Gillian Malin

A biogeochemical study of the coccolithophore, Emiliania huxleyi, in the North Atlantic

The biogeochemical properties of an extensive bloom (∼250,000 km2) of the coccolithophore, Emiliania huxleyi, in the north east Atlantic Ocean were investigated in June 1991. Satellite (NOAA-AVHRR) imagery showed that the bloom was centered initially at 60°–63°N by 13°–28°W and lasted approximately 3 weeks. Spatial variations in satellite-measured reflectance were well correlated with surface measurements of the beam attenuation coefficient, levels of particulate inorganic carbon, and coccolith density. Rates of both photosynthesis and calcification were typically relatively low within the coccolithophore-rich waters, suggesting the population was in a late stage of development at the time of the field observations. Levels of dimethyl sulphide (DMS) in surface waters were high compared to average ocean values, with the greatest concentrations in localized areas characterized by relatively high rates of photosynthesis, calcification, and grazing by microzooplankton. The estimated spatially averaged flux of DMS to the atmosphere was 1122 nmol m−2 h−1, somewhat greater than that determined for the same region in June-July 1987. Coccolith production (1 × 106 tonnes calcite-C) had a significant impact on the state of the CO2 system, causing relative increases of up to 50 μatm in surface pCO2 in association with alkalinity and water temperature changes. Gradients in pCO2 were as great as 100 μatm over horizontal distances of 20–40 km. The environmental implications of these findings are discussed in relation to the spatial and temporal distributions of E. huxleyi.

Dimethylsulphide and dimethylsulphoniopropionate in the Northeast atlantic during the summer coccolithophore bloom

Abstract Concentrations of dimethylsulphide (DMS) and dissolved and particulate pools of its precursor, dimethylsulphoniopropionate (DMSP), were surveyed at the time of the summer bloom of coccolithophores in the Northeast Atlantic. The average DMS concentration was 12 nmol dm −3 ( n = 158, range 1.06−93.8 nmol dm −3 , σn − 1 = 12.4). Statistically significant positive correlations between particulate DMSP and chlorophyll were found for samples from areas where coccolithophores accounted for 50% or more of the total carbon biomass. In these areas correlations between DMS and chlorophyll were not as strong but still significant. An estimate of the flux of DMS from the Northeast Atlantic in June–July (721 nmol m −2 h −1 ) is of the same order as estimates for the southern North Sea at the same time of year (646 nmol m −2 h −1 ). The data provide strong evidence for the importance of coccolithophores in the emission of DMS to the atmosphere. Comparison of flux data with budgets for airborne sulphur in Europe, reported by the European Monitoring and Evaluation Programme (EMEP), suggests that in summer the Northeast Atlantic may be a source of the sulphur deposited on adjacent land areas not strongly affected by anthropogenic sulphur sources. The overall results are discussed in relation to present knowledge of the global distribution of coccolithophores.

ALGAL PRODUCTION OF DIMETHYL SULFIDE AND ITS ATMOSPHERIC ROLE1

It has long been recognized that algae play a highly significant role in the global biogeochemical cycles of oxygen, carbon, nitrogen, phosphorus, and sulfw. The key chemical compounds driving these cycles are often low molecular weight and/or volatile species. In the case of sulfur, a dominant compound is dimethyl sulfide (DMS), which derives from dimethylsulfoniopropionate (DMSP) , an organic osmolyte that acts as a compatible solute in algal cells. High concentrations of DMS and DMSP can be found in areas of high algal productivity, such as marine phytoplankton blooms. In recent years research has unveiled greater detail concerning the biosynthesis and turnover of DMSP, DMS, and related compounds in the biosphere and with respect to the role of DMS in the global sulfur cycle and climate. In this minireview our aim is to concentrate mainly on these more recent discoveries.

Coccolithovirus (Phycodnaviridae): Characterisation of a new large dsDNA algal virus that infects Emiliana huxleyi

Summary. Emiliania huxleyi-specific viruses (EhV) were isolated from E. huxleyi blooms off the coast of Plymouth, UK, in July 1999 and July/August 2001, and from an E. huxleyi bloom induced during a mesocosm experiment in a fjord off Bergen, Norway, during June 2000. Transmission electron microscopy revealed that all 10 virus isolates are 170–200 nm in diameter with an icosahedral symmetry. Their density is approximately 1.2 in CsCl gradients and they have large double stranded DNA genomes approximately 410 kb in size. Phylogenetic analysis of the DNA polymerase genes of these viruses suggests that EhV belongs to a new genus within the family of algal viruses, Phycodnaviridae. We propose to name this new virus genus Coccolithovirus. Differences within members of the Coccolithovirus were elucidated by host range analysis of the virus isolates and sequence analysis of a gene fragment encoding part of their putative major capsid protein. All 10 virus isolates within this new genus only infected E. huxleyi strains that have previously been shown to exhibit low dimethylsulphoniopropionate lyase (DMSP-lyase) activity (CCMP1516, CCMP374 and L), while E. huxleyi strains with high DMSP-lyase activity (CCMP373 and CCMP379) were resistant to infection.

Isolation of viruses responsible for the demise of an Emiliania huxleyi bloom in the English Channel

This study used analytical flow cytometry (AFC) to monitor the abundance of phytoplankton, coccoliths, bacteria and viruses in a transect that crossed a high reflectance area in the western English Channel. The high reflectance area, observed by satellite, was caused by the demise of an Emiliania huxleyi bloom. Water samples were collected from depth profiles at four stations, one station outside and three stations inside the high reflectance area. Plots of transect data revealed very obvious differences between Station 1, outside, and Stations 2–4, inside the high reflectance area. Inside, concentrations of viruses were higher; E. huxleyi cells were lower; coccoliths were higher; bacteria were higher and virus:bacteria ratio was lower than at Station 1, outside the high reflectance area. This data can simply be interpreted as virus-induced lysis of E. huxleyi cells in the bloom causing large concentrations of coccoliths to detach, resulting in the high reflectance observed by satellite imagery. This interpretation was supported by the isolation of two viruses, Eh V84 and Eh V86, from the high reflectance area that lysed cultures of E. huxleyi host strain CCMP1516. Basic characterization revealed that they were lytic viruses approximately 170 nm–190 nm in diameter with an icosahedral symmetry. Taken together, transect and isolation data suggest that viruses were the major contributor to the demise of the E. huxleyi population in the high reflectance area. Close coupling between microalgae, bacteria and viruses contributed to a large organic carbon input. Consequent cycling influenced the succession of an E. huxleyi -dominated population to a more characteristic mixed summer phytoplankton community.

Sulfur: The plankton/climate connection

Question (Aiken et al.): Given the uncertainties involved in the calculation of air-sea exchange of gases such as DMS, is it possible to assess by other methods the importance of natural versus anthropogenic sulfur emissions? Answer: In our paper we discuss various approaches that have been taken to estimate the rate of emission of DMS from the oceans including models (Erikson et al. 1990, Thompson et al. 1990) and the use of observed concentration fields of DMS combined with knowledge of air-sea transfer velocity (Andreae 1986, Bates et al. 1987b). In addition measurements of MSA in atmospheric aerosols can be used to infer the emission of DMS into the atmosphere (J. Prospero, University of Miami, pers. commun.). Many studies (not reviewed in this paper) have investigated the emissions of sulfur from manmade sources. In all these attempts, an assessment is often made of the relative importance of the two sources, although the methodology used to calculate biogenic and anthropogenic sources are fundamentally different. The only consistent approach we are aware of, which has the potential to directly ascribe sulfur in the atmosphere to its major sources, is through the use of sulfur isotope signature measurements. The method relies on the sulfur isotope signature of fossil fuels being significantly different from that of DMS and its oxidation products. This approach shows great promise and is currently being investigated in our laboratory.

Interlaboratory calibration and sample analysis of dimethyl sulphide in water

An intercalibration exercise carried out by the Universities of Stockholm and East Anglia, for the determination of natural levels of dimethyl sulphide in aqueous samples, is described. Good agreement between the two groups was obtained for natural samples, but for cultures containing high numbers of marine phytoplankton some differences were observed. The problems associated with the analysis of samples with high densities of phytoplankton are discussed. Four calibration techniques were tested, and their relative merits are assessed.

Virus Succession Observed during an Emiliania huxleyi Bloom

ABSTRACT Denaturing gradient gel electrophoresis was used as a molecular tool to determine the diversity and to monitor population dynamics of viruses that infect the globally important coccolithophorid Emiliania huxleyi. We exploited variations in the major capsid protein gene from E. huxleyi-specific viruses to monitor their genetic diversity during an E. huxleyi bloom in a mesocosm experiment off western Norway. We reveal that, despite the presence of several virus genotypes at the start of an E. huxleyi bloom, only a few virus genotypes eventually go on to kill the bloom.

Viral infection of Emiliania huxleyi (Prymnesiophyceae) leads to elevated production of reactive oxygen species

The effect of viral infection of Emiliania huxleyi (Lohman) Hay and Mohler on the concentration of intracellular reactive oxygen species (ROS), hydrogen peroxide (H2O2) excretion and cell photosynthetic capacity (CPC) was examined. During the crash of an E. huxleyi culture induced by viruses intracellular ROS concentrations were generally elevated and reached levels of approximately double those observed in non‐infected control cultures. H2O2 concentrations also increased in the media of the infected cultures from background levels of around 130 nM to approximately 580 nM while levels in the controls decreased. These data suggest that oxidative stress is elevated in infected cells. Although the precise mechanism for ROS production was not identified, a traditional defense related oxidative burst was ruled out, as no evidence of a rapid intracellular accumulation of ROS following addition of the virus was found. CPC declined substantially in the infected culture from a healthy 0.6–0 arbitrary units. Clearly infection disrupted normal photosynthetic processes, which could lead to the production of ROS via interruption of the electron transport chain at the PSII level. Alternatively, ROS may also be a necessary requirement for viral replication in E. huxleyi, possibly due to a link with viral‐induced cell death or associated with general death processes.

A genetic marker to separate Emiliania huxleyi (Prymnesiophyceae) morphotypes

Emiliania huxleyi (Lohm.) Hay and Mohler is a ubiquitous unicellular marine alga surrounded by an elaborate covering of calcite platelets called coccoliths. It is an important primary producer involved in oceanic biogeochemistry and climate regulation. Currently, E. huxleyi is separated into five morphotypes based on morphometric, physiological, biochemical, and immunological differences. However, a genetic marker has yet to be found to characterize these morphotypes. With the use of sequence analysis and denaturing gradient gel electrophoresis, we discovered a genetic marker that correlates significantly with the separation of the most widely recognized A and B morphotypes. Furthermore, we reveal that the A morphotype is composed of a number of distinct genotypes. This marker lies within the 3′ untranslated region of a coccolith associated protein mRNA, which is implicated in regulating coccolith calcification. Consequently, we tentatively termed this marker the coccolith morphology motif.