Martha Gledhill

GEOMAR Helmholtz Centre for Ocean Research Kiel · Division of Chemical Oceanography

Trichodesmium spp. is a colonial diazotrophic cyanobacterium found in the oligotrophic (sub)tropical oceans, where its distribution is strongly regulated by the availability of phosphorus and iron. The bulk carbon : nitrogen : phosphorus elemental composition of phytoplankton has previously been shown to depart from classical “Redfield” values unde...

The marine cyanobacterium Trichodesmium has the remarkable ability to interact with and utilize air‐borne dust as a nutrient source. However, dust may adversely affect Trichodesmium through buoyancy loss and exposure to toxic metals. Our study explored the effect of desert dust on buoyancy and mortality of natural Red Sea puff‐shaped Trichodesmium...

The marine cyanobacterium Trichodesmium has a remarkable ability to interact with and utilize air-borne dust as a nutrient source. However, dust may adversely affect Trichodesmium through buoyancy loss and exposure to toxic metals. Our study explored the effect of desert dust on buoyancy and mortality of natural Red Sea puff-shaped Trichodesmium th...

Marine dissolved organic matter (DOM) presents key thermodynamic properties that are not yet fully constrained. Here, we report the distribution of binding sites occupied by protons (i.e., proton affinity spectra) and parametrize the median intrinsic proton binding affinities (log K̅H) and heterogeneities (m), for DOM samples extracted from the Nor...

The photosynthetic and diazotrophic cyanobacterium Trichodesmium is a key contributor to marine biogeochemical cycles in the subtropical-oligotrophic oceans. Trichodesmium form colonies that harbor a distinct microbial community in comparison to the surrounding seawater. The presence of their associated bacteria can expand Trichodesmium’s functiona...

An insufficient supply of the micronutrient iron (Fe) limits phytoplankton growth across large parts of the ocean. Ambient Fe speciation and solubility are largely dependent on seawater physico‐chemical properties. We calculated the apparent Fe solubility (SFe(III)app) at equilibrium for ambient conditions, where SFe(III)app is defined as the sum o...

The photosynthetic and diazotrophic cyanobacterium Trichodesmium is a key contributor to marine biogeochemical cycles in the subtropical-oligotrophic oceans. Trichodesmium forms colonies that harbor a distinct microbial community, which expands their functional potential and is predicted to influence the cycling of carbon, nitrogen, phosphorus and...

Tropical peat swamps are essential ecosystems, which provide numerous services, and also serve as a rich source of dissolved organic carbon (DOC), hydrogen ions and trace elements to peat draining rivers. However, not much is known about trace element export from tropical peat swamps. We investigated trace element dynamics in rivers and estuaries d...

Reactive iron (Fe) oxides in marine sediments may represent a source of bioavailable Fe to the ocean via reductive dissolution and sedimentary Fe release or can promote organic carbon preservation and long-term burial. Furthermore, enrichments of reactive Fe (sum of Fe oxides, carbonates and sulfides normalized to total Fe) in ancient sediments are...

The abundance of plastic debris (PDs) and its correlation with phthalic acid esters (PAEs), a class of pollutants associated with plastics, is not well understood, although PDs have been reported in relation to the release and distribution of aquatic pollutants such as PAEs. Few studies have linked the distribution of these pollutants in seawater....

Dissolved organic matter (DOM) is a distinct component of Earth’s hydrosphere and provides a link between the biogeochemical cycles of carbon, nutrients, and trace metals (TMs). Binding of TMs to DOM is thought to result in a TM pool with DOM-like biogeochemistry. Here, we determined elemental stoichiometries of aluminum, iron, copper, nickel, zinc...

Surface ocean pH is declining due to anthropogenic atmospheric CO2 uptake with a global decline of ~0.3 possible by 2100. Extracellular pH influences a range of biological processes, including nutrient uptake, calcification and silicification. However, there are poor constraints on how pH levels in the extracellular microenvironment surrounding phy...

In this study, the leaching of six phthalic acid esters (PAEs) from three common consumer plastics was investigated: low and high density polyethylene (LDPE, HDPE) and recycled polyethylene (RP). The effects of salinity, temperature, and ultraviolet irradiation (UVR) on leaching were investigated. The study of leaching of phthalates in aqueous envi...

Siderophores are low molecular weight high affinity iron chelates found at low concentrations in seawater. In this study we determined the total concentrations and identities of siderophores in extracts isolated from a shelf sea environment on the Northwest European shelf by high performance liquid chromatography coupled to high resolution inductiv...

Trichodesmium spp. is a colonial diazotrophic cyanobacterium found in the oligotrophic (sub)tropical oceans, where dissolved inorganic phosphorus (DIP) can be depleted. To cope with low P concentrations, P can be scavenged from the dissolved organic P (DOP) pool. This requires the deployment of multiple enzymes activated by trace metals, potentiall...

Here we archive a protocol that can be used to determine competition between a siderophore (ferrioxamine B) and humic like binding sites that are present in marine DOM. We use the NICA-Donnan model to describe binding by humic like binding sites in DOM. Constants for Fe binding to marine DOM are taken from Zhu et al., (2021). Thermodynamic constant...

Ultratrace concentrations of Ti were determined by catalytic differential pulse adsorptive stripping voltammetry (cDPAdSV) in samples collected in the Pará and Amazon estuaries and plume into the Atlantic Ocean. Different end members including rivers Tocantins, Amazon and Pará and Atlantic seawater were sampled as well as the salinity gradients in...

The physicochemical characteristics of dissolved organic matter (DOM) strongly influence its interactions with inorganic species such as protons and trace elements in natural waters. We collected water samples at Boknis Eck, a time series station in the Baltic Sea with a low exposure to freshwater inputs, to investigate how seasonal fluctuations im...

We used a combined ion pairing - organic matter speciation model (NICA-Donnan) to predict the organic complexation of iron (Fe) at ambient pH and temperature in the Celtic Sea. We optimized our model by direct comparison with Fe speciation determined by Adsorptive Cathodic Stripping Voltammetry using the added Fe-binding ligand 1-nitroso-2-naphthol...

Relic munitions are a hazardous legacy of the two world wars present in coastal waters worldwide. The southwest Baltic Sea has an especially high prevalence of unexploded ordnance and dumped munition material, which represent a large potential source of toxic explosive chemicals (munition compounds, MC). In the current study, diverse biota (plankto...

Competitive ligand exchange–adsorptive cathodic stripping voltammetry (CLE-AdCSV) is used to determine the conditional concentration ([L]) and the conditional binding strength (logKcond) of dissolved organic Fe-binding ligands, which together influence the solubility of Fe in seawater. Electrochemical applications of Fe speciation measurements vary...

Lateral fluxes (i.e. outwelling) of dissolved organic (DOC) and inorganic (DIC) carbon and total alkalinity were estimated using radium isotopes at the groundwater, mangrove creek and continental shelf scales in the Amazon region. Observations of salinity and radium isotopes in the creek indicated tidally-driven groundwater exchange as the main sou...

The Amazon River has the largest drainage basin in the world, making it a major source of trace elements and dissolved organic matter (DOM) to the Atlantic Ocean. However, despite the increasing anthropogenic impacts to the Amazon basin, few recent studies exist quantifying trace element data in this region. The aim of the study was to analyze the...

Competitive ligand exchange–adsorptive cathodic stripping voltammetry (CLE-AdCSV) is used to determine the conditional concentration ([L]) and the conditional binding strength (logKcond) of dissolved organic Fe-binding ligands, which together influence the solubility of Fe in seawater. Electrochemical applications of Fe speciation measurements vary...

The Amazon is Earth's largest river by volume output, making it an important source of trace metals and dissolved organic matter (DOM) to the Atlantic Ocean. Despite major recent anthropogenic disruptions to the Amazon catchment area, data for trace metals such as copper (Cu) in the Amazon River estuary and associated mixing plume are still rare. F...

Competitive ligand exchange – adsorptive cathodic stripping voltammetry (CLE-AdCSV) is a widely used technique to determine dissolved iron (Fe) speciation in seawater, and involves competition for Fe of a known added ligand (AL) with natural organic ligands. Three different ALs were used, 2-(2-thiazolylazo)-p-cresol (TAC), salicylaldoxime (SA) and...

In this study, we identify and quantify processes that lead to sedimentary potassium (K) sequestration and phosphorus (P) release on the Amazon shelf. To this end, seven short sediment cores were recovered from the Amazon shelf during R/V Meteor cruise M147. All of the sediment cores investigated in this study are characterized by elevated K to alu...

We investigated trace element stoichiometries of the nitrogen-fixing marine cyanobacterium Crocosphaera subtropica ATCC51142 under steady-state growth conditions. We utilized exponentially fed batch cultures and varied iron (Fe) concentrations to establish nutrient limitation in C. subtropica growing at a constant growth rate (0.11 d ⁻¹ ). No stati...

The major part of dissolved iron (DFe) in seawater is bound to organic matter, which prevents iron from adsorptive removal by sinking particles and essentially regulates the residence time of DFe and its availability for marine biota. Characteristics of iron‐binding ligands highly depend on their biological origin and physico‐chemical properties of...

There is a paucity of data on Fe-binding ligands in the Arctic Ocean. Here we investigate the distribution and chemical properties of natural Fe-binding ligands in Fram Strait and over the northeast Greenland shelf, shedding light on their potential sources and transport. Our results indicate that the main sources of organic ligands to surface wate...

Due to its accurate and precise character, spectrophotometric pH detection is a common technique applied in measurement methods for carbonate system parameters. However, impurities in the used pH indicator dyes can influence the measurements quality. During our work described here, we focused on impacts of impurities in the pH indicator dye bromocr...

Marine dissolved organic matter (DOM) plays a key role in the current and future global carbon cycle, which supports life on Earth. Trace metals such as iron, an essential micronutrient, compete with protons and major ions for the binding to DOM. These competitive effects and the DOM binding capacity are related to the DOM acid-base properties, whi...

Plain Language Summary Some trace metals, including iron, are essential micronutrients for phytoplankton growth. However, the solubility of iron is very low under oxygenated conditions. Consequently, restricted iron availability in oxygen‐rich seawater can limit phytoplankton growth in the ocean, including in the Eastern Tropical South Pacific. Und...

The speciation of dissolved iron (DFe) in the ocean is widely assumed to consist almost exclusively of Fe(III)-ligand complexes. Yet in most aqueous environments a poorly defined fraction of DFe also exists as Fe(II), the speciation of which is uncertain. Here we deploy flow injection analysis to measure in situ Fe(II) concentrations during a serie...

Estuarine systems are of key importance for the riverine input of silicon (Si) to the ocean, which is a limiting factor of diatom productivity in coastal areas. This study presents a field dataset of surface dissolved Si isotopic compositions (δ³⁰SiSi(OH)4) obtained in the estuaries of three of the world’s largest rivers, the Amazon (ARE), Yangtze...

Heme b is an iron-containing cofactor in hemoproteins that participates in the fundamental processes of photosynthesis and respiration in phytoplankton. Heme b concentrations typically decline in waters with low iron concentrations but due to lack of field data, the distribution of heme b in particulate material in the ocean is poorly constrained....

Abstract. Due to its accurate and precise character, the spectrophotometric pH detection is a common technique applied in measurement methods for carbonate system parameters. However, impurities in the used pH indicator dyes can influence the measurements quality. The work described here focuses on influences from impurities in the pH indicator dye...

The pelagic ocean receives terrigenous inputs of a range of organic compounds; however, the role that this terrigenous material plays in the ocean carbon cycle and biological pump is not entirely understood, and questions remain as to how oceanic cycles of terrigenous and autochthonous carbon interact. A significant portion of organic carbon that c...

The availability of the micronutrient iron (Fe) in surface waters determines primary production, N2 fixation, and microbial community structure in large parts of the world's ocean, and thus it plays an important role in ocean carbon and nitrogen cycles. Eastern boundary upwelling systems and the connected oxygen minimum zones (OMZs) are typically a...

Trichodesmium is a globally important marine nitrogen fixing cyanobacteria which forms colonies and utilizes atmospherically derived dust as a source for the limiting micro-nutrient iron. Here we report the identification of metallophores isolated from incubations of natural Trichodesmium colonies collected from the Gulf of Aqaba in the Red Sea. Th...

Iron (Fe) bioavailability limits phytoplankton growth in vast ocean regions. Iron-rich dust uplifted from deserts is transported in the atmosphere and deposited on the ocean surface. However, this dust is a poor source of iron for most phytoplankton since dust-bound Fe is poorly soluble in seawater and dust rapidly sinks out of the photic zone. An...

Heme b is an iron-containing co-factor in hemoproteins. Heme b concentrations are low (<1 pmol L⁻¹) in iron limited phytoplankton in cultures and in the field. Here, we determined heme b in marine particulate material (>0.7 μm) from the North Atlantic Ocean (GEOVIDE cruise – GEOTRACES section GA01), which spanned several biogeochemical regimes. We...

N2-fixing cyanobacteria mediate H2 fluxes through the opposing processes of H2 evolution, which is a by-product of the N2 fixation reaction, and H2 uptake, which is driven by uptake hydrogenases. Here, we used microelectrodes to characterize H2 and O2 dynamics in single natural colonies of the globally important N2 fixer Trichodesmium collected fro...

Underwater munitions containing millions of tons of toxic explosives are present worldwide in coastal marine waters as a result of unexploded ordnance and intentional dumping. Dissolution flux of solid explosives following corrosion of metal munition housings controls exposure of biological receptors to toxic munition compounds (MC; including TNT:...

Atmospheric deposition is an important source of micronutrients to the ocean, but atmospheric deposition fluxes remain poorly constrained in most ocean regions due to the limited number of field observations of wet and dry atmospheric inputs. Here we present the distribution of dissolved aluminium (dAl), as a tracer of atmospheric inputs, in surfac...

As a consequence of World War II, large amounts of munition have been deposited in coastal waters. Deterioration of the mines and bombs is resulting in a release of munition compounds (MCs) like trinitrotoluene to the surrounding marine environment, with potential implications to ecosystems. Analytical methods have thus far been unable to detect th...

The availability of the micronutrient iron (Fe) in surface waters determines primary production, N2 fixation and microbial community structure in large parts of the world's ocean, and thus plays an important role in ocean carbon and nitrogen cycles. Eastern boundary upwelling systems and the connected oxygen minimum zones (OMZs) are typically assoc...

The speciation of dissolved iron (DFe) in the ocean is widely assumed to consist exclusively of Fe(III)-ligand complexes. Yet in most aqueous environments a poorly defined fraction of DFe also exists as Fe(II). Here we deploy flow injection 15 analysis to measure in-situ Fe(II) concentrations during a series of mesocosm/microcosm experiments in coa...

The distribution of dissolved aluminium (dAl) in the water column of the North Atlantic and Labrador Sea was studied along GEOTRACES section GA01 to unravel the sources and sinks of this element. Surface water dAl concentrations were low (median of 2.5 nM) due to low aerosol deposition and removal by biogenic particles (i.e. phytoplankton cells). H...

Iron (Fe), cobalt (Co), and vitamin B12 addition experiments were performed in the eastern Equatorial Pacific/Peruvian upwelling zone during the 2015 El Niño event. Near the Peruvian coastline, apparent photosystem II photochemical efficiencies (Fv/Fm) were unchanged by nutrient addition and chlorophyll a tripled in untreated controls over 2 days,...

Atmospheric deposition is an important source of micronutrients to the ocean, but atmospheric deposition fluxes remain poorly constrained in most ocean regions due to the limited number of field observations of wet and dry atmospheric inputs. Here we present the distribution of dissolved aluminium (dAl), as a tracer of atmospheric inputs, in surfac...

Coastal marine environments are contaminated globally with a vast quantity of unexploded ordnance and munitions from intentional disposal. These munitions contain organic explosive compounds as well as a variety of metals, and represent point sources of chemical pollution to marine waters. Most underwater munitions originate from World Wars at the...

We have assessed how varying CO2 (180, 380, and 720 μatm) and growth light intensity (40 and 400 μmol photons m⁻² s⁻¹) affected Trichodesmium erythraeum IMS101 growth and photophysiology over free iron (Fe′) concentrations between 20 and 9,600 pM. We found significant i