Production In Aquatic Environments Research Articles

More than just geosmin and 2‐methylisoborneol? Off‐flavours associated with recirculating aquaculture systems

AbstractThe consumption of seafood is driven by flavour, yet achieving its high quality remains a challenge for many species reared in recirculating aquaculture systems (RAS). A comprehensive knowledge of off‐odour sources in aquatic foods is indispensable in ensuring flavour quality standards. At the beginning of the production chain, early post‐harvest lipid oxidation products develop into endogenous off‐odours and accumulate over time. These malodours add to those already absorbed exogenously, namely from the water and feeds, although the information on the interactions between these sources is currently scarce. Despite geosmin and 2‐methylisoborneol receiving significant attention in relation to fish off‐flavour, only limited knowledge on the molecular and ecological mechanisms driving their production in aquatic environments has been reported. Moreover, RAS‐hosted bacteria have been associated with a wide range of other odour‐active compounds, such as pyrazines, terpenoids, and other degradation byproducts that are frequently overlooked when studying flavour taint in fish. The influence of aquaculture feeds on the flavour of fish flesh has been underestimated, too, both as a source of off‐odours but also as a novel modulator strategy to achieve desirable aquatic food flavours. Finally, the influence of water treatment processes widely used in RAS operations, such as advance oxidation process, ozone, ultraviolet and hydrogen peroxide disinfections, is greatly underexplored with respect to odour quality. This article reviews the current scientific evidence with supporting data on the chemical diversity of off‐odours found in aquaculture fish worldwide and their potential sources and highlights knowledge gaps that should be addressed in future research.

Functional genes and microorganisms controlling in situ methylmercury production and degradation in marine sediments: A case study in the Eastern China Coastal Seas

Dominant microorganisms and functional genes, including hgcA, hgcB, merA, and merB, have been identified to be responsible for mercury (Hg) methylation or methylmercury (MeHg) demethylation. However, their in situ correlation with MeHg levels and the processes of Hg methylation and MeHg demethylation in coastal areas remains poorly understood. In this study, four functional genes related to Hg methylation and MeHg demethylation (hgcA, hgcB, merA, and merB) were all detected in the sediments of the Eastern China Coastal Seas (ECCSs) (representative coastal seas highly affected by human activities) using metagenomic approaches. HgcA was identified to be the key gene controlling the in situ net production of MeHg in the ECCSs. Based on metagenomic analysis and incubation experiments, sulfate-reducing bacteria were identified as the dominant microorganisms controlling Hg methylation in the ECCSs. In addition, hgcA gene was positively correlated with the MeHg content and Hg methylation rates, highlighting the potential roles of Hg methylation genes and microorganisms influenced by sediment physicochemical properties in MeHg cycling in the ECCSs. These findings highlighted the necessity of conducting similar studies in other natural systems for elucidating the molecular mechanisms underlying MeHg production in aquatic environments.

Assessment of Common Cyanotoxins in Cyanobacteria of Biological Loess Crusts.

Cyanotoxins are a diverse group of bioactive compounds produced by cyanobacteria that have adverse effects on human and animal health. While the phenomenon of cyanotoxin production in aquatic environments is well studied, research on cyanotoxins in terrestrial environments, where cyanobacteria abundantly occur in biocrusts, is still in its infancy. Here, we investigated the potential cyanotoxin production in cyanobacteria-dominated biological loess crusts (BLCs) from three different regions (China, Iran, and Serbia) and in cyanobacterial cultures isolated from the BLCs. The presence of cyanotoxins microcystins, cylindrospermopsin, saxitoxins, and β-N-methylamino-L-alanine was analyzed by liquid chromatography-tandem mass spectrometry (LC-MS/MS) method, while the presence of cyanotoxin-encoding genes (mcyE, cyrJ, sxtA, sxtG, sxtS, and anaC) was investigated by polymerase chain reaction (PCR) method. We could not detect any of the targeted cyanotoxins in the biocrusts or the cyanobacterial cultures, nor could we amplify any cyanotoxin-encoding genes in the cyanobacterial strains. The results are discussed in terms of the biological role of cyanotoxins, the application of cyanobacteria in land restoration programs, and the use of cyanotoxins as biosignatures of cyanobacterial populations in loess research. The article highlights the need to extend the field of research on cyanobacteria and cyanotoxin production to terrestrial environments.

Arthur James McComb 1936–2017

Professor Arthur McComb conducted pioneer research on the occurrence and mode of action of the plant growth hormones gibberellins for fifteen years. He then applied his experimental skills and physiological knowledge to develop a whole ecosystem approach to the study of aquatic systems. He was passionate in wanting to improve the state of environmental management, based on rational, logical and well-founded biological principles. He and his team focused primarily on the mechanisms controlling plant growth and productivity in aquatic environments, and especially the effects of nutrient enrichment and its consequences, eutrophication. He became a leader in nutrient analysis of water systems, with innovations in how to determine nutrient pathways into waterways and strategies for fixing these issues. This important research has informed the long-term management of several important aquatic systems in Western Australia: the Blackwood River Estuary, the Peel Harvey Estuary, and the protection of seagrasses in Shark Bay, the Swan River and Cockburn Sound. Arthur McComb had a seminal influence on a generation of researchers. Thirty-nine students completed their higher degrees under his supervision and they are spread internationally and throughout Australia in universities, state government departments and consulting firms, confirming his influence on driving our understanding and management of marine, estuarine and freshwater systems.

Effects of silver nanoparticles on nitrification and associated nitrous oxide production in aquatic environments

Silver nanoparticles (AgNPs) are the most common materials in nanotechnology-based consumer products globally. Because of the wide application of AgNPs, their potential environmental impact is currently a highly topical focus of concern. Nitrification is one of the processes in the nitrogen cycle most susceptible to AgNPs but the specific effects of AgNPs on nitrification in aquatic environments are not well understood. We report the influence of AgNPs on nitrification and associated nitrous oxide (N2O) production in estuarine sediments. AgNPs inhibited nitrification rates, which decreased exponentially with increasing AgNP concentrations. The response of nitrifier N2O production to AgNPs exhibited low-dose stimulation (<534, 1476, and 2473 μg liter-1 for 10-, 30-, and 100-nm AgNPs, respectively) and high-dose inhibition (hormesis effect). Compared with controls, N2O production could be enhanced by >100% at low doses of AgNPs. This result was confirmed by metatranscriptome studies showing up-regulation of nitric oxide reductase (norQ) gene expression in the low-dose treatment. Isotopomer analysis revealed that hydroxylamine oxidation was the main N2O production pathway, and its contribution to N2O emission was enhanced when exposed to low-dose AgNPs. This study highlights the molecular underpinnings of the effects of AgNPs on nitrification activity and demonstrates that the release of AgNPs into the environment should be controlled because they interfere with nitrifying communities and stimulate N2O emission.

Extensive Dark Biological Production of Reactive Oxygen Species in Brackish and Freshwater Ponds.

Within natural waters, photodependent processes are generally considered the predominant source of reactive oxygen species (ROS), a suite of biogeochemically important molecules. However, recent discoveries of dark particle-associated ROS production in aquatic environments and extracellular ROS production by various microorganisms point to biological activity as a significant source of ROS in the absence of light. Thus, the objective of this study was to explore the occurrence of dark biological production of the ROS superoxide (O2(-)) and hydrogen peroxide (H2O2) in brackish and freshwater ponds. Here we show that the ROS superoxide and hydrogen peroxide were present in dark waters at comparable concentrations as in sunlit waters. This suggests that, at least for the short-lived superoxide species, light-independent processes were an important control on ROS levels in these natural waters. Indeed, we demonstrated that dark biological production of ROS extensively occurred in brackish and freshwater environments, with greater dark ROS production rates generally observed in the aphotic relative to the photic zone. Filtering and formaldehyde inhibition confirmed the biological nature of a majority of this dark ROS production, which likely involved phytoplankton, particle-associated heterotrophic bacteria, and NADH-oxidizing enzymes. We conclude that biological ROS production is widespread, including regions devoid of light, thereby expanding the relevance of these reactive molecules to all regions of our oxygenated global habit.

Reservoirs Act as a Source for Greenhouse Gases

Scientists examine nitrous oxide production in aquatic environments and the conditions that drive it.

Alkaline phosphatase activity at the southwest coast of India: A comparison of locations differently affected by upwelling

The realization of the potential importance of phosphorus (P) as a limiting nutrient in marine ecosystem is increasing globally. Hence, the contribution of biotic variables in mobilizing this nutrient would be relevant especially in productive coastal waters. As alkaline phosphatase activity (APA) indicates the status of P for primary production in aquatic environments, we asked the following question: is the level of APA indicative of P sufficiency or deficiency in coastal waters, especially, where upwelling is a regular phenomenon? Therefore, we have examined the total APA, chlorophyll a along with phosphatase producing bacteria (PPB) and related environmental parameters from nearshore to offshore in coastal waters off Trivandrum and Kochi regions differently affected by upwelling during the onset of monsoon. Off Trivandrum, APA in the offshore waters of 5-m layer at 2.23μMPh−1 was >4 times higher than nearshore. Thus, low APA could be indicative of P sufficiency in coastal waters and higher activity suggestive of deficiency in offshore waters off Trivandrum. In contrast, there was less difference in APA between near and offshore surface waters off Kochi. Our results show that the regions differently affected by upwelling respond differently according to ambient P concentration, distance from shore or depth of water. These observations could apparently be applicable to other coastal systems as well, where gradients in upwelling and phosphate runoff have been noticed. Further studies on other transects would throw more light on the extent and direction of the relationship between APA and ambient P concentration. Such studies would help in understanding the level of control of this nutrient on the productivity of coastal waters.

In situ production of charophyte communities under reduced light conditions in a brackish-water ecosystem

INTRODUCTION Charophyte communities are an important element in shallow enclosed fresh- and brackish-water ecosystems (Mathieson and Nienhuis, 1991; van den Berg et al., 1998; Pelechaty et al., 2006). They provide shelter and habitat for numerous species including epiphytic microalgae, filamentous macroalgae, as well as various crustacean and insect species (Linden et al., 2003; Schmieder et al., 2006; Torn et al., 2010). Besides, charophytes are an important component in the food web as part of the diet of benthic invertebrates (Kotta et al., 2004, 2013), waterfowl (Noordhuis et al., 2002; Schmieder et al., 2006), and fish and fish larvae (de Winton et al., 2002; Dugdale et al., 2006). Declining distribution and diversity of charophytes have been observed in many regions worldwide including the brackish Baltic Sea (Blindow, 2000, 2001; Schubert and Blindow 2003; Munsterhjelm, 2005). Eutrophication is assumed to be the most important threat to charophytes causing their decline (e.g. Blindow, 1992; Auderset Joye et al., 2002). The main effect associated with eutrophication is the bloom of ephemeral planktonic algae, which leads to increased sedimentation, water turbidity and, as a result, reduced light availability. The shortage of light may reduce the photosynthetic production and growth of charophytes down to the level where their sustainable development becomes impossible (Blindow et al., 2002; Johnsen and Sosik, 2004; Hautier et al., 2009; Dickey et al., 2011). On the other hand, charophytes often prefer soft bottom habitats where even moderate wind may cause sediment resuspension and sedimentation of particles on the plant surface. In such habitats underwater light climate is naturally very variable (Schneider et al., 2006 and references therein). Thus, charophytes are adapted to periodic stress of low light intensities. Nevertheless, the interactive effect of elevated eutrophication and weather variables may result in poorer light conditions than expected from their separate effects (Blindow et al., 2003; Kling et al., 2003). So far, the studies concerning photosynthesis of charophytes are mainly based on laboratory experiments with either detached pieces or single individuals (e.g. Blindow et al., 2003; Marquardt and Schubert, 2009). Very few have been carried out in the natural environments, especially in brackish bodies of water. As compared to their freshwater counterparts, charophytes are often naturally stressed at elevated salinity and therefore are expected to respond differently to changes in light conditions (e.g. Blindow et al., 2003). The existing data on in situ primary production of charophytes related to light limitation are scarce and hardly comparable because of difference in methodologies and the environmental conditions among habitats (Kufel and Kufel, 2002). Light is a key limiting factor for photosynthetic production in aquatic environments (Kurtz et al., 2003; Asaeda et al., 2004, Binzer et al., 2006; Zhang et al., 2010). Earlier experimental studies carried out at the community level have also shown that canopy density and canopy structure significantly affect the photosynthetic production of marine macroalgae (Middelboe et al., 2006). This suggests that macroalgal communities are largely light-limited and such light limitation increases with canopy height and/or community biomass (Parnoja et al., 2013). Altough the photosynthetic production of marine macroalgae at the community level has been increasingly studied (Middelboe and Binzer, 2004; Middelboe et al., 2006; Parnoja et al., 2013), to the best of our knowledge, there is only a single study on charophyte communities (Libbert and Walter, 1985). Based on the above, our goal was to determine the primary production of a charophyte community under manipulated in situ light conditions. We hypothesized that (a) the community would have higher responses under more severe light limitation and (b) the recovery of charophyte photosynthetic performance would be faster under less severe disturbances. …

Seasonal variation and interaction of photodegradation and microbial metabolism of DOC in black water Amazonian ecosystems

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 70:157-168 (2013) - DOI: https://doi.org/10.3354/ame01651 Seasonal variation and interaction of photodegradation and microbial metabolism of DOC in black water Amazonian ecosystems João Henrique F. Amaral1,*, Albert L. Suhett2,3, Sérgio Melo1,4, Vinicius F. Farjalla2 1Laboratório de Ecossistemas Aquáticos, Instituto Nacional de Pesquisas da Amazônia INPA-V8, CPEC, Av. Ephigênio Sales, 2239 Aleixo - Manaus, Amazonas 69060-020, Brazil 2Laboratório de Limnologia, Depto de Ecologia, Instituto de Biologia, Universidade Federal do Rio de Janeiro, Ilha do Fundão, Rio de Janeiro, Rio de Janeiro 21941-590, Brazil 3Present address: Unidade de Biotecnologia e Ciências Biológicas, Centro Universitário Estadual da Zona Oeste, Av. Manuel Caldeira de Alvarenga 1203, Campo Grande, Rio de Janeiro, Rio de Janeiro 23070-200, Brazil 4Present address: Instituto de Ciências e Tecnologia das Águas, Universidade Federal do Oeste do Pará, Campus Tapajós, Av. Vera Paz, s/n, Bairro Salé, Santarém, Pará 68035-110, Brazil *Email: jh.amaral@gmail.com ABSTRACT: Photodegradation of dissolved organic carbon (DOC) can generate labile substrates readily available for microbial consumption, thus increasing DOC removal, especially in freshwater humic ecosystems. While a few studies have evaluated the effects of sunlight on DOC removal and CO2 production in aquatic environments, none have investigated the seasonal variation and interaction of photodegradation and microbial metabolism of DOC in a large tropical black-water river system. We present the results of experiments designed to evaluate the rates of photodegradation and subsequent microbial metabolism of DOC in the Negro River and an associated floodplain lake (Lake Tupé) in the central Brazilian Amazon Basin. Water samples collected in both environments at different phases of the river hydrological cycle were filtered and exposed to natural sunlight to estimate photodegradation; they were then inoculated with natural bacteria and incubated in the dark to evaluate bacterial metabolism. Changes in incident solar radiation and in DOC concentration and quality throughout the hydrological cycle directly affected the DOC photodegradation rates and microbial metabolism. Total DOC mineralization (photodegradation plus bacterial consumption) was more intense in the falling water period. DOC photodegradation generally stimulated further microbial DOC degradation, enhancing total DOC removal in samples exposed to solar radiation in both ecosystems. While direct photodegradation represented only a small part of the total DOC mineralization (6.7% in the high water period in the Negro River), the combined effect of photodegradation and stimulus of bacterial metabolism could account for a significant part of the CO2 production in Amazonian black water ecosystems. KEY WORDS: DOC photodegradation · Bacterial metabolism · Amazon · Carbon mineralization · Seasonality · Hydrological cycle Full text in pdf format PreviousNextCite this article as: Amaral JHF, Suhett AL, Melo S, Farjalla VF (2013) Seasonal variation and interaction of photodegradation and microbial metabolism of DOC in black water Amazonian ecosystems. Aquat Microb Ecol 70:157-168. https://doi.org/10.3354/ame01651 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 70, No. 2. Online publication date: August 22, 2013 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2013 Inter-Research.

Anaerobic oxidation of Hg(0) and methylmercury formation by Desulfovibrio desulfuricans ND132

The transformation of inorganic mercury (Hg) to methylmercury (MeHg) plays a key role in determining the amount of Hg that is bioaccumulated in aquatic food chains. An accurate knowledge of Hg methylation mechanisms is required to predict the conditions that promote MeHg production in aquatic environments. In this study, we conducted experiments to examine the oxidation and methylation of dissolved elemental mercury [Hg(0)] by the anaerobic bacterium Desulfovibrio desulfuricans ND132. Anoxic cultures of D. desulfuricans ND132 were exposed to Hg(0) in the dark, and samples were collected and analyzed for the loss of Hg(0), formation of non-purgeable Hg, and formation of MeHg over time. We found that D. desulfuricans ND132 rapidly transformed dissolved gaseous mercury into non-purgeable Hg, with bacterial cultures producing approximately 40μg/L of non-purgeable Hg within 30min, and as much as 800μg/L of non-purgeable Hg after 36h. Derivatization of the non-purgeable Hg in the cell suspensions to diethylmercury and analysis of Hg(0)-reacted D. desulfuricans ND132 cells using X-ray absorption near edge structure (XANES) spectroscopy demonstrated that cell-associated Hg was dominantly in the oxidized Hg(II) form. Spectral comparisons and linear combination fitting of the XANES spectra indicated that the oxidized Hg(II) was covalently bonded to cellular thiol functional groups. MeHg analyses revealed that D. desulfuricans ND132 produced up to 118μg/L of methylmercury after 36h of incubation. We found that a significant fraction of the methylated Hg was exported out of the cell and released into the culture medium. The results of this work demonstrate a previously unrecognized pathway in the mercury cycle, whereby anaerobic bacteria produce MeHg when provided with dissolved Hg(0) as their sole Hg source.

From sink to resurgence: the buffering capacity of a cave system in the Tongass national forest, USA

The Tongass National Forest of Southeast Alaska, USA, provides a unique environment for monitoring the impact of the cave system on water quality and biological productivity. The accretionary terrane setting of the area has developed into a complex and heterogeneous geologic landscape which includes numerous blocks of limestone with intense karstification. During the Wisconsian glaciation, there were areas of compacted glacial sediments and silts deposited over the bedrock. Muskeg peatlands developed over these poorly drained areas. The dominant plants of the muskeg ecosystem are Sphagnum mosses, whose decomposition leads to highly acidic waters with pH as low as 2.4. These waters drain off the muskegs into the cave systems, eventually running to the ocean. In accordance with the Tongass Land Management Plan, one of the research priorities of the National Forest is to determine the contributions of karst groundwater systems to productivity of aquatic communities. On Northern Prince of Wales Island, the Conk Canyon Cave insurgence and the Mop Spring resurgence were continuously monitored to understand the buffering capacity of the cave system. Over the length of the system, the pH increases from an average 3.89 to 7.22. The insurgence water temperature, during the summer months, ranged from between 10oC to 17oC. After residence in the cave system, the resurgence water had been buffered to 6oC to 9oC. Over the continuum from insurgence to resurgence, the specific conductance had increased by an order of magnitude with the resurgence waters having a higher ionic strength. The cave environment acts as a buffer on the incoming acidic muskeg water to yield resurgence water chemistry of a buffered karst system. These buffered waters contribute to the productivity in aquatic environments downstream. The waters from this system drain into Whale Pass, an important location for the salmon industry. The cool, even temperatures, as well as buffered flow rates delivered by the karst systems are associated with higher productivity of juvenile coho salmon.

A Novel Photosynthetic Strategy for Adaptation to Low-Iron Aquatic Environments

Iron (Fe) availability is a major limiting factor for primary production in aquatic environments. Cyanobacteria respond to Fe deficiency by derepressing the isiAB operon, which encodes the antenna protein IsiA and flavodoxin. At nanomolar Fe concentrations, a PSI-IsiA supercomplex forms, comprising a PSI trimer encircled by two complete IsiA rings. This PSI-IsiA supercomplex is the largest photosynthetic membrane protein complex yet isolated. This study presents a detailed characterization of this complex using transmission electron microscopy and ultrafast fluorescence spectroscopy. Excitation trapping and electron transfer are highly efficient, allowing cyanobacteria to avoid oxidative stress. This mechanism may be a major factor used by cyanobacteria to successfully adapt to modern low-Fe environments.

Dynamic chlorophyll and nitrogen:carbon regulation in algae optimizes instantaneous growth rate

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 402:81-96 (2010) - DOI: https://doi.org/10.3354/meps08333 Dynamic chlorophyll and nitrogen:carbon regulation in algae optimizes instantaneous growth rate Kai W. Wirtz1,*, Markus Pahlow2 1Institute for Coastal Research, GKSS Research Centre, Max-Planck-Strasse 1, 21502 Geesthacht, Germany 2Leibniz-Institut für Meereswissenschaften (IFM-GEOMAR), Düsternbrooker Weg 20, 24105 Kiel, Germany *Email: wirtz@gkss.de ABSTRACT: Photoacclimation models are a prerequisite for accurate estimates of primary production in aquatic environments under typically variable light conditions. They generally start from empirical functions of the internal chlorophyll a (chl a) or nutrient quota (e.g. the Droop model). We propose that physiological variations in phytoplankton reflect phenotypic adaptation which maximizes the growth rate. Growth maximization has to account for indirect effects of the enhancement of carbon (C) acquisition by acclimation, primarily through concomitant changes in the intracellular nitrogen (N) budget. Our model expresses, for the first time, the indirect effect of alterations in N uptake on C assimilation by a parameter-free trade-off between the 2 uptake functions. The model explicitly prescribes optimal protein partitioning between N and C uptake and sub-partitioning into carboxylation (1,5-bisphosphate carboxylase/oxygenase, Rubisco) and light harvesting. Applications to various published experimental data for different phytoplankton species support the validity of the optimality hypothesis and point to different flexibility in the re-organization of chloroplasts between taxa as well as to different time-scales on which photoacclimation operates. Simulations of a batch culture with the haptophyte Isochrysis galbana show that a decoupling in pigment N:C from cellular N:C may explain observed lag phases in chl a:C regulation. For diatoms, seemingly stronger constraints in intra-cellular stoichiometry determine the photoacclimative response to variable light regimes, as simulated and reported for Skeletonema costatum. N and chl a quotas correlate well in nutrient-limited chemostats of Thalassiosira fluviatilis, but in part decouple under light limitation. In N limited growth, non-linearity in N:C as expressed by the Droop function results from a combination of a linear quota dependency, down-regulation of relative carboxylation capacity, and increasing N costs of chl a synthesis at elevated growth rates. Our optimality assumption that includes indirect feed-backs through the concept of protein partitioning generates an accurate model for adaptation in physiological traits. KEY WORDS: Photoacclimation · Trait-based model · Adaptive dynamics · Protein partitioning · Stoichiometry Full text in pdf format PreviousNextCite this article as: Wirtz KW, Pahlow M (2010) Dynamic chlorophyll and nitrogen:carbon regulation in algae optimizes instantaneous growth rate. Mar Ecol Prog Ser 402:81-96. https://doi.org/10.3354/meps08333 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 402. Online publication date: March 08, 2010 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2010 Inter-Research.

Some thoughts on the concept of colimitation: Three definitions and the importance of bioavailability

We discuss the concept of colimitation of primary productivity in aquatic environments, with an emphasis on reconciling this concept with recent advances in marine bioinorganic chemistry. Colimitations are divided into three categories on the basis of their mathematical formulations and visualizations: type I, independent nutrient colimitation (e.g., N and P); type II, biochemical substitution colimitation (e.g., Co and Zn); and type III, biochemically dependent colimitation (e.g., Zn and C), where the ability to acquire one nutrient is dependent upon sufficient supply of another. The potential for colimitation occurring in the marine environment and the critical importance of understanding nutrient bioavailability are discussed.

Cobalt Limitation of Growth and Mercury Methylation in Sulfate-Reducing Bacteria

Sulfate-reducing bacteria (SRB) have been identified as the primary organisms responsible for monomethylmercury (MeHg) production in aquatic environments, but little is known of the physiologyand biochemistry of mercury(Hg) methylation. Corrinoid compounds have been implicated in enzymatic Hg methylation, although recent experiments with a vitamin B12 inhibitor indicated that incomplete-oxidizing SRB likely do not use a corrinoid-enzyme for Hg methylation, whereas experiments with complete-oxidizing SRB were inconclusive due to overall growth limitation. Here we explore the role of corrinoid-containing methyltransferases, which contain a cobalt-reactive center, in Hg methylation. To this end, we performed cobalt-limitation experiments on two SRB strains: Desulfococcus multivorans, a complete-oxidizer that uses the acetyl-CoA pathway for major carbon metabolism, and Desulfovibrio africanus, an incomplete-oxidizer that does not contain the acetyl-CoA pathway. Cultures of D. multivorans grown with no direct addition of Co or B12 became cobalt-limited and produced 3 times less MeHg per cell than control cultures. Differences in growth rate and Hg bioavailability do not account for this large decrease in MeHg production upon Co limitation. In contrast, the growth and Hg methylation rates of D. africanus cultures remained nearly constant regardless of the inorganic cobalt and vitamin B12 concentrations in the medium. These results are consistent with mercury being methylated by different pathways in the two strains: catalyzed by a B12-containing methyltransferase in D. multivorans and a B12-independent methyltransferase in D. africanus. If complete-oxidizing SRB like D. multivorans account for the bulk of MeHg production in coastal sediments as reported, the ambient Co concentration and speciation may control the rate of Hg methylation.

Mercury Biogeochemical Cycling and Bioaccumulation in Aquatic Environments: A Review

Over the last century the mercury (Hg) concentration in the environment has been increased by human activities with inputs from sources such as atmospheric deposition, urban runoff, and industrial effluents. Mercury can be transformed to methylmercury (MeHg) in anaerobic conditions by sulfate reducing bacteria (SRB) and sediments are the principal location for MeHg production in aquatic environments. Interest in bioaccumulation of Hg and MeHg into lower trophic levels of benthic and pelagic organisms stems from public health concerns as these organisms provide essential links for higher trophic levels of food chains such as fish and larger invertebrates. Fish consumption is the major exposure route of MeHg to humans. Recently, it was reported that blood samples in Korea showed much higher Hg levels (5-8 times) than those in USA and Germany. Although this brings much attention to Hg research in Korea, there are very few studies on Hg biogeochemical cycling and bioaccumulation in aquatic environments. Given the importance of Hg methylation and MeHg transfer through food chains in aquatic environments, it is imperative that studies should be done in much detail looking at the fate, transport, and bioaccumulation of Hg and MeHg in the environment. Moreover, there should be long-term monitoring plans in Korea to evaluate the environmental and health effects of Hg and MeHg.

Sulfur and primary production in aquatic environments: an ecological perspective

Sulfur is one of the critical elements in living matter, as it participates in several structural, metabolic and catalytic activities. Photosynthesis is an important process that entails the use of sulfur during both the light and carbon reactions. Nearly half of global photosynthetic carbon fixation is carried out by phytoplankton in the aquatic environment. Aquatic environments are very different from one another with respect to sulfur content: while in the oceans sulfate concentration is constantly high, freshwaters are characterized by daily and seasonal variations and by a wide range of sulfur concentration. The strategies that algal cells adopt for energy and resource allocation often reflect these differences. In the oceans, the amount and chemical form of sulfur has changed substantially during the course of the Earth's history; it is possible that sulfur availability played a role in the evolution of marine phytoplankton communities and it may continue to have appreciable effects on global biogeochemistry and ecology. Phytoplankton is also the main biogenic source of sulfur; sulfur can be released into the atmosphere by algal cells as dimethylsulfide, with possibly important repercussions on global climate. These and related matters are discussed in this review.

A fluorometric method for the differentiation of algal populations in vivo and in situ.

Fingerprints of excitation spectra of chlorophyll (Chl) fluorescence can be used to differentiate 'spectral groups' of microalgae in vivo and in situ in, for example, vertical profiles within a few seconds. The investigated spectral groups of algae (green group, Chlorophyta; blue, Cyanobacteria; brown, Heterokontophyta, Haptophyta, Dinophyta; mixed, Cryptophyta) are each characterised by a specific composition of photosynthetic antenna pigments and, consequently, by a specific excitation spectrum of the Chl fluorescence. Particularly relevant are Chl a, Chl c, phycocyanobilin, phycoerythrobilin, fucoxanthin and peridinin. A laboratory-based instrument and a submersible instrument were constructed containing light-emitting diodes to excite Chl fluorescence in five distinct wavelength ranges. Norm spectra were determined for the four spectral algal groups (several species per group). Using these norm spectra and the actual five-point excitation spectrum of a water sample, a separate estimate of the respective Chl concentration is rapidly obtained for each algal group. The results of dilution experiments are presented. In vivo and in situ measurements are compared with results obtained by HPLC analysis. Depth profiles of the distribution of spectral algal groups taken over a time period of few seconds are shown. The method for algae differentiation described here opens up new research areas, monitoring and supervision tasks related to photosynthetic primary production in aquatic environments.

Are the actively respiring cells (CTC+) those responsible for bacterial production in aquatic environments?

The 5-cyano-2,3-ditolyl tetrazolium chloride (CTC) staining method is commonly and increasingly used to detect and to enumerate actively respiring cells (CTC+ cells) in aquatic systems. However, this method remains controversial since some authors promote this technique while others pointed out several drawbacks of the method. Using flow cytometry (FCM), we showed that CTC staining kinetics vary greatly from one sample to another. Therefore, there is no universal staining protocol that can be applied to aquatic bacterial communities. Furthermore, using (3)H-leucine incorporation, it was shown that the CTC dye has a rapid toxic effect on bacterial cells by inhibiting protein synthesis, a key physiological function. The coupling of radioactive labelling with cell sorting by FCM suggested that CTC+ cells contribute to less than 60% of the whole bacterial activity determined at the community level. From these results, it is clearly demonstrated that the CTC method is not valid to detect active bacteria, i.e. cells responsible for bacterial production.