Daniel C. O. Thornton

Diatom aggregation in the sea: mechanisms and ecological implications

Marine snow is a ubiquitous feature of the ocean and an important agent in the transport of energy and nutrients through marine ecosystems. Diatom aggregates, which form during blooms and, to a lesser extent, by the resuspension of benthic biofilms, are a primary source of marine snow. Genera commonly found in diatom aggregates are: Nitzschia, Chaetoceros, Rhizosolenia, Leptocylindricus, Skeletonema and Thalassionema. Most fieldwork has been restricted to a limited number of locations in the Northern Hemisphere. To quantify the global impact of diatom aggregation there is a need to conduct fieldwork in a wider range of areas, particularly in the Southern Hemisphere. Aggregates form when particles collide and stick together. Collisions in the water column are affected by turbulence, differential settlement and animal feeding, whereas diatom stickiness is affected by extracellular polymeric substances (EPS). Laboratory experiments have demonstrated that diatoms produce more EPS under nutrient limitation, although little is known about how limitation by different nutrients affects the quantity and composition of EPS and subsequent stickiness. EPS form three pools in the environment: cell coatings, soluble EPS and transparent exopolymeric particles (TEP). There is a need to investigate the dynamics of conversion between the pools of EPS by both abiotic and biological processes and how these conversions affect aggregate concentration and structure. Processes governing disaggregation have been largely overlooked, although they are as important as aggregation in determining the dynamics of aggregate concentrations in the water column. The biogeochemical significance of diatom aggregates as a means of transporting carbon and other nutrients from the euphotic zone to the seabed is well established. However, the internal biogeochemistry of aggregates is not well understood. Aggregates contain anaerobic microsites and further work is required to establish whether aggregates are significant sinks for nitrogen in the water column through anaerobic denitrification. Several hypotheses have been proposed to explain diatom aggregation in the field, but many of these are flawed because the mechanisms and adaptive explanations proposed require natural selection to operate at the level of populations rather than genes or individuals.

Dissolved organic matter (DOM) release by phytoplankton in the contemporary and future ocean

The partitioning of organic matter (OM) between dissolved and particulate phases is an important factor in determining the fate of organic carbon in the ocean. Dissolved organic matter (DOM) release by phytoplankton is a ubiquitous process, resulting in 2–50% of the carbon fixed by photosynthesis leaving the cell. This loss can be divided into two components: passive leakage by diffusion across the cell membrane and the active exudation of DOM into the surrounding environment. At present there is no method to distinguish whether DOM is released via leakage or exudation. Most explanations for exudation remain hypothetical; as while DOM release has been measured extensively, there has been relatively little work to determine why DOM is released. Further research is needed to determine the composition of the DOM released by phytoplankton and to link composition to phytoplankton physiological status and environmental conditions. For example, the causes and physiology of phytoplankton cell death are poorly understood, though cell death increases membrane permeability and presumably DOM release. Recent work has shown that phytoplankton interactions with bacteria are important in determining both the amount and composition of the DOM released. In response to increasing CO2 in the atmosphere, climate change is creating increasingly stressful conditions for phytoplankton in the surface ocean, including relatively warm water, low pH, low nutrient supply and high light. As ocean physics and chemistry change, it is hypothesized that a greater proportion of primary production will be released directly by phytoplankton into the water as DOM. Changes in the partitioning of primary production between the dissolved and particulate phases will have bottom-up effects on ecosystem structure and function. There is a need for research to determine how these changes affect the fate of organic matter in the ocean, particularly the efficiency of the biological carbon pump.

Positive priming of terrestrially derived dissolved organic matter in a freshwater microcosm system

The role of priming processes in the remineralization of terrestrially derived dissolved organic carbon (TDOC) in aquatic systems has been overlooked. We provide evidence for TDOC priming using a lab-based microcosm experiment in which TDOC was primed by the addition of 13C-labeled algal dissolved organic carbon (ADOC) or a 13C-labeled disaccharide (trehalose). The rate of TDOC remineralization to carbon dioxide (CO2) occurred 4.1 ± 0.9 and 1.5 ± 0.3 times more rapidly with the addition of trehalose and ADOC, respectively, relative to experiments with TDOC as the sole carbon source over the course of a 301 h incubation period. Results from these controlled experiments provide fundamental evidence for the occurrence of priming of TDOC by ADOC and a simple disaccharide. We suggest that priming effects on TDOC should be considered in carbon budgets for large-river deltas, estuaries, lakes, hydroelectric reservoirs, and continental shelves.

Pollen as atmospheric cloud condensation nuclei

Anemophilous (wind-dispersed) pollen grains are emitted in large quantities by vegetation in the midlatitudes for reproduction. Pollen grains are coarse particles (5–150 µm) that can rupture when wet to form submicron subpollen particles (SPP) that may have a climatic role. Laboratory CCN experiments of six fresh pollen samples show that SPP activate as CCN at a range of sizes, requiring supersaturations from 0.81 (± 0.07)% for 50 nm particles, 0.26 (± 0.03)% for 100 nm particles, and 0.12 (± 0.00)% for 200 nm particles. Compositional analyses indicate that SPP contain carbohydrates and proteins. The SPP contribution to global CCN is uncertain but could be important depending on pollen concentrations outside the surface layer and the number of SPP generated from a single pollen grain. The production of hygroscopic SPP from pollen represents a novel, biologically driven cloud formation pathway that may influence cloud optical properties and lifetimes, thereby influencing climate.

Effect of low pH on carbohydrate production by a marine planktonic diatom (Chaetoceros muelleri).

Rising carbon dioxide (CO2) concentrations in the atmosphere due to human activity are causing the surface ocean to become more acidic. Diatoms play a pivotal role in biogeochemical cycling and ecosystem function in the ocean. pH affected the quantum efficiency of photosystem II and carbohydrate metabolism in a planktonic diatom (Chaetoceros muelleri), representative of a widely distributed genus. In batch cultures grown at low pH, the proportion of total carbohydrate stored within the cells decreased and more dissolved carbohydrates were exuded from the cells into the surrounding medium. Changes in productivity and the way in which diatoms allocate carbon into carbohydrates may affect ecosystem function and the efficiency of the biological carbon pump in a low pH ocean.

Prediction of sporadic mucilaginous algal blooms in the Northern adriatic sea

In the eutrophic waters of the Northern Adriatic, occasional, large scale mucilage events cause economic and ecological damage. A hypothesis linking biology, hydrography and water chemistry at the mouth of the Po River is proposed to explain the sporadic occurrence of mucilaginous algal blooms. We propose that the formation of stable mucilaginous macroaggregates associated with the blooms occurs in the following temporal sequence: low flow in the Po River leads to nutrient limitation offshore. Mucilage is produced by nutrient limited phytoplankton. Stable hydrographic conditions (which are consonant with low river flows) allow the development of mucous macroaggregates under conditions of low calcium concentration. A subsequent period of hydrographic mixing with the relatively high calcium concentrations in seawater causes stabilisation of the macroaggregates. Seawater circulation then causes the movement of stable mucilage aggregates in a southerly direction parallel to the coast or onto the beaches.

Using Raman Microspectroscopy to Determine Chemical Composition and Mixing State of Airborne Marine Aerosols over the Pacific Ocean

Chemical composition and mixing state of aerosols collected over an 11,000 km latitudinal cruise in the Pacific Ocean are reported here as determined by a new application of Raman spectroscopy. The Raman microspectroscopy technique employs a Raman spectrometer coupled to an optical microscope to identify the chemical composition and internal mixing state of single particles. By analyzing multiple particles in a collected ensemble, the degree of external mixing of particles was also determined. To lend context to the Pacific aerosol population sampled, atmospheric aerosol concentration, and the critical supersaturation required for the aerosols to activate as cloud condensation nuclei, and chlorophyll a concentration in the underlying water (a metric for phytoplankton biomass in the ocean) were also obtained. Our results indicate that long chain organic molecules were prevalent in the marine aerosol samples throughout the cruise, including during coastal and open ocean locations, in both hemispheres, and in the seasons of autumn and spring. Long chain organic compounds tended to be present in internal mixtures with other organic and inorganic components. Although variations in the fraction of aerosols activated as CCN were observed, no simple correlation between organics and CCN activation was found. According to our measurements, marine aerosol in the Pacific Ocean may be generally characterized as multicomponent aerosol containing and often dominated by a high organic fraction. Our results suggest that the prevalence of organics and the high degree of internal mixing of aerosol must be accounted for in accurate modeling of the role of marine aerosols in cloud formation and climate. Copyright 2014 American Association for Aerosol Research

Individuals, clones or groups? Phytoplankton behaviour and units of selection

Over the last 20 years, several aspects of phytoplankton physiology and behaviour have been explained as mechanisms of nutrient acquisition, defence and dispersal that require the pooling of resources by populations. The aim of this paper is to review these hypotheses, which assume that selection is operating at a higher level than the gene or individual, thus implying group selection or that individual phytoplankton cells are genetically identical. Group selection is considered to be a weak force, often in opposition to individual selection, therefore it has been assumed that phytoplankton populations are essentially clonal. However, evidence is growing that phytoplankton blooms are not clones. Given the importance of phytoplankton to biogeochemical cycles and energy fluxes through the biosphere, there is a need to reconcile hypotheses explaining phytoplankton physiology and behaviour with the genetic data. Phytoplankton blooms should be regarded as populations of individuals rather than a single genetically homogeneous entity.

Atmospheric Ammonia Mixing Ratios at an Open-Air Cattle Feeding Facility

Abstract Mixing ratios of total and gaseous ammonia were measured at an open-air cattle feeding facility in the Texas Panhandle in the summers of 2007 and 2008. Samples were collected at the nominally upwind and downwind edges of the facility. In 2008, a series of far-field samples was also collected 3.5 km north of the facility. Ammonium concentrations were determined by two complementary laboratory methods, a novel application of visible spectrophotometry and standard ion chromatography (IC). Results of the two techniques agreed very well, and spectrophotometry is faster, easier, and cheaper than chromatography. Ammonia mixing ratios measured at the immediate downwind site were drastically higher (∼2900 parts per billion by volume [ppbv]) than those measured at the upwind site (≤200 ppbv). In contrast, at 3.5 km away from the facility, ammonia mixing ratios were reduced to levels similar to the upwind site (≤200 ppbv). In addition, PM10 (particulate matter <10 μm in optical diameter) concentrations obtained at each sampling location using Grimm portable aerosol spectrometers are reported. Time-averaged (1-hr) volume concentrations of PM10 approached 5 × 1012 nm3 cm−3. Emitted ammonia remained largely in the gas phase at the down-wind and far-field locations. No clear correlation between concentrations of ammonia and particles was observed. Overall, this study provides a better understanding of ammonia emissions from open-air animal feeding operations, especially under the hot and dry conditions present during these measurements.

Dissolved Organic Matter Composition Drives the Marine Production of Brominated Very Short-Lived Substances

Brominated very short-lived substances (BrVSLS), such as bromoform, are important trace gases for stratospheric ozone chemistry. These naturally derived trace gases are formed via bromoperoxidase-mediated halogenation of dissolved organic matter (DOM) in seawater. Information on DOM type in relation to the observed BrVSLS concentrations in seawater, however, is scarce. We examined the sensitivity of BrVSLS production in relation to the presence of specific DOM moieties. A total of 28 model DOM compounds in artificial seawater were treated with vanadium bromoperoxidase (V-BrPO). Our results show a clear dependence of BrVSLS production on DOM type. In general, molecules that comprise a large fraction of the bulk DOM pool did not noticeably affect BrVSLS production. Only specific cell metabolites and humic acid appeared to significantly enhance BrVSLS production. Amino acids and lignin phenols suppressed enzyme-mediated BrVSLS production and may instead have formed halogenated nonvolatile molecules. Dibromomethane production was not observed in any experiments, suggesting it is not produced by the same pathway as the other BrVSLS. Our results suggest that regional differences in DOM composition may explain the observed BrVSLS concentration variability in the global ocean. Ultimately, BrVSLS production and concentrations are likely affected by DOM composition, reactivity, and cycling in the ocean.