms of carbon transport and transformation in aquatic environments.

• SDG 13 (Climate Action): By elucidating the fate of organic carbon—whether it is respired as CO2 or sequestered in refractory forms—this study contributes to more accurate global carbon budgets and climate models.
• SDG 14 (Life Below Water): The composition of DOM directly affects the health and productivity of marine ecosystems by controlling nutrient availability and the structure of microbial food webs. Understanding these dynamics is vital for protecting marine biodiversity.
• SDG 6 (Clean Water and Sanitation): DOM from terrestrial sources, including agricultural runoff and wastewater, influences the quality of freshwater and coastal systems. This research helps identify the sources and transformations of these inputs.
• SDG 15 (Life on Land): The study highlights the profound impact of terrestrial ecosystems on aquatic carbon pools, reinforcing the need for integrated land and water management strategies.

Key Findings on DOM Transformation Across Aquatic Ecosystems

Ecosystem-Specific DOM Composition and Dynamics

The molecular composition of DOM varied significantly across the four ecosystems studied. A clear trend was observed along the glacier-to-ocean continuum, characterized by a progressive shift in chemical properties.

• Molecular Richness: The number of unique DOM formulae declined from glaciers (18,110) to the open ocean (5,925), indicating a filtering or selective removal process.
• Chemical Properties: Weighted averages of oxygen-to-carbon ratios (O/C_wa), aromaticity (AI_wa), and double bond equivalents (DBE_wa) increased from glaciers to rivers before declining in marine environments, reflecting the addition and subsequent transformation of terrestrial organic matter.
• Compound Classes: The relative intensity of bioavailable compounds, such as lipid-like molecules, decreased sharply from 43% in glacial DOM to just 0.5% in the open ocean. This demonstrates the preferential consumption of labile organic matter during transit.

Universal DOM: Homogenization Driven by Terrestrial Inputs

A core pool of 4,042 DOM molecules was identified in all four ecosystems, termed “universal DOM.” This pool represents a stable fraction of organic matter that persists across diverse environmental conditions. Its dynamics suggest a process of chemical convergence driven by non-biological factors.

1. Increasing Dominance: The relative intensity of the universal DOM pool increased significantly along the gradient, from 65 ± 20% in glaciers to 97 ± 0.7% in the open ocean. This indicates a selective preservation and accumulation of these molecules.
2. Terrestrial Origin: This pool was dominated by lignin-like compounds (averaging 77%), which are biopolymers derived from vascular plants on land. Their proportion rose from 38% in glaciers to 91% in the open ocean.
3. Physicochemical Drivers: The composition of universal DOM was strongly correlated with environmental parameters reflecting terrestrial inputs, such as mineral dust ions (Ca²⁺, Mg²⁺) in glaciers and silicate weathering products in rivers. In marine systems, its distribution was governed by water mass properties like temperature and salinity, which trace the transport of terrestrial material.

Non-Universal DOM: Uniqueness Driven by Microbial Communities

In contrast to the universal pool, the remaining 18,903 molecules, or “non-universal DOM,” represent the unique and dynamic component of each ecosystem’s carbon pool. This fraction is primarily shaped by biological activity.

1. Decreasing Proportion: The relative intensity of non-universal DOM showed a consistent decline, from 82 ± 31% in glaciers to only 3 ± 0.7% in the open ocean. This reflects the rapid turnover of labile, ecosystem-specific molecules.
2. Microbial Control: Statistical modeling revealed that microbial community composition was the primary driver of non-universal DOM variation, particularly in the biologically active, nutrient-limited environments of glaciers and the open ocean. In these systems, microbes explained 70% and 55.7% of the variation, respectively.
3. Functional Differentiation: In glaciers, microbes like Polaromonas and Rhodoferax targeted a diverse range of bioavailable compounds (lipids, proteins). In the open ocean, microbial communities sustained by primary production from cyanobacteria transformed labile exudates, ultimately converging toward more refractory molecular profiles.

Implications for Sustainable Development Goals

SDG 13 (Climate Action): Carbon Cycling and Climate Change

The findings provide a mechanistic understanding of how carbon is sequestered in aquatic systems. The transport and transformation of DOM result in the selective preservation of refractory, terrestrial-derived carbon (universal DOM), which contributes to long-term carbon storage in the ocean. As climate change accelerates glacial melting and alters precipitation patterns, the flux of terrestrial DOM into aquatic systems will change, impacting the global carbon cycle. Incorporating the dual mechanisms of DOM processing into climate models is essential for accurate predictions under SDG 13.

SDG 14 (Life Below Water) and SDG 6 (Clean Water): Aquatic Ecosystem Health

The health of aquatic ecosystems is intrinsically linked to DOM quality. This study shows a clear gradient from highly bioavailable DOM in glaciers to highly refractory DOM in the open ocean. This shift directly controls the metabolic activity of microbial communities, which form the base of aquatic food webs.

• For SDG 14, understanding this transformation is critical for assessing how marine ecosystems will respond to changes in terrestrial inputs, which can alter nutrient cycles and primary productivity.
• For SDG 6, the research highlights how terrestrial runoff, including pollutants that associate with DOM, is processed in rivers and coastal zones. This knowledge can inform better water quality management and protect vital freshwater and estuarine resources.

SDG 15 (Life on Land): The Land-Ocean Connection

The dominance of terrestrial signals in the universal DOM pool across all ecosystems underscores the profound connection between land and water, a central theme of SDG 15. The chemical signature of rivers and oceans is fundamentally shaped by processes occurring on land, such as soil erosion, vegetation cover, and anthropogenic activities. Sustainable land management practices are therefore not only crucial for terrestrial ecosystems but are also a prerequisite for maintaining the health and biogeochemical balance of downstream aquatic environments.

Conclusion and Recommendations

This report concludes that the composition of Dissolved Organic Matter along the land-to-ocean aquatic continuum is governed by two distinct but interacting mechanisms. Physicochemical processes and terrestrial inputs drive the homogenization of DOM, leading to the accumulation of a persistent, universal pool of refractory carbon. Simultaneously, microbial transformations create unique, non-universal DOM pools that reflect the specific biological activity of each ecosystem. This dual-control framework provides critical insights for predicting carbon cycling dynamics in a changing world.

To advance global sustainability targets, it is recommended that:

1. Ecosystem models should be updated to differentiate between the drivers of universal (physicochemical) and non-universal (biological) DOM to improve predictions of carbon sequestration and ecosystem responses to climate change.
2. Water management strategies under SDG 6 and SDG 14 should adopt an integrated approach that considers the impact of land use (SDG 15) on the quality and composition of DOM entering aquatic systems.
3. Future research should focus on quantifying the rates of these transformation processes to better constrain the role of aquatic ecosystems in the global carbon budget and support efforts to achieve climate neutrality under SDG 13.

Analysis of Sustainable Development Goals (SDGs) in the Article

1. Which SDGs are addressed or connected to the issues highlighted in the article?

The article on Dissolved Organic Matter (DOM) and carbon cycling across aquatic ecosystems connects to several Sustainable Development Goals (SDGs) by providing critical scientific understanding of environmental processes that underpin them.

• SDG 6: Clean Water and Sanitation

  The research investigates the composition of DOM in freshwater ecosystems, including glaciers, which are described as “solid reservoirs of freshwater,” and mountain rivers. By analyzing the sources and transformations of organic matter in these water bodies, the study provides foundational knowledge relevant to protecting and managing water quality and the health of water-related ecosystems.

• SDG 13: Climate Action

  This is a central theme of the article. The abstract explicitly states that DOM “plays a critical role in global carbon cycling” and underscores its “pivotal role in carbon cycling under climate change.” The entire study is framed around understanding the mechanisms of carbon transfer and storage in aquatic systems, which is fundamental knowledge for modeling climate change impacts and developing mitigation strategies.

• SDG 14: Life Below Water

  The study extends its analysis to coastal zones and the open ocean, examining the composition and drivers of marine DOM. It discusses how “coastal DOM is influenced by riverine inputs and human activities” and analyzes the unique microbial communities and metabolic processes in the ocean. This research contributes to understanding the biogeochemical health of marine ecosystems and the impact of land-based pollution on them.

• SDG 15: Life on Land

  The article establishes a direct link between terrestrial and aquatic ecosystems. It identifies that “terrestrial inputs” are a primary driver of DOM composition, noting that riverine DOM is “primarily derived from vascular plants, soil leachates and sewage.” This highlights how the health and management of terrestrial ecosystems, including soils and forests, directly impact the carbon cycle and the condition of downstream aquatic environments.

2. What specific targets under those SDGs can be identified based on the article’s content?

Based on the article’s focus, the following specific SDG targets can be identified:

1. Target 6.6: Protect and restore water-related ecosystems.

   The article provides a detailed molecular-level analysis of glaciers and mountain rivers, which are critical water-related ecosystems. By “tracking its changes along the aquatic continuum,” the study offers insights into the biogeochemical functioning of these systems, which is essential for developing strategies to protect and restore them from the impacts of climate change and pollution.

2. Target 13.3: Improve education, awareness-raising and human and institutional capacity on climate change mitigation, adaptation, impact reduction and early warning.

   As a scientific study, the article directly contributes to the body of knowledge required to achieve this target. It “unveils key mechanisms driving dissolved organic matter turnover across ecosystems,” which is crucial for “predicting future carbon cycling.” This type of fundamental research enhances the scientific capacity to understand and address climate change.

3. Target 14.1: By 2025, prevent and significantly reduce marine pollution of all kinds, in particular from land-based activities, including… nutrient pollution.

   The research identifies that DOM in rivers is derived from sources like “sewage” and that “coastal DOM is influenced by riverine inputs and human activities.” It also measures nutrient concentrations such as “PO₄³⁻ and NO₃⁻” in coastal areas. This directly addresses the need to understand the pathways and impacts of land-based pollution on marine environments.

4. Target 15.1: Ensure the conservation, restoration and sustainable use of terrestrial and inland freshwater ecosystems and their services…

   The study’s investigation of the “aquatic continuum” from terrestrial sources (glaciers, rivers) to the ocean directly relates to this target. It emphasizes how “terrestrial inputs” from “plant and soil sources” shape the entire continuum, reinforcing the interconnectedness of land and freshwater ecosystems and the need for integrated management.

3. Are there any indicators mentioned or implied in the article that can be used to measure progress towards the identified targets?

The article does not explicitly mention official SDG indicators, but it uses and implies several scientific metrics that can serve as powerful proxy indicators for assessing ecosystem health and pollution levels.

• Indicators for Water Quality and Ecosystem Health (Targets 6.6, 14.2, 15.1):
  • Dissolved Organic Carbon (DOC) concentrations: The article measures DOC levels across all ecosystems, which is a fundamental indicator of water quality and carbon content.
  • Molecular Composition of DOM: The study uses advanced metrics from FT-ICR MS analysis, such as H/C and O/C ratios, Aromaticity Index (AI), and Double Bond Equivalents (DBE). Changes in these values, as shown when the article states “O/C_wa, AI_wa and DBE_wa increased from glaciers to mountain rivers,” can indicate shifts in the source and lability of organic matter, reflecting ecosystem health.
  • Microbial Community Composition: The article uses 16S rRNA gene sequencing to characterize microbial communities. It notes that “Microbial communities differed significantly across ecosystems,” implying that the composition and diversity of these communities can be an indicator of environmental conditions and ecosystem function.
• Indicators for Nutrient Pollution (Target 14.1):
  • Nutrient Concentrations: The article directly measures concentrations of “PO₄³⁻ and NO₃⁻” in coastal waters. These are key components of the official SDG indicator for coastal eutrophication and serve as direct measures of nutrient pollution from land-based sources.
• Indicators for Carbon Cycle Monitoring (Target 13.3):
  • Relative Intensity of DOM Pools: The study quantifies the proportion of “universal” and “non-universal” DOM. The finding that the “relative intensity of the universal DOM pool exhibited a significant increase along the glacier-to-ocean gradient” provides a novel metric for tracking the processing and stabilization of carbon as it moves through aquatic systems.

4. Summary Table of SDGs, Targets, and Indicators

Source: nature.com