About

Nicolas Cassar is the Lee Hill Snowdon Professor of Biogeochemistry and a faculty member in the Earth and Climate Sciences at the Nicholas School of the Environment at Duke University. His research focuses on environmental biogeochemistry and physiology, with the aim of constraining the mechanisms governing carbon cycling and climate. His current research interests include ocean carbon cycles and productivity, carbon acquisition mechanisms in marine phytoplankton and their implications for climate change and paleo-CO2 reconstruction, as well as global carbon cycle and ocean-atmosphere fluxes. Cassar's scientific approach is interdisciplinary, integrating field observations, laboratory experiments, modeling, and theory to address these complex environmental processes.

Research topics

• Oceanography
• Ecology
• Environmental science
• Geology
• Geography
• Biology
• Mathematics
• Physics
• Meteorology
• Animal science

Selected publications

• Comment on egusphere-2025-6336

  2026-02-05

  peer-reviewOpen access

  <strong class="journal-contentHeaderColor">Abstract.</strong> Nickel (Ni) is an essential micronutrient for marine microorganisms, being involved in enzymes controlling the nitrogen cycle and metabolic responses to oxidative stress. In this study, we examine the covariation between the abundance of Ni-related enzymes and Ni isotope fractionation. To do so, dissolved Ni concentrations and isotope compositions are presented together with metagenomics on samples from the Antarctic Circumnavigation Expedition. Overall, results reveal lower Ni concentrations and higher &delta;⁶⁰Ni values in surface waters north of the Sub-Antarctic Front compared to southerly stations. One exception is seen near the high-latitude Mertz Glacier, where the systematics between Ni and &delta;⁶⁰Ni better resemble those of low-latitude stations. Relative abundances of urease and Ni-SOD in metagenomes are found to correlate with &delta;⁶⁰Ni, potentially suggesting preferential biological uptake of Ni by the organisms using these enzymes. We find a particularly high abundance of urease in diatoms and alphaproteobacteria near the Mertz Glacier, matching the surprisingly high &delta;⁶⁰Ni. We thus hypothesise that urea could serve as a nitrogen source for microbial organisms in the late stage of polynya diatom blooms, perhaps causing the observed Ni drawdown and isotope fractionation. This study represents an initial exploration of the influence of biological processes on Ni and &delta;⁶⁰Ni distributions. It constitutes a first step towards the further analyses (<em>e.g</em>., culture experiments and metatranscriptomics) needed to determine which exact processes lead to the &delta;⁶⁰Ni biogeochemical divide observed between low-latitude and high-latitude waters.

  PublisherDOI

• Decoding Surface Ocean Net Community Production: Primary Production as the Dominant Predictor in the Surface Ocean

  2026-04-13

  articleOpen accessSenior author

  Deciphering the balance between primary production and respiration is critical to understanding ocean metabolism and developing an accurate global carbon budget.However, predicting this balance across large spatiotemporal scales remains challenging because the biogeochemical and ecological drivers of net community production (NCP) vary widely among ocean regions.To help address this gap, the NASA EXport Processes in the Ocean from RemoTe Sensing program carried out campaigns in the subarctic North Pacific and the North Atlantic, two contrasting ecosystems spanning low-biomass, low-productivity to high-biomass, high-productivity conditions.Here, we identify representative metrics to predict NCP within contrasting ocean basins.We compared biogeochemical rates (NCP, primary production, microbial respiration), hydrographic parameters (sea surface temperature), particulate organic carbon properties (concentration, particle size), and phytoplankton characteristics (chlorophyll a, photosynthetic efficiency) within the surface ocean.Our study found that NCP was highly temporally variable, particularly in the North Atlantic which had a higher maximum rate (36.2 mmol C m -2 d -1 ) within the mixed layer, whereas the North Pacific which had a higher mean rate within the mixed layer (12.3 mmol C m -2 d -1 ) due to consistently lower metabolic rates through time.Despite substantially different characteristics per region, net primary production (NPP) and POC concentrations were stronger indicators of temporal trends in NCP than microbial respiration, chlorophyll concentrations, or temperature, supporting the use of NCP reconstructions from satellite-derived NPP and POC.The consistency of this finding between contrasting regions suggests that this relationship should be further investigated to test whether it will hold throughout the global ocean.

  PublisherOA PDFDOI

• Response of biological nitrogen fixation in sub-Arctic Peltigera lichens to environmental perturbations

  Zenodo (CERN European Organization for Nuclear Research) · 2026-02-24

  datasetOpen accessSenior author
  PublisherDOI

• Supplementary material to "Influence of Ni-related enzymes on the Ni cycle in the Southern Ocean: insights from isotopes and metagenomics"

  2026-01-19

  articleOpen access

  Supplementary information about nitrogen fixation 1) Estimating the potential for nitrogen fixationAnvi'o v7.1 was used to estimate the metabolic potential for nitrogen fixation in euphotic layer samples where significant Ni isotope fractionation was detected (stations 8, 11 and 25), using the commands and parameters described in https://anvio.org/blog/targeted-binning/.This approach relies on anvi-estimate-metabolism to estimate the completeness of KEGG modules in contigs of individual metagenomic samples, working at operon-level.It allows to detect complete or partial nitrogen fixation operons instead of traditional single-gene nifH detection. 2) Nitrogen fixation and Ni fractionationAmong the 13 omic samples corresponding to high 60 , each with two size fractions except 25-15 m which has three), none showed a contig bearing more than 50 % of the genes from the M00175 KEGG module, corresponding to the catalytic part of the nif based nitrogen fixation.Nitrogen fixation potential could have been missed due to single assemblies not properly recovering contigs of nitrogen-fixing populations.Yet, similar metagenomic-based approaches have demonstrated their efficiency (Delmont et al. 2022), and recent data has shown evidence

  PublisherDOI

• Monitoring, reporting, and verification of marine carbon dioxide removal: Exploring scientific consensus and divergences across continents

  Elementa Science of the Anthropocene · 2026-01-01

  articleOpen access

  Marine Carbon Dioxide Removal (mCDR) approaches are increasingly considered for climate change mitigation as a supplement to rapid emissions reduction. However, it is still unclear if mCDR approaches could be effective, safe, and accountable. A critical requirement for mCDR to work, potentially, is robust monitoring, reporting, and verification (MRV) of greenhouse gas(es) removed through mCDR activities. MRV frameworks and protocols are currently developed in a scattered manner by individual stakeholders, so that the principles they are built upon may or may not appeal to the broader international community that is interested in the ocean commons. International agreement and consolidation on MRV for mCDR seem crucial to legitimize and validate mCDR, considering that the ocean is a globally interconnected fluid and that activities by some may affect many others. Here, we undertake a step toward consolidation of MRV by consulting the international scientific community. We established a global network of scientists organized into 6 “continental” nodes, each of which addressed the same set of MRV-related questions and whose thoughts were equally weighted in the synthesis. Our consultation shows that while there are many converging views on MRV (e.g., the importance of modeling for MRV), there are also differences in the regional MRV priorities (e.g., the importance of regional vs global models). The areas of consensus and divergence identified herein may be instrumental in the design of more widely accepted MRV frameworks, informed equally by scientists from 6 continents.

  PublisherDOI

• Comment on egusphere-2025-6336

  2026-03-17

  peer-reviewOpen access

  <strong class="journal-contentHeaderColor">Abstract.</strong> Nickel (Ni) is an essential micronutrient for marine microorganisms, being involved in enzymes controlling the nitrogen cycle and metabolic responses to oxidative stress. In this study, we examine the covariation between the abundance of Ni-related enzymes and Ni isotope fractionation. To do so, dissolved Ni concentrations and isotope compositions are presented together with metagenomics on samples from the Antarctic Circumnavigation Expedition. Overall, results reveal lower Ni concentrations and higher &delta;⁶⁰Ni values in surface waters north of the Sub-Antarctic Front compared to southerly stations. One exception is seen near the high-latitude Mertz Glacier, where the systematics between Ni and &delta;⁶⁰Ni better resemble those of low-latitude stations. Relative abundances of urease and Ni-SOD in metagenomes are found to correlate with &delta;⁶⁰Ni, potentially suggesting preferential biological uptake of Ni by the organisms using these enzymes. We find a particularly high abundance of urease in diatoms and alphaproteobacteria near the Mertz Glacier, matching the surprisingly high &delta;⁶⁰Ni. We thus hypothesise that urea could serve as a nitrogen source for microbial organisms in the late stage of polynya diatom blooms, perhaps causing the observed Ni drawdown and isotope fractionation. This study represents an initial exploration of the influence of biological processes on Ni and &delta;⁶⁰Ni distributions. It constitutes a first step towards the further analyses (<em>e.g</em>., culture experiments and metatranscriptomics) needed to determine which exact processes lead to the &delta;⁶⁰Ni biogeochemical divide observed between low-latitude and high-latitude waters.

  PublisherDOI

• Water mass specific genes dominate the Southern Ocean microbiome

  Nature Communications · 2026-03-09

  articleOpen access

  The Southern Ocean (SO) plays a key role in regulating global biogeochemical cycles and climate, yet microbial genes sustaining its biological activity remain poorly characterized. We introduce a microbial genes collection from 218 metagenomes sampled during the Antarctic Circumnavigation Expedition, the majority of which are missing from functional databases. 38% even lack homologs in current reference marine gene catalogs, defining a singular genetic seascape. We show that SO gene assemblages exhibit a common polar signature with the Arctic Ocean while being structured by water masses at the SO-scale. We analyze genomic markers of diverse SO biomes, focusing on dimethylsulphoniopropionate (DMSP) cleavage by polar-adapted bacteria, organic matter consumption in the blooming Mertz polynya and adaptation to polar conditions in the ubiquitous bacteria Pelagibacter. Our work takes a step towards a comprehensive understanding of SO's plankton ecology and evolution, capturing the current state of the unique microbial diversity in this rapidly changing Ocean.

  PublisherOA PDFDOI

• Response of biological nitrogen fixation in sub-Arctic Peltigera lichens to environmental perturbations

  Open MIND · 2026-02-24

  datasetSenior author
  DOI

• Influence of Ni-related enzymes on the Ni cycle in the Southern Ocean: insights from isotopes and metagenomics

  2026-01-19

  articleOpen access

  Abstract. Nickel (Ni) is an essential micronutrient for marine microorganisms, being involved in enzymes controlling the nitrogen cycle and metabolic responses to oxidative stress. In this study, we examine the covariation between the abundance of Ni-related enzymes and Ni isotope fractionation. To do so, dissolved Ni concentrations and isotope compositions are presented together with metagenomics on samples from the Antarctic Circumnavigation Expedition. Overall, results reveal lower Ni concentrations and higher δ60Ni values in surface waters north of the Sub-Antarctic Front compared to southerly stations. One exception is seen near the high-latitude Mertz Glacier, where the systematics between Ni and δ60Ni better resemble those of low-latitude stations. Relative abundances of urease and Ni-SOD in metagenomes are found to correlate with δ60Ni, potentially suggesting preferential biological uptake of Ni by the organisms using these enzymes. We find a particularly high abundance of urease in diatoms and alphaproteobacteria near the Mertz Glacier, matching the surprisingly high δ60Ni. We thus hypothesise that urea could serve as a nitrogen source for microbial organisms in the late stage of polynya diatom blooms, perhaps causing the observed Ni drawdown and isotope fractionation. This study represents an initial exploration of the influence of biological processes on Ni and δ60Ni distributions. It constitutes a first step towards the further analyses (e.g., culture experiments and metatranscriptomics) needed to determine which exact processes lead to the δ60Ni biogeochemical divide observed between low-latitude and high-latitude waters.

  PublisherOA PDFDOI

• Ecosystem technology (ecotech): Harnessing natural processes to address global challenges

  Science Advances · 2026-05-06

  articleOpen access

  Over the past 80 years, biotechnology has advanced agriculture, health care, and economic development by harnessing biological processes from the organism inward, i.e., from the organ system to the molecular scale. Today's global challenges, including biodiversity loss, climate change, and pollution, demand a complementary technological expansion inspired by processes operating from the organism outward, i.e., at the levels of populations, communities, ecosystems, and the biosphere. Here, we present the components of this technological expansion through ecosystem technology, or ecotech. We propose a framework for ecotech to integrate elements of ecology, engineering, and earth science and to function as a practical and conceptual convergence accelerator. Ecotech will advance critical environmental solutions by uniting independently evolving technologies, generating diverse fields of inquiry (e.g., ecomimicry, ecosystem materials science, ecosystem sensing and signaling), and inspiring innovation. To harness this innovation to improve nature restoration, carbon storage, water quality, ecosystem energy, and infrastructure resistance to disasters, ecotech is guided by cross-cutting actions to ensure scalability, equity, and accountability. When integrated into strategies across nongovernmental organizations, business, and governments, ecotech offers a pathway to advance climate adaptation, biodiversity recovery, and economic diversification and growth. By uniting ecology, engineering, and earth sciences at scale, ecotech transforms technology into a tool to confront humanity's most urgent existential needs and secure a livable future.

  PublisherDOI

Recent grants

• PHYSIOLOGICAL AND ECOSYSTEM STRUCTURE FORCINGS ON CARBON FLUXES IN THE SOUTHERN OCEAN MIXED LAYER

  NSF · $492k · 2011–2015

• CAREER: Method Development for High-Resolution Underway N2 Fixation Measurements

  NSF · $790k · 2014–2021

• EAGER: Method development for the assessment of spatiotemporal variability of non-symbiotic N2 fixation

  NSF · $248k · 2011–2013

• Biological and Physical Drivers of Oxygen Saturation and Net Community Production Variability along the Western Antarctic Peninsula

  NSF · $468k · 2017–2023

Frequent coauthors

• Yajuan Lin

  267 shared

• Hugo Berthelot

  Institut Universitaire Européen de la Mer

  224 shared

• Stéphane L’Helguen

  Institut de Recherche pour le Développement

  168 shared

• Jean‐François Maguer

  Laboratoire des Sciences de l'Environnement Marin

  168 shared

• Hélène Planquette

  Laboratoire des Sciences de l'Environnement Marin

  73 shared

• Weiyi Tang

  Princeton University

  71 shared

• Éric Gilson

  Gilson (France)

  62 shared

• Éric Douville

  62 shared

Awards & honors

• Lee Hill Snowdon Professor of Biogeochemistry (2025)