Marine Colloids Research Articles

Marine Colloids Boost Nitrogen Fixation in Trichodesmium erythraeum by Photoelectrophy.

Nitrogen fixation by the diazotrophic cyanobacterium Trichodesmium contributes up to 50% of the bioavailable nitrogen in the ocean. N2 fixation by Trichodesmium is limited by the availability of nutrients, such as iron (Fe) and phosphorus (P). Although colloids are ubiquitous in the ocean, the effects of Fe limitation on nitrogen fixation by marine colloids (MC) and the related mechanisms are largely unexplored. In this study, we found that MC exhibit photoelectrochemical properties that boost nitrogen fixation by photoelectrophy in Trichodesmium erythraeum. MC efficiently promote photosynthesis in T. erythraeum, thus enhancing its growth. Photoexcited electrons from MC are directly transferred to the photosynthetic electron transport chain and contribute to nitrogen fixation and ammonia assimilation. Transcriptomic analysis revealed that MC significantly upregulates genes related to the electron transport chain, photosystem, and photosynthesis, which is consistent with elevated photosynthetic capacities (e.g., Fv/Fm and carboxysomes). As a result, MC increase the N2 fixation rate by 67.5-89.3%. Our findings highlight a proof-of-concept electron transfer pathway by which MC boost nitrogen fixation, broadening our knowledge on the role of ubiquitous colloids in marine nitrogen biogeochemistry.

Marine Colloids Promote the Adaptation of Diatoms to Nitrate Contamination by Directional Electron Transfer.

Nitrate contamination from human activities (e.g., domestic pollution, livestock breeding, and fertilizer application) threatens marine ecosystems and net primary productivity. As the main component of primary productivity, diatoms can adapt to high nitrate environments, but the mechanism is unclear. We found that electron transfer from marine colloids to diatoms enhances nitrogen uptake and assimilation under visible-light irradiation, providing a new pathway for nitrogen adaptation. Under irradiation, marine colloids exhibit semiconductor properties (e.g., the separation of electron-hole pairs) and can trigger the generation of free electrons and singlet oxygen. They also exhibit electron acceptor and donor properties, with the former being stronger than the latter, reacting with polysaccharides in extracellular polymeric substances (EPSs) under high nitrogen stress, enhancing the elasticity and permeability of cells, and promoting nitrogen assimilation and electron transfer to marine diatom EPSs. Electron transfer promotes extracellular-to-intracellular nitrate transport by upregulating membrane nitrate transporters and nitrate reductase. The upregulation of anion transport genes and unsaturated fatty acids contributes to nitrogen assimilation. We estimate that colloids may increase the nitrate uptake efficiency of marine diatoms by 10.5-82.2%. These findings reveal a mechanism by which diatoms adapt to nitrate contamination and indicate a low-cost strategy to control marine pollution.

Sir Dai Rees (28 April 1936 to 10 June 2021)

Sir Dai Rees (28 April 1936 to 10 June 2021)

Marine colloids, agents of the self-cleansing capacity of aquatic systems: Historical perspective and new discoveries

Colloids in the ocean are mainly composed of natural organic matter (NOM), with trace amounts of metals, metalloids, radionuclides, pollutants, and variable amounts of anthropogenically produced engineered nanoparticles and nanoplastics. State-of-the-art sampling and instrumental techniques were required to unravel both size spectra and chemical composition.Chemical components of colloidal macromolecular organic matter (COM) include aquagenic substances (composed of microbially produced Exopolymeric Substances (EPS), gels, transparent exopolymer particles (TEP) and TEP precursors), and pedogenic substances (mostly humic and fulvic matter). COM can be bioavailable and biodegradable. EPS in aquatic systems can form flocs of Marine Snow (MS) or Marine Oil Snow (MOS). EPS is thus important in initiating and maintaining aggregation, flocculation, and sedimentation processes of particles and microorganisms that act also as sorbents of pollutants, a process that contributes in a major way to the ‘self-cleansing capacity’ of aquatic systems.COM contains a diverse array of functionalities which can make them good metal chelating agents, amphiphilic and surface active pH-sensitive and redox-active. Due to numerous electrostatic and hydrophobic interactions between different moieties and ligand groups within a macromolecule, equilibrium constants, Ki, will become distribution functions for most reactions rather than fixed constants, as is common for low molecular weight organic molecules.Colloidal pumping of trace substances bound to EPS and other reactive macromolecular NOM compounds is counteracting the prevailing pathway of degradation of the bulk macromolecular NOM. Due to strong sorption capabilities for stable and radioactive metal ions, particle- and colloid-reactive radionuclides are useful to trace particle and organic carbon fluxes in the ocean.Open research questions include the role of different mechanisms that affect size spectra and composition of EPS in response to changing conditions, the role of macromolecular organic carrier molecules of radioactive and other trace substances that are used as oceanographic tracers; the potential use of the protein/carbohydrate ratio as a predictor of colloid or particle stickiness, aggregation or emulsification processes, and the importance of reactive oxygen species (ROS) mediated polymerization reactions of proteins and other molecules, which are part of EPS, for MS and MOS formation.

Light-induced aggregation of microbial exopolymeric substances

Sunlight can inhibit or disrupt the aggregation process of marine colloids via cleavage of high molecular weight compounds into smaller, less stable fragments. In contrast, some biomolecules, such as proteins excreted from bacteria can form aggregates via cross-linking due to photo-oxidation. To examine whether light-induced aggregation can occur in the marine environment, we conducted irradiation experiments on a well-characterized protein-containing exopolymeric substance (EPS) from the marine bacterium Sagitulla stellata. Our results show that after 1 h sunlight irradiation, the turbidity level of soluble EPS was 60% higher than in the dark control. Flow cytometry also confirmed that more particles of larger sized were formed by sunlight. In addition, we determined a higher mass of aggregates collected on filter in the irradiated samples. This suggests light can induce aggregation of this bacterial EPS. Reactive oxygen species hydroxyl radical and peroxide played critical roles in the photo-oxidation process, and salts assisted the aggregation process. The observation that Sagitulla stellata EPS with relatively high protein content promoted aggregation, was in contrast to the case where no significant differences were found in the aggregation of a non-protein containing phytoplankton EPS between the dark and light conditions. This, together with the evidence that protein-to-carbohydrate ratio of aggregates formed under light condition is significantly higher than that formed under dark condition suggest that proteins are likely the important component for aggregate formation. Light-induced aggregation provides new insights into polymer assembly, marine snow formation, and the fate/transport of organic carbon and nitrogen in the ocean.

Binding of Th, Pa, Pb, Po and Be radionuclides to marine colloidal macromolecular organic matter

To study the binding mechanisms of radionuclides to organic moieties in colloidal organic matter (COM), marine colloids (1kDa–0.2μm) were isolated by cross-flow ultrafiltration from seawater of the west Pacific Ocean and the northern Gulf of Mexico. For the same purpose, exopolymeric substances (EPS) produced by laboratory-cultured diatoms were collected as well. In our study areas, colloidal organic carbon (COC) concentrations ranged from 6.5 to 202μg-C/L in the Pacific Ocean, and were 808μg-C/L in the Gulf of Mexico. The COM compositions (organic carbon, organic nitrogen, proteins, total hydrolysable amino acids, total polysaccharides, uronic acids, hydroxamate siderophores, hydroquinone) were quantified to examine the relationships between partition coefficients (Kc) of five different radionuclides, 234Th, 233Pa, 210Pb, 210Po and 7Be, and concentration ratios to COC of individual chelating biomolecules that could potentially act as a chelating moiety. The range of partition coefficients (Kc, reported as logKc) of radionuclides between water and the different colloidal materials was 5.12 to 5.85 for 234Th, 5.19 to 6.01 for 233Pa, 4.21 to 4.85 for 210Pb, 4.87 to 5.68 for 210Po, and 4.49 to 4.92 for 7Be, similar to values previously reported for lab and field determinations under different particle concentrations. While any relationship obtained between Kc and abundance of specific moieties could not be taken as proving the existence of colloidal organic binding ligands for the different radionuclides, it could suggest possible organic moieties involved in the scavenging of these natural radionuclides. Together with results from isoelectric focusing of radiolabeled COM, we conclude that binding to different biomolecules is nuclide-specific, with colloidal hydroxamate siderophoric moieties being important for the binding of Th and Pa radionuclides. Hydroquinones/quinone (HQ/Q) facilitated redox and chelation reactions seem to be involved in the binding of Pa and Be. However, the actual mechanisms are not clear. Individual amino acids, proteins, total polysaccharides and uronic acids did not yield significant relationships with logKc values of the different radionuclides. Nonetheless, our results provide new insights into the relative importance of different potential ligand moieties in COM in the binding and possible scavenging of specific radionuclides in the ocean.

Association of macroaggregates and metals in coastal waters

Macroaggregates (macrogels) occurring episodically in the northern Adriatic, offer a rare opportunity to study interactions between marine organic macromolecules and metals and metalloids (Me). Macrogels are formed by agglomeration of dissolved heteropolysaccharides, mostly of phytoplankton origin. Filtration and centrifugation were used to discriminate between macrogel matrix and interstitial water colloids. The interstitial fraction was then used in an incubation experiment to follow the progress of microbial degradation of colloids. The colloids were ultrafiltered through membranes and each size class of colloids (>30kDa, 10–30kDa and 5–10kDa) was analyzed for metal, metalloid, organic carbon (Corg) and total nitrogen (Ntot) content. Higher Me concentrations and lower Me/Corg ratios in matrix, which are an indicator of Me bound to organic matter, suggest that Me in a matrix can also be bound onto Me containing particles. All the studied Me in macroaggregate colloidal fraction, except Se, proved to be bound to large macromolecules (>30kDa), mainly glycoproteins and aminopolysaccharides. During the rapid degradation of all, except the 5–10kDa fraction, the Me associated with the >30kDa fraction were markedly preserved. Consequently, the Me/Corg ratios increased due to the loss of Corg and relative increase of metal-binding sites. The observed differences between our Me/Corg ratios and the Irving–Williams series can be explained by variable background concentrations of the studied Me in the northern Adriatic. Comparison between Me levels in macroaggregate colloids and dissolved marine colloids, considered to be macroaggregate precursors, revealed similar values. Distribution coefficients (Kd) between colloidal and truly dissolved Me were lower than Kd for particulate matter, suggesting that organic colloids, including macroaggregates, exhibit rather weak Me binding capacity and that Me are not distributed homogenously in the various particle pools in the northern Adriatic coastal waters.

Picomonas judraskeda Gen. Et Sp. Nov.: The First Identified Member of the Picozoa Phylum Nov., a Widespread Group of Picoeukaryotes, Formerly Known as ‘Picobiliphytes’

In 2007, a novel, putatively photosynthetic picoeukaryotic lineage, the ‘picobiliphytes’, with no known close eukaryotic relatives, was reported from 18S environmental clone library sequences and fluorescence in situ hybridization. Although single cell genomics later showed these organisms to be heterotrophic rather than photosynthetic, until now this apparently widespread group of pico-(or nano-)eukaryotes has remained uncultured and the organisms could not be formally recognized. Here, we describe Picomonas judraskeda gen. et sp. nov., from marine coastal surface waters, which has a ‘picobiliphyte’ 18S rDNA signature. Using vital mitochondrial staining and cell sorting by flow cytometry, a single cell-derived culture was established. The cells are biflagellate, 2.5–3.8×2–2.5 µm in size, lack plastids and display a novel stereotypic cycle of cell motility (described as the “jump, drag, and skedaddle”-cycle). They consist of two hemispherical parts separated by a deep cleft, an anterior part that contains all major cell organelles including the flagellar apparatus, and a posterior part housing vacuoles/vesicles and the feeding apparatus, both parts separated by a large vacuolar cisterna. From serial section analyses of cells, fixed at putative stages of the feeding cycle, it is concluded that cells are not bacterivorous, but feed on small marine colloids of less than 150 nm diameter by fluid-phase, bulk flow endocytosis. Based on the novel features of cell motility, ultrastructure and feeding, and their isolated phylogenetic position, we establish a new phylum, Picozoa, for Picomonas judraskeda, representing an apparently widespread and ecologically important group of heterotrophic picoeukaryotes, formerly known as ‘picobiliphytes’.

Ion-exchange reactions on clay minerals coupled with advection/dispersion processes. Application to Na+/Ca2+ exchange on vermiculite: Reactive-transport modeling, batch and stirred flow-through reactor experiments

The present study aims at testing the validity of using an Na+/Ca2+ ion-exchange model, derived from batch data to interpret experimental Ca2+-for-Na+ exchange breakthrough curves obtained on vermiculite (a common swelling clay mineral in surface environments). The ion-exchange model was constructed considering the multi-site nature of the vermiculite surface as well as the exchange of all aqueous species (Mg2+ derived from the dissolution of the solid and H+). The proposed ion-exchange model was then coupled with a transport model, and the predicted breakthrough curves were compared with the experimental ones obtained using a well stirred flow-through reactor. For a given solute residence time in the reactor (typically 50min), our thermodynamic model based on instantaneous equilibrium was found to accurately reproduce several of the experimental breakthrough curves, depending on the Na+ and Ca2+ concentrations of the influents pumped through the reactor. However the model failed to reproduce experimental breakthrough curves obtained at high flow rates and low chemical gradient between the exchanger phase and the solution. An alternative model based on a hybrid equilibrium/kinetic approach was thus used and allowed predicting experimental data. Based on these results, we show that a simple parameter can be used to differentiate between thermodynamic and kinetic control of the exchange reaction with water flow. The results of this study are relevant for natural systems where two aquatic environments having contrasted chemistries interact. Indeed, the question regarding the attainment of a full equilibrium in such a system during the contact time of the aqueous phase with the particle/colloid remains most often open. In this context, we show that when a river (a flow of fresh water) encounters marine colloids, a systematic full equilibrium can be assumed (i.e., the absence of kinetic effects) when the residence time of the solute in 1m3 of the system is ⩾6200h.

Role of organic matter in the accumulation of platinum in oceanic ferromanganese deposits

In order to elucidate possible processes leading to platinum accumulation in ferromanganese deposits, we analyzed published data on the interaction of dissolved platinum species in different valence states with iron and manganese oxyhydroxides under oceanic conditions and experimentally studied the kinetics of sorption of inorganic and organic complexes of platinum (II) and platinum (IV) on synthetic iron and manganese oxyhydroxides and natural materials (marine colloids, and ferromanganese crust samples). The role of dissolved and suspended particulate aquatic organic matter in the sorption accumulation of platinum was evaluated. Possible reasons for the preferential (compared with other noble metals) accumulation of platinum in oceanic ferromanganese deposits were discussed.

Characterization of exopolysaccharides in marine colloids by capillary electrophoresis with indirect UV detection

A method was established using capillary electrophoresis with indirect UV detection for analysis of monosaccharides liberated from exopolysaccharides by acidic hydrolysis. Tangential flow filtration was used to isolate high molecular weight polysaccharides from seawater. The capillary electrophoresis method included the use of a background electrolyte consisting of 2,6-dimethoxyphenol and cetyltrimethylammonium bromide. Several neutral sugars commonly existing in marine polysaccharides were separated under optimized conditions. The relative standard deviations were between 1.3% and 2.3% for relative migration time and 1.3–2.5% for peak height. Detection limits (at S/N 3) were in the range of 27.2–47.8 μM. The proposed approach was applied to the analysis of hydrolyzed colloidal polysaccharides in seawater collected from the Baltic Sea. Nanomolar levels of liberated monosaccharides in seawater samples can be detected by preconcentration up to 30,000 times.

Variation in colloidal chromophoric dissolved organic matter in the Damariscotta Estuary, Maine

The relationship between chromophoric dissolved organic matter (CDOM), colloidal DOM (.1 kDa), and phytoplankton biomass in marine waters of the Damariscotta estuary, Maine, was studied using flow field‐flow fractionation (flow FFF). The goal was to determine whether temporal variability in CDOM in this region is due in part to rapid cycling of marine colloids. CDOM absorption of filtered (0.2 µm) seawater correlated positively with colloidal absorbance (r2 = 0.772, p = 0.021). We observed changes in colloidal size spectra and CDOM optical properties associated with phytoplankton biomass. In particular, a significant positive correlation was found between the < 18 kDa colloidal fraction and chlorophyll a (Chl a) (r2 = 0.819, p = 0.013). While we did not find significant relationships between Chl a and either CDOM absorption or the spectral slope of filtered seawater (r2 and p values of 0.079, 0.590 and 0.111, 0.519, respectively), we did observe patterns of higher CDOM absorption and spectral slopes following periods of increased phytoplankton biomass. The spectral slope of colloidal material often was significantly lower than that of filtered seawater and normally varied significantly with colloidal size. These findings show that colloidal processes can contribute to CDOM optical signatures in coastal waters.

Fluorescence variability of marine and terrestrial colloids: Examining size fractions of chromophoric dissolved organic matter in the Damariscotta River estuary

Marine chromophoric dissolved organic matter (CDOM) imparts highly variable optical signatures in surface waters over short spatial and temporal scales, but the cause of that variability is poorly understood. A major fraction of dissolved organic matter in seawater is colloidal in size and can cycle quite rapidly, potentially contributing to the observed variability in CDOM. The relationship between marine colloids and CDOM optical variability was examined using flow field-flow fractionation (FlFFF) to partition the colloidal organic phase into a continuum of molecular sizes for optical characterization by excitation emission matrix spectroscopy (EEMS). Colloidal organic matter in surface seawater of the Damariscotta River estuary showed 2 major peaks in apparent abundance, spanning at ∼ 1–5 kDa and ∼ 15–150 kDa in size, respectively. The relative magnitude of these peaks changed systematically with the phase of phytoplankton blooms during 2003 and 2004, implying a relationship between colloid size distribution and bloom dynamics. Of the two colloidal sizes, the 1–5 kDa fraction was far more variable in apparent abundance than the larger colloidal matter. EEMS results reveal a compositional partitioning of protein-like and humic-like fluorescence between size fractions. Protein-like materials occurred primarily in the smallest colloid size fraction while humic-type materials resided mainly in the larger colloidal phase. These findings suggest that the fluorescence signature of bulk dissolved organic matter results from a collage of chromophores having optical characteristics that differ according to size of the molecular constituents. The colloidal-sized fluorescence characteristics of marine derived CDOM were contrasted with bulk CDOM to provide fundamental information on the distribution and forms of CDOM in Maine coastal waters. The findings here indicate that colloidal processes will have significant effects on the character and variability in the optical signature of surface seawaters.

On the role of colloidal particles in light scattering in the ocean

We calculated the optical scattering and backscattering coefficients for assemblages of marine colloids from the measured size distribution and the assumed refractive index of colloids. The comparison of pure seawater scattering and backscattering coefficients with our results from 11 samples of small colloids (sized between 0.01 and 0.2 µm) and 10 samples of large colloids (~0.4™1 µm) suggests that (1) the role of colloids in light scattering in the ocean can vary from insignificant to very important as a result of variations in the concentration, size distribution, and refractive index of particles; (2) whereas small colloids generally play an insignificant role in particulate scattering, large colloids can make a sizable contribution because their scattering can exceed that of pure water by more than an order of magnitude; and (3) small colloids appear to play an important role in the overall colloidal backscattering. The combined backscattering of small and large colloids is typically higher than that of pure seawater over most of the visible spectrum (e.g., by a factor of 2.5 at 550 nm and 5.6 at 700 nm from our average results). This indicates that the contribution of colloids to the particulate backscattering is typically significant.

The effect of marine colloids on the growth of photosysthetic bacteria

Marine colloids could be an important source of nitrogen for bacteria and photoplankton. But elevated concentration of colloids may stimulate algal growth and lead to red tides in coastal waters. The effects of colloidal organic carbon (COC) concentration on the growth of photosysthetic bacteria (PSB) were investigated under different colloidal treatments in the laboratory. The PSB growth was inversely proportional to COC concentration and was restricted by high-molecular-weight (HMW) colloids (>10 KDa) in treatments with non-nutrient or just inorganic nutrient with low COC concentration (⩽5 μMC). However, the PBS growth was enhanced in the presence of HMW colloids in the treatment with inorganic nutrient and high COC (127 and 255 μMC) or with both inorganic nutrient and low-molecular-weight organic matter. Both bacteria number and bacteria growth ratio increased significantly when the concentration of COC was ⩾5 μMC. Our results suggest that marine colloids can be utilized by bacteria and might regulate primary productivity in coastal waters.

Dynamic colloid--water partitioning of pyrene through a coastal Baltic spring bloom.

Colloidal organic particles constitute the dominant portion of particulate organic matter in surface seawater, but their influence on the phase speciation and bioavailability of hydrophobic organic compounds (HOCs) is sparsely evaluated. Studies on colloid-water partitioning have been focused on other regimes and have largely been performed on chemically defined subportions of total colloids such as the humic fraction. Available estimates of colloid-water partition coefficients (Kcoc) are highly variable and not easily explained by regularly applied Kow-Koc relationships. Here, pyrene was partitioned to bulk natural colloids isolated using cross-flow ultrafiltration techniques from the surface water of a coastal bay. A key objective was to elucidate biogeochemical controls on the changing colloid-sorbent qualities over the course of the dynamic allochtonous-autochtonous transition of a well-constrained boreal coastal spring bloom. The pyrene Kcoc was found to decrease from 12.9+/-0.9 x 10(3) Lw/kg(oc) in the terrestrial runoff dominated regime to values around 2.9+/-0.7 x 10(3) Lw/kg(oc), once phytoplankton production became the governing source of organic matter to the surface waters. The changing Kcoc was well correlated with the molar extinction coefficient at 280 nm of the colloidal organic carbon. This study supports other reports of an improved prediction of HOC phase speciation through this simple molecular proxy of the "quality" of organic sorbents. While being poor sorbents on a carbon atom basis, relative to soils and sediments, coastal marine colloids, by their shear abundance, may significantly attenuate the truly dissolved exposures of HOCs with log Kow above 5.

The distribution of colloidal and particulate bioactive metals in Narragansett Bay, RI

Abstract The bioactive metals Fe, Mn, Cu, Zn and Ni in Narragansett Bay, RI, were partitioned into soluble, colloidal and particulate size fractions using a combination of conventional and cross-flow filtrations. Particulate samples (0.2–8.0 μm; >8 μm) were chemically fractionated into acetic-acid reactive and non-reactive metals. Conventional “dissolved” samples ( Mn>Zn>Cu>Ni with concentrations in the 0.2–0.8 μm fraction being generally higher than in the >8.0 μm fraction. The acid leachable fraction of the particulate phase increased from ∼32%–80% in the order Fe 8.0 μm) being generally less labile than small particulates (0.2–8.0 μm). The colloidal phase represented an average 4%–96% of the “dissolved” metals, ranging in importance from Fe (96%)>Cu (44%)>Ni (25%)>Zn (7%)>Mn (4%). Although generally small, the colloidal fraction of Zn and Mn was highest in a region of the bay where biomass typically is high. Changes in soluble and colloidal fractions along a transect through the bay indicate that a significant proportion of Fe, Cu and Ni were transferred from “dissolved” to particulate size fractions via colloid aggregation. Predicting colloidal metal concentrations from measurements of particulate mass ( C p ) and literature values of colloid metal partition coefficients ( K c ) underestimated the measured concentrations by 5–50×. Acetic acid leachable metal concentrations in the small particle (0.2–8 μm) phase correlated well with metal concentrations in the larger (8 kDa–0.2 μm) colloid fraction ( r =0.91–0.99). In contrast, metals in the smaller colloid fraction (1–8 kDa) were for the most part independent of any measured parameters. Metals were not distributed equally between colloidal size classes; colloidal Zn was associated with larger colloids (>90%), Fe and Ni were associated primarily with larger colloids (∼70–85%) but also with the smaller colloid fraction, while ≳70% of colloidal Cu was associated with smaller colloids. The non-uniform distribution of metals within colloidal size classes indicates that metal:colloid associations are regulated by specific interactions. These findings suggest that it is inappropriate to employ single, non-specific sorbing metal tracers (e.g. Th) to delimit the pathways and kinetics of bioactive metal interactions with marine colloids.

Trace metal composition of colloidal organic material in marine environments

Marine colloidal material (1 kDa–0.2 μm) was isolated by cross-flow ultrafiltration followed by diafiltration and freeze-drying from surface waters of the Gulf of Mexico and the Middle Atlantic Bight (MAB), as well as from estuarine waters of Galveston Bay. Elemental characterization of isolated colloidal material included organic carbon (OC) and selected trace metal (Cu, Pb, Zn, Cd, Co, Ni, Cr, Be, Fe, Al, Mn, V, Ba, and Ti) determinations. It was found that levels of these metals in marine colloids ranged from <0.1 to ∼50 μg/g colloidal matter, except for Fe which generally had a concentration >120 μg/g. Most metals (Cu, Pb, Zn, Ni, Al, Mn, V, and Ti) had an average concentration >1 μg/g while concentrations of Cd, Co and Be were usually <1 μg/g. Metal concentrations (μg/g) in isolated colloids were, in general, higher in Galveston Bay than in the Gulf of Mexico, suggesting either high abundance of trace metals in estuarine waters or differences in organic matter composition. Higher colloidal metal concentrations in the MAB than in the Gulf of Mexico might be due to higher terrestrial inputs in the MAB. Colloidal metal concentrations (μg/g) were generally lower than those in average soils, continental crust and suspended particles. However, metal/aluminum ratios (Me/Al) in isolated marine colloids were significantly higher than those for average soils and continental crust. Most importantly, colloids had a metal composition and metal/OC ratio (Me/C) similar to humic substances and marine plankton, suggesting that marine colloids largely originate from planktonic sources and are composed of predominately organic components. The Me/C ratios of Galveston Bay colloids followed the sequence of Cu>Ni, Cr, Zn>Mn>Co>Pb, Cd, which is similar to the Irving–Williams order except for Mn, suggesting that the interaction of metals with marine colloids is determined by the affinity of metals for specific organic ligands.

Lipid class and carbohydrate concentrations in marine colloids

Cross-flow filtration (CFF) was used to concentrate marine colloids from microalgal cultures and surface seawater. Lipid classes in different size fractions were determined using the Chromarod-Iatroscan thin-layer chromatography with the flame ionization detection method. Total carbohydrate concentrations were determined colorimetrically using methyl benzothiazolinone hydrazone reagent. Extensive determinations of CFF blanks and recoveries were conducted for each lipid class to optimize CFF operating procedures. The total lipid blank in the CFF system was approximately 6-7 μg (~0.5 μM C). Recoveries for the major lipid classes in marine samples varied from 80% to 120%. -- Both colloidal lipid and carbohydrate concentrations in microalgal cultures were found to be 120-140 μg/L. Triacylglycerol (TAG) and polar lipids were the major lipid classes in colloids from microalgal cultures. Concentrations of colloidal (> 10,000 dalton) lipids and carbohydrates in Newfoundland seawater ranged from 0.9 μg/L to 8.7 μg/L and from 21 μg/L to 30 μg/L, respectively. The major lipid classes present in coastal seawater colloids were TAG, free fatty acids, and phospholipids. -- Data from both algal cultures and actual seawater samples showed that marine colloids are characterized by having high proportions of free fatty acids, phospholipids and hydrocarbons by comparison with truly dissolved material. The free fatty acids may be produced by microbial degradation, the phospholipids are probably derived from biological membranes, and the hydrocarbons are likely preferentially adsorbed from the surrounding water onto the colloids.

Spontaneous assembly of marine dissolved organic matter into polymer gels

A large pool of organic carbon resides in the world's oceans in the form of dissolved organic matter (DOM)1,2. DOM is operationally defined as the fraction of organic matter that passes through a filter with a given pore size (which can range from less than 0.1 μm to 0.46 μm). This fraction has a longer oceanic residence time — and is generally less biodegradable — than particulate organic matter (POM)1,2,3,4. Processes transforming DOM into POM are therefore crucial for our understanding of the cycling of organic material in the oceans1,2,3,4. The aggregation of marine colloids, which constitute 10–40% of DOM2,3,5, is thought to be an important step in the transformation of DOM into POM3. It has been suggested that colloids, as well as transparent exopolymer particles and large aggregates (‘marine snow’) can be viewed as polymer gels6,7,8. Whether free DOM polymers can indeed spontaneously assemble to form polymer gels has, however, not yet been shown. Here we present experimental observations that demonstrate that marine polymer gels can assemble from free DOM polymers, and that their formation mechanism, physical characteristics and mineralization can be understood in terms of polymer gel theory9,10,11. The principles and methods of polymer gel physics thus have the potential to provide profound new insights into the processes controlling the exchange between the DOM and POM pools and the cycling of marine organic matter.