About

Christopher Martens is a Distinguished Professor in the Department of Earth, Marine and Environmental Sciences at the University of North Carolina at Chapel Hill. He holds a Ph.D. from Florida State University, obtained in 1972. His research focuses on aquatic biogeochemical processes, biological oceanography, and marine ecology. His work encompasses global and regional cycles, coastal resources, conservation, and the development of emerging tools for environmental research. Martens is actively involved in advancing understanding of Earth's chemical cycles, fluid processes, and ecosystem health, contributing significantly to the field through his research and academic leadership.

Research topics

• Biology
• Oceanography
• Ecology
• Geography
• Chemistry
• Environmental chemistry
• Meteorology
• Environmental science
• Geology

Selected publications

• Linking microbes to in situ methane oxidation rates in a eutrophic freshwater lake

  Frontiers in Microbiology · 2026-03-06

  articleOpen access

  Introduction Aerobic methanotrophs and non-methanotrophic methylotrophs drive methane cycling in oxic freshwater lakes. Most knowledge about biological aerobic methane oxidation (MOx) comes from ex situ rate experiments, laboratory cultures, and static measurements of natural abundances. Methods We investigated the link between MOx rate constants measured with a novel in situ incubation device and the microbial community in Jordan Lake, a methane-rich freshwater lake in NC, USA. We coupled relative abundances of 16S rRNA genes and quantitative PCR of particulate methane monooxygenase subunit A ( pmoA ) to methane, oxygen, temperature, and in situ MOx rate constants, all collected using the novel iBag in situ incubation system. Results In 16 incubations spread across 13 months, Methylococcaceae , whose cultured members are obligate aerobic methanotrophs, strongly and inversely correlate with naturally-varying oxygen but not with methane. Non-methanotrophic methylotrophs and facultative aerobic methanotrophs are more abundant (up to 15.4% of amplicons), but do not correlate with either dissolved gas. Methylococcaceae correlate better than all other families in the methane-oxidizing community with the first-order MOx rate constants obtained from the in situ incubation data. Changes in the methane-oxidizing community across incubations were inconsistent between experiments but replicable within parallel incubations. The lack of response of the methanotrophic community to ammonium and organic carbon additions suggest these are not limiting. Discussion Our results suggest Methylococcaceae primarily drive MOx in Jordan lake, despite often not being the most abundant methanotrophic group, and that high oxygen concentrations may suppress this group independently of their association with lower methane concentrations.

  PublisherOA PDFDOI

• In situ aerobic methane oxidation rates in a stratified lake

  Limnology and Oceanography · 2024 · 4 citations

  Senior authorCorresponding
  • Environmental science
  • Environmental chemistry
  • Oceanography

  Abstract Microbial aerobic methane oxidation is an important sink for aquatic methane worldwide. Despite its importance to global methane fluxes, few aerobic methane oxidation rates have been obtained in freshwater or marine environments without imposing changes to the microbial community through use of ex situ methods. A novel in situ incubation method for continuous time‐series measurements was used in Jordan Lake, North Carolina, during 2020–2021, to determine reaction kinetics for aerobic methane oxidation rates across a wide range of naturally varying methane (55–1833 nM) and dissolved oxygen (DO; 28–366 μ M) concentrations and temperatures (17–30°C). Methane oxidation began immediately at the start of each of 21 incubations and methane oxidation rates were 1 st order with respect to methane. The data density allowed for accurate calculation of 1 st ‐order rate constants, k , that ranged from 0.018 to 0.462 h −1 ( R 2 > 0.967). Addition of ammonium (20–45 μ M) to natural concentrations ranging from 0.057 to 2.4 μ M did not change aerobic methane oxidation rate kinetics, suggesting that the natural population of aerobic methane oxidizers in this eutrophic lake was not nitrogen limited. Values of k inversely correlated most strongly with initial DO concentrations ( R 2 = 0.82) rather than temperature. Values for k increased with Julian day throughout our sampling period, suggesting seasonal influences on methane oxidation via responses to geochemical changes or shifts in microbial community abundance and composition. These experiments demonstrate a high variability in the enzymatic capacity for 1 st ‐order methane oxidation rates in this eutrophic lake that is tightly and inversely coupled to oxygen concentrations. Measurements of in situ aerobic methane oxidation rate constants allow for the direct quantification and modeling of the microbial community's capacity for methane oxidation over a wide range of natural methane concentrations.

  PublisherOA PDFDOI

• Chemistry of aerosols, cloud droplets, and rain in the Puerto Rican marine atmosphere

  UNC Libraries · 2022-10-14

  articleOpen accessSenior author

  The chemical changes occurring during the transformation of aerosols to cloud droplets and rain were observed in orographic clouds on the eastern end of the island of Puerto Rico. Comparisons of elemental ratios in particles and rain and of elemental particle size distributions in and out of the clouds demonstrated scavenging efficiencies of >100% for the elements I, Br, and Cl relative to Na and of <25% for the elements Al, Mn, and V relative to Na. These different scavenging efficiencies are consistent with predictions based on existing cloud physics theory and are a reflection of the geochemical properties of the aerosol consisting partly of a hygroscopic sea salt component containing I, Br, Cl, and Na and partly of a soil dust component containing Al, Mn, and possibly V.

  PublisherDOI

• BORON IN COASTAL NORTH FLORIDA RAINFALL.

  UNC Libraries · 2022-10-14

  articleOpen access1st authorCorresponding

  Dissolved boron and sodium concentrations were determined in rainfall collected on the Gulf Coast of northern Florida in order to investigate chemical and physical processes influencing natural concentration levels in the coastal marine atmosphere. Rainfall samples were collected during summer showers, during drizzle and heavy downpours associated with winter frontal activities, and sequentially during tropical storm Becky. Mean calculated B enrichments E//N//a(B) for summer and winter sample sets were 32 and 11, respectively. A surprisingly constant concentration of 'excess' B above that expected from direct seawater injection of approximately 6 mu g 1** minus **1 was observed in the sequential Becky samples. The lower winter B/Na ratios, and thus the calculated E//N//a(B) values, appear related to higher Na concentrations; however, several alternative hypotheses can be advanced to explain the results, including greater particulate sea-salt injection during winter months, incorporation of soil materials with a B/Na ratio above the seawater value, and possible influences of temperature variation on gaseous B incorporation in rain and evaporation from the sea surface.

  PublisherDOI

• The Role of Benthic Fluxes of Dissolved Organic Carbon in Oceanic and Sedimentary Carbon Cycling

  UNC Libraries · 2021-07-02

  articleOpen access1st authorCorresponding

  Benthic fluxes (sediment-water exchange) of dissolved organic carbon (DOC) represent a poorly quantified component of sedimentary and oceanic carbon cycling. In this paper the authors use pore water DOC data and direct DOC benthic flux measurements to begin to quantitatively examine this problem. These results; suggest that marine sediments represent a significant source of DOC to the oceans, as a lower limit of the globally-integrated benthic DOC flux is comparable in magnitude to riverine inputs of organic carbon to the oceans. Benthic fluxes of DOC also appear to be similar in magnitude to other sedimentary processes such as organic carbon oxidation (remineralization) in surface sediments and organic carbon burial with depth. 31 refs., 3 figs., 3 tabs.

  PublisherDOI

• Sponges represent a major source of inorganic nitrogen in Florida Bay (U.S.A.)

  Limnology and Oceanography · 2019-12-10 · 6 citations

  articleSenior author

  Abstract Florida Bay nutrient budgets have shown that the majority of existing and influent nitrogen (N) is in organic forms. Consequently, local remineralization processes have been found to regulate the supply of dissolved inorganic nitrogen (DIN). Sponges have dominated benthic animal biomass in Florida Bay and are known to influence local DIN concentrations through remineralization organic matter, yet the role of these organisms in local N budgets is largely unaddressed. We quantified the role of sponges in N cycling in Florida Bay during 2012–2013 by constructing an N budget for a sponge‐rich basin. Surveys of sponge biomass conducted in Mystery Basin found sponges at 57 of the 59 assessed stations. Sponge population maxima reached 21 individuals m −2 and biomass contributions as high as 4.4 L sponge m −2 . We estimated an average areal DIN contribution from total sponge biomass of 0.59 ± 0.28 mmol N m −2 d −1 . However, calculated fluxes from the 59 stations exhibited significant spatial variability associated with changes in the size and species composition of the sponge community; peak N fluxes reached 3.5 ± 0.9 mmol N m −2 d −1 in areas with large populations of high microbial abundance sponges. The average flux from the sponge community was the largest of the estimated sources of DIN to Mystery Basin, representing roughly half of the overall N sourcing. This N satisfied more than half of the demand by primary productivity. These results indicate that sponges are important sources of inorganic N to Florida Bay environments.

  PublisherDOI

• Dissolved inorganic nitrogen fluxes from common Florida Bay (U.S.A.) sponges

  Limnology and Oceanography · 2018-08-07 · 18 citations

  articleOpen accessSenior author

  Abstract Sponge biomass represents the largest heterotrophic component of benthic biota in the Florida Bay ecosystem. These organisms can significantly alter the water quality of their surrounding environment through biogeochemical transformations of nutrient elements resulting from their dynamic pumping, water filtration, and respiration processes. Ammonium (NH 4 + ) and nitrate plus nitrite (NO 3 − + NO 2 − ; NO x − ) fluxes were obtained for 11 ecologically important species at three sites within Florida Bay, Florida (U.S.A.) utilizing chamber incubations on undisturbed individual sponges. Significant dissolved inorganic nitrogen (DIN) effluxes ranging between 9.0 ± 2.2 μ mol N h −1 L sponge −1 and 141 ± 26 μ mol N h −1 L sponge −1 were observed for eight of the 11 tested sponges; specifically, from six of eight tested high‐microbial abundance (HMA) sponges, and from two of three tested low‐microbial abundance (LMA) sponges. The abundant HMA species Chondrilla nucula showed the highest, volume‐normalized rate of DIN release. These fluxes represent a continuation of the previously observed dichotomy in the chemical speciation of DIN in exhalent waters of LMA and HMA sponges, with NH 4 + and NO x − dominating their respective exhalent jets. Surprisingly, we found that dissolved organic matter (DOM) appeared to make a negligible contribution to the total released N, but we hypothesize that the lack of DOM utilization or production was due to methodological limitations. Our flux data combined with sponge biomass estimates indicate that sponges, particularly HMA species, are a large, and potentially dominant, source of inorganic nitrogen to Florida Bay waters.

  PublisherOA PDFDOI

• Consumption of dissolved organic carbon by Caribbean reef sponges

  Limnology and Oceanography · 2017-08-01 · 73 citations

  articleOpen accessSenior authorCorresponding

  Abstract Sponges are conspicuous and abundant within the benthic fauna on Caribbean reefs. The ability of these organisms to efficiently capture carbon from particulate sources is well known and the importance of dissolved organic carbon (DOC) uptake has been recognized for several species. We surveyed DOC ingestion by seven sponge species common to Florida Keys reefs using nondisruptive sampling methods on undisturbed individuals. Three of the seven species exhibited significant DOC removal ranging from 13% to 24% of ambient concentrations. The tested species that removed DOC host large microbial consortia within their tissues, while the converse was observed for those that did not. This divergent behavior may suggest an important role for sponge associated microbes in the utilization of DOC by these species. The feeding behaviors of individuals of Xestospongia muta were then monitored over time to investigate its respiratory consumption of particulate and DOC. The uptake rates of dissolved oxygen (DO) and organic carbon by two undisturbed individuals revealed that DOC represented 96% of removed C, and that the tested individuals removed approximately equal quantities of C and DO. This demonstrates that X. muta largely satisfies its respiration demands through DOC consumption, and that DOC likely represents the dominant C source for biomass production and cell overturn in this species. These results further illustrate the metabolic importance of DOC to sponges, and suggest that these organisms are an important pathway for remineralizing organic matter on Caribbean reefs.

  PublisherOA PDFDOI

• Manganese enrichments near a large gas‐hydrate and cold‐seep field: a record of past redox and sedimentation events

  The Depositional Record · 2016-07-26 · 8 citations

  articleOpen accessSenior author

  Abstract The spatial distribution, mineralogy, and origin of manganese enrichments surrounding a large gas hydrate and cold seep field (Mississippi Canyon 118, Gulf of Mexico) are investigated in this study, to better constrain their biogeochemical context in deep‐sea sediments and to assess how gas hydrates may alter such records. Manganese depth profiles from 10 sediment cores, documented using centimetre‐scale X‐ray fluorescence core scanning, display highly‐enriched 1 to 10 cm thick layers. These manganese‐rich layers are more numerous, but of lower concentration, in close proximity to the field, and show no consistent relationship with sedimentology (clay vs. carbonate content) or the established chronostratigraphic framework at the site. X‐ray diffraction and sequential dissolution procedures indicate that the manganese enrichments are authigenic carbonates, which formed along a palaeo redox boundary during periods of prolonged steady‐state conditions. The hypothesis that spatial heterogeneity of this manganese record is linked to the nearby gas hydrate and cold seep field, by influencing redox conditions and/or sedimentation processes, is investigated here. Results are consistent with more frequent interruption of steady‐state sedimentation in closer proximity to the salt‐tectonic induced bathymetric mound, which contains the active cold seeps and gas hydrate deposits. Thus, spatial mapping of manganese enrichment horizons provides a tool to reconstruct sedimentation surrounding these volatile sea bed features, yielding a measure of past activity of gas hydrates and cold seeps.

  PublisherOA PDFDOI

• Chasing Sources and Transports of Methane Plumes In the Northern Gulf of Mexico Using In Situ Sensors On Untethered Landers

  Aquila Digital Community (University of Southern Mississippi) · 2016-02-01

  article1st authorCorresponding

  In situ time-series measurements of light hydrocarbons, oxygen, temperature and bottom currents from landers and elevators in the benthic boundary layer (BBL) at multiple sites in the northern Gulf of Mexico reveal spatial and temporal variability in methane concentrations controlled by horizontal advection of methane-rich plumes originating from nearby natural oil and gas seeps. Multi-sensor systems deployed for several weeks within 1m of the seafloor at depths from 882 to 1622m revealed methane concentrations ranging from near atmospheric saturation (&lt;3 nM) to over 4000 nM depending on seep proximity, current speed and direction. Methane concentrations observed in the BBL equal or exceed maximum near-bottom values seen in shipboard water column profiles analyzed by conventional gas chromatography. Continuous laser sensor methane measurements from mini-landers deployed in September 2015 at our Horn Dome and Bush Hill sites featuring numerous gas seeps revealed methane concentrations ranging from &lt;3 to over 300 nM over two-week periods. Net current speeds in the BBL at our six lander sites in blocks GC600, OC26 and MC118 ranged from near zero to over 5 cm/s; instantaneous speeds ranged from near zero to over 30 cm/s. Near real-time acquisition of continuous hydrocarbon concentration and current data within the BBL and friction layer from untethered platforms provides important new opportunities for monitoring the impacts of natural seeps and accidental hydrocarbon releases. The instrumented approaches we have developed to simultaneously monitor methane sources and physical processes controlling plume development and transport will enable more effective responses to further accidental hydrocarbon releases.

  Publisher

Recent grants

• Biogeochemical Cycling in the Organic-Rich Coastal Environment

  NSF · $247k · 1982–1985

• Collaborative Research: The Role of Sponges in the Coastal Nitrogen Cycle

  NSF · $973k · 2011–2015

• Collaborative Proposal: In Situ Quantification of Sponge N Cycling in Coastal Ecosystems

  NSF · $647k · 2006–2010

Frequent coauthors

• Jeffrey P. Chanton

  Florida State University

  39 shared

• L. Lapham

  Aarhus University

  22 shared

• Marc J. Alperin

  University of North Carolina at Chapel Hill

  21 shared

• J. Robert Woolsey

  17 shared

• Daniel B. Albert

  Infineon Technologies (Germany)

  16 shared

• Paul Higley

  Specialty Devices (United States)

  16 shared

• Hans W. Jannasch

  Monterey Bay Aquarium Research Institute

  16 shared

• Nancy B. Dise

  UK Centre for Ecology & Hydrology

  10 shared

Labs

• Earth, Marine and Environmental SciencesPI