Kenneth Mopper

The amino acid and sugar composition of diatom cell-walls

The cell walls of diatoms consist of a silica frustule encased in an organic coating. Biochemical characterization of this coating should allow insight into: (1) the mechanism of silicification; (2) taxonomy and evolution of diatoms; (3) preservation of fossil frustules. The amino acid and sugar composition of cell walls from 6 diatom species have been elucidated. When compared to cellular protein, cell-wall protein is enriched in serine plus threonine and glycine, and depleted in acidic, sulfur-containing and aromatic amino acids. The sugars of the cell-wall carbohydrates are quite variable and fucose tends to replace glucose in estuarine species. Condensation of silicic acid, in epitaxial order, on a protein template enriched in serine and threonine, is suggested as the Si-depositing mechanism in diatoms. The nature of this template and the polysaccharides in the cell wall may determine the solubility of diatom frustules in various environments. There is sufficient variability in cell-wall amino acids to warrant further investigation of their taxonomic utility. The sugars appear to be related to environmental factors, but they may also serve in biosystematic studies.

Fluorescence as a possible tool for studying the nature and water column distribution of DOC components

Abstract Seawater samples from the Hawaiian HOTS station (used in the DOC/DON intercalibration exercise), and from a depth profile from the Sargasso Sea were studied by fluorescence spectroscopy using excitation-emission matrices based on corrected spectra. Our results suggest that marine dissolved organic matter (DOM) fluorescence can be categorized into two distinct groups of fluorescing compounds. One group is distinguished by a protein-type fluorescence (excitation about 220 and 270 nm and emission about 300–350 nm) and the other group is characterized by a humic-type fluorescence (excitation about 230 and 310 nm and emission about 420–450 nm). Chemical evidence supports the interpretation that proteins and humic substances are mainly responsible for these two fluorescence types in seawater. At both the Atlantic and Pacific stations, protein-type fluorescence was dominant in the upper water column, while humic-type fluorescence was dominant in the deeper samples. These results suggest that fluorescence can be used to study the nature and distribution of two major DOM fractions, i.e. recent biochemical versus old humic substances, in the open ocean. The relevance of our results to the controversial ‘new’ or ‘missing’ DOC, as revealed by high temperature combustion oxidation DOC analyzers, is discussed.

The role of surface-active carbohydrates in the flocculation of a diatom bloom in a mesocosm

A study was undertaken to evaluate the role of exocellular polysaccharides in the flocculation of a marine diatom bloom in a large tank mesocosm. Surface-active organic matter was extracted from 1.0 μ-filtered tank water by bubble adsorption each day for 7 days of the experiment. In agreement with past studies, particles (3–51 urn equivalent spherical diameter) were readily formed by bubbling and became concentrated in the foam. At the beginning of the bubbling (0–0.5 h), both particles and surface-active carbohydrates were extracted at high rates; however, these rates dropped off steeply after about 0.5 h of bubbling. The rate of particle formation by bubbling could be modeled fairly well by second order kinetics. The extracted, surface-active material was enriched in deoxysugars and galactose, while the residual material was enriched in glucose. Extracted surface-active carbohydrates reached a maximum of 33% of the total dissolved sugars ( 0.99) with particle stickiness (alpha). In addition, the concentration of surface-active carbohydrates was well correlated (2 = 0.91) with the concentration of transparent exopolymer particles (TEP) in the tank, and it was demonstrated that TEP could be copiously formed by bubbling of 1.0 μm-filered seawater. The finding of a highly surface-active, deoxysugar-rich polysaccharide material that can be rapidly (<0.5 h) and selectively extracted by bubble adsorption is significant, as it is apparent that this material played important roles in particle stickiness and TEP formation in the tank, and thus it may, at times, play similar roles in particle aggregation in the sea.

Photochemical degradation of dissolved organic matter and dissolved lignin phenols from the Congo River

[1] Photochemical degradation of Congo River dissolved organic matter (DOM) was investigated to examine the fate of terrigenous DOM derived from tropical ecosystems. Tropical riverine DOM receives greater exposure to solar radiation, particularly in large river plumes discharging directly into the open ocean. Initial Congo River DOM exhibited dissolved organic carbon (DOC) concentration and compositional characteristics typical of organic rich blackwater systems. During a 57 day irradiation experiment, Congo River DOM was shown to be highly photoreactive with a decrease in DOC, chromophoric DOM (CDOM), lignin phenol concentrations (S8) and carbon-normalized yields (L8), equivalent to losses of � 45, 85–95, >95 and >95% of initial values, respectively, and a +3.1 % enrichment of the d 13 C-DOC signature. The loss of L8 and enrichment of d 13 C-DOC during irradiation was strongly correlated (r = 0.99, p < 0.01) indicating tight coupling between these biomarkers. Furthermore, the loss of CDOM absorbance was correlated to the loss of L8 (e.g., a355 versus L8; r = 0.98, p < 0.01) and d 13 C-DOC (e.g., a355 versus d 13 C; r = 0.97, p < 0.01), highlighting the potential of CDOM absorbance measurements for delineating the photochemical degradation of lignin and thus terrigenous DOM. It is apparent that these commonly used measurements for examination of terrigenous DOM in the oceans have a higher rate of photochemical decay than the bulk DOC pool. Further process-based studies are required to determine the selective removal rates of these biomarkers for advancement of our understanding of the fate of this material in the ocean.

Chapter 9 – Photochemistry and the Cycling of Carbon, Sulfur, Nitrogen and Phosphorus

This chapter presents a discussion on photochemistry and the cycling of carbon, sulfur, nitrogen, and Phosphorus. Dissolved organic matter (DOM) plays a dominant role in the absorption of ultraviolet (UV) and visible light in the open ocean. As DOM absorbance is regulated in part by photobleaching processes, light availability for photosynthesis and the penetration of UV radiation within the marine environment are influenced by photochemical transformations. In addition to its control on UV light fields, DOM photochemistry strongly impacts the biogeochemical cycling of biologically important elements in surface seawater. By the conversion of DOM into volatile species such as carbonyl sulfide, DOM photochemistry influences atmospheric chemistry and climate. The chapter illustrates an important light-driven chemical reaction in the photic zone—that is, the reduction of trace metals such as iron, manganese, and copper. The chemistry of these reduced species is quite different from their oxidized counterparts. Photochemical oxidation of DOM also produces a suite of free radicals and other short-lived species including the superoxide anion, carbonate radical, singlet oxygen, hydroxyl radical, di-bromide radical anion, and a number of poorly described organic radicals and excited state triplets. These species are much more reactive than their corresponding diamagnetic and ground state species and are expected to influence biological and chemical processes in sunlit surface waters. Furthermore, DOM photolysis is an important source or sink of a variety of atmospherically important gases that are emitted from the ocean, some of which affect the Earths radioactive balance. Concentrations—and hence emissions—of carbon monoxide (CO), carbon dioxide (CO 2 ), carbonyl sulfide (OCS), and di-methyl sulfide (DMS) are all partly regulated through photochemical processes involving DOM.

Fluorescence contouring analysis of DOC intercalibration experiment samples: a comparison of techniques

Abstract The technique of 3-D fluorescence contouring was applied to samples from the DOC (dissolved organic carbon) Intercalibration Experiment using two different instrument types: a SPEX Fluorolog 2 and a Hitachi F3010. A comparison of results from the two instruments is presented for humic-like and protein-like fluorescence signals. Distinct differences in the composition of fluorescent DOC were observed between surface and deep water which may affect results of DOC measurements obtained by different analytical techniques.

Determination of photochemically produced hydroxyl radicals in seawater and freshwater

A variety of short-lived, reactive chemical species (i.e. free radicals and excited state species) are known to be photochemically produced in natural waters. Some of these transients may strongly affect chemical and biological processes, and they have been implicated in the degradation of organic pollutants and natural organic compounds in aqueous environments. Previous studies demonstrated that the highly reactive hydroxyl radical (OH) is photochemically formed in seawater. However, the quantitative importance of this key species in the sea has not been previously studied because of past analytical limitations. By using a highly sensitive probe based on α-H atom abstraction from methanol, we were able to measure production rates and steady-state concentrations of photochemically produced OH radicals in coastal and open ocean seawater and freshwaters. The validity of the method was tested by intercalibrating with an independent, OH-specific reaction, hydroxylation of benzoic acid, and also by competition kinetics experiments. Our OH production rates and steady-state concentrations for freshwaters are in excellent agreement with those measured by previous investigators for similar waters. In contrast, for seawater, the values we measured are 1–3 orders of magnitude higher than previously predicted by models, indicating that there is a major unknown photochemical OH source (s) in seawater.

Photochemical production of low-molecular-weight carbonyl compounds in seawater and surface microlayer and their air-sea exchange

Abstract Coastal and oceanic surface microlayer samples were collected using a stainless steel screen, along with subsurface bulk seawater, and were analyzed for low-molecular-weight (LMW) carbonyl compounds, including formaldehyde, acetaldehyde, propanal, glyoxal, methylglyoxal, glyoxylic acid and pyruvic acid. The enrichment factor in surface microlayer compared to corresponding subsurface seawater ranged from 1.2 to 21. A time-series measurement at a coastal site showed strong diurnal variations in concentrations of the LMW carbonyl compounds in the surface microlayer and in the enrichment factor, with maxima in the early afternoon and minima in the early morning. Exposure of samples to sunlight resulted in the higher yields of these compounds in the surface microlayer than in the bulk seawater, by a factor of 1.1–25, suggesting that the higher photoproduction rate of LMW carbonyl compounds in the surface microlayer accounts for the majority of the observed enrichment in these samples. Potential sinks include biological uptake and mixing. Air-sea exchange may be a source for soluble compounds and a sink for less soluble compounds. The enrichment of the LMW carbonyl in surface microlayer may alter their net air-sea exchange direction e.g., from the ocean as a potential sink to a source for atmospheric acetaldehyde and acetone. The residence times of the LMW carbonyl compounds in the microlayer were estimated to be on the order of tens of seconds to minutes using a modified two-layer model. However, to maintain the observed microlayer enrichment factor, the residence time should be on the order of ~ 1 hour. This prolonged residence time may be due to organic enrichment in the surface microlayer (‘organic film’) which inhibited molecular transfer of carbonyl compounds into and out of the microlayer. The deviated behavior from model prediction may also be due to changes in the apparent partition coefficients of these species as a result of thier physical and chemical interactions with organic matrix in the surface microlayer.

Sources and sinks of low molecular weight organic carbonyl compounds in seawater

Abstract Results from laboratory studies indicated that low molecular weight (LMW) carbonyl compounds, especially formaldehyde, acetaldehyde, acetone and glyoxal, can be formed in seawater by photochemical processes. Once formed, these compounds appear to be readily consumed by biota. These results suggest that concentrations of LMW carbonyl compounds should undergo diurnal variations in the illuminated layer of the sea. In support of this, diurnal fluctuations of LMW carbonyl concentrations were observed in humic-rich surface waters off the west coast of Florida over a three day sampling period using a shipboard HPLC system. Fluctuations in acetaldehyde were particularly strong and reproducible, with steady night-time concentrations of 2–3 nM and day-time concentrations reaching a maximum of 20–30 nM in the early afternoon. In contrast, diurnal fluctuations in formaldehyde were less distinct, ranging from 15 to 50 nM. The laboratory and field results are discussed in terms of biotic/abiotic sources and sinks of LMW carbonyl compounds in surface seawater. It is speculated that photooxidative cleavage of biologically refractory dissolved organic matter (DOM) in seawater to yield LMW organic fragments, such as carbonyl compounds, may be important in the breakdown and geochemical cycling of DOM in the ocean.

Effect of Humic Substance Photodegradation on Bacterial Growth and Respiration in Lake Water

ABSTRACT This study addresses how humic substance (HS) chemical composition and photoreactivity affect bacterial growth, respiration, and growth efficiency (BGE) in lake water. Aqueous solutions of HSs from diverse aquatic environments representing different dissolved organic matter sources (autochthonous and allochthonous) were exposed to artificial solar UV radiation. These solutions were added to lake water passed through a 0.7-μm-pore-size filter (containing grazer-free lake bacteria) followed by dark incubation for 5, 43, and 65 h. For the 5-h incubation, several irradiated HSs inhibited bacterial carbon production (BCP) and this inhibition was highly correlated with H2O2 photoproduction. The H2O2 decayed in the dark, and after 43 h, nearly all irradiated HSs enhanced BCP (average 39% increase relative to nonirradiated controls, standard error = 7.5%, n = 16). UV exposure of HSs also increased bacterial respiration (by ∼18%, standard error = 5%, n = 4), but less than BCP, resulting in an average increase in BGE of 32% (standard error = 10%, n = 4). Photoenhancement of BCP did not correlate to HS bulk properties (i.e., elemental and chemical composition). However, when the photoenhancement of BCP was normalized to absorbance, several trends with HS origin and extraction method emerged. Absorbance-normalized hydrophilic acid and humic acid samples showed greater enhancement of BCP than hydrophobic acid and fulvic acid samples. Furthermore, absorbance-normalized autochthonous samples showed ∼10-fold greater enhancement of BCP than allochthonous-dominated samples, indicating that the former are more efficient photoproducers of biological substrates.