No abstract

The cyanobacterium Microcystis can produce microcystins, a family of toxins that are of major concern in water management. In several lakes, the average microcystin content per cell gradually declines from high levels at the onset of Microcystis blooms to low levels at the height of the bloom. Such seasonal dynamics might result from a succession of toxic to nontoxic strains. To investigate this hypothesis, we ran competition experiments with two toxic and two nontoxic Microcystis strains using light-limited chemostats. The population dynamics of these closely related strains were monitored by means of characteristic changes in light absorbance spectra and by PCR amplification of the rRNA internal transcribed spacer region in combination with denaturing gradient gel electrophoresis, which allowed identification and semiquantification of the competing strains. In all experiments, the toxic strains lost competition for light from nontoxic strains. As a consequence, the total microcystin concentrations in the competition experiments gradually declined. We did not find evidence for allelopathic interactions, as nontoxic strains became dominant even when toxic strains were given a major initial advantage. These findings show that, in our experiments, nontoxic strains of Microcystis were better competitors for light than toxic strains. The generality of this finding deserves further investigation with other Microcystis strains. The competitive replacement of toxic by nontoxic strains offers a plausible explanation for the gradual decrease in average toxicity per cell during the development of dense Microcystis blooms.Blooms of the cyanobacterium Microcystis can be a major hazard in recreational lakes, drinking water reservoirs, and protected wetland areas (6,47,49). Microcystis often forms dense blooms that may cause anoxia when cells die off massively. Moreover, Microcystis can produce the toxin microcystin. This hepatotoxin poses serious health risks for animals and humans (3, 7). Especially in dense scums, the concentration of microcystins may increase dramatically. Microcystin concentrations up to 25,000 g liter Ϫ1 have been reported (10), exceeding the guideline values for recreational waters of 20 g liter Ϫ1 by more than 3 orders of magnitude (5). Microcystis populations often consist of mixtures of microcystin-producing and non-microcystin-producing strains (10, 23, 48, 52). Interestingly, several studies show that the average microcystin content expressed per cell is typically high at the onset of Microcystis blooms but much lower at the height of these blooms (22,51,53). In other words, with increasing Microcystis biomass, the Microcystis cells become, on average, less toxic. Examples from three Microcystis-dominated Dutch lakes are shown in Fig. 1. This striking seasonal variability in microcystin content of Microcystis blooms exceeds the physiological variability in cellular microcystin content reported for isolated Microcystis strains in laboratory experiments (13,29,54). Thus, it seems that the changes in...

Mechanism of formation of polychlorinated dibenzo-p-dioxins and dibenzfurans in the catalyzed combustion of carbon. Luijk, R.; Akkerman, D.M.; Slot, P.C.; Olie, K.; Kapteijn, F. Published in:Environmental Science and Technology DOI:10.1021/es00051a019Link to publication Citation for published version (APA):Luijk, R., Akkerman, D. M., Slot, P. C., Olie, K., & Kapteijn, F. (1994). Mechanism of formation of polychlorinated dibenzo-p-dioxins and dibenzfurans in the catalyzed combustion of carbon. Environmental Science and Technology, 28, 312-321. DOI: 10.1021/es00051a019 General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. Combustion experiments of an activated carbon (Norit RX Extra), catalyzed by CuC12, were conducted in triplicate in a flow of moist air containing 5 vol % HC1 at 300 "C to reveal the mechanism of formation of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs). The catalyst concentration was varied from 0, 0.1,0.5,1.0, to 5.0 wt %. After partial combustion (1 h), the samples were analyzed on the presence of PCDDs and PCDFs. The total yield of these compounds amounted to 19 (0.1 wt % CuC12) and 2 (5.0 wt % CuClz) pg/g of carbon. The PCDD/PCDF ratio showed a decrease from 33 to 0.2. At low copper concentrations, predominantly PCDDs are formed with an isomer pattern that can be explained completely as a product of chlorophenol condensation reactions. At high copper concentrations, the PCDD/PCDF isomer distribution pattern shifts toward a characteristic waste incineration fly ash pattern. A proposal for the mechanism of formation of PCDDs and PCDFs in the catalyzed combustion of carbon will be presented. Condensation reactions of 2,4,6-trichlorophenol on carbon ( O w t 9% CuC12)

Anaerobic microorganisms enriched from Rhine River sediments are able to remove chlorine substituents from poly‐chlorinated dibenzo‐p‐dioxines (PCDDs). A model PCDD, 1,2,3,4‐tetrachlorodibenzo‐p‐dioxin (1,2,3,4–TeCDD) was reduc‐tively dechlorinated to both 1,2,3–and l,2,4–trichlorodibenzo‐/>‐dioxins (1,2,3–and 1,2,4–TrCDD). These compounds were further dechlorinated to 1,3–and 2,3–dichlorodibenzo‐p‐dioxins and traces of 2–monochlorodibenzo‐p‐dioxin. This is the first report in the literature of the anaerobic microbial dechlorination of PCDDs. The same enrichment culture was previously found to deSchlorinate chlorinated benzenes (CBs) and polychlorinated biphenyls (PCBs). An anaerobic culture able to remove aryl chlorines from three classes of compounds has not been reported before. The rate at which the culture dechlorinates 1,2,3,4–TeCDD (t1/2 = 15.5 d) was between those observed for CBs and PCBs. This study shows that reductive dechlorination may have an effect on PCDDs in sediments, as has been demonstrated for CBs and PCBs. The formation of metabolites with a conserved 2,3‐substitution pattern from 1,2,3,4–TeCDD indicates that dechlorination of highly chlorinated dibenzo‐p‐dioxins may result in metabolites that are potentially more toxic than the parent compounds.

Rising atmospheric CO 2 concentrations are likely to affect many ecosystems worldwide. However, to what extent elevated CO 2 will induce evolutionary changes in photosynthetic organisms is still a major open question. Here, we show rapid microevolutionary adaptation of a harmful cyanobacterium to changes in inorganic carbon (C i ) availability. We studied the cyanobacterium Microcystis, a notorious genus that can develop toxic cyanobacterial blooms in many eutrophic lakes and reservoirs worldwide. Microcystis displays genetic variation in the C i uptake systems BicA and SbtA, where BicA has a low affinity for bicarbonate but high flux rate, and SbtA has a high affinity but low flux rate. Our laboratory competition experiments show that bicA + sbtA genotypes were favored by natural selection at low CO 2 levels, but were partially replaced by the bicA genotype at elevated CO 2 . Similarly, in a eutrophic lake, bicA + sbtA strains were dominant when C i concentrations were depleted during a dense cyanobacterial bloom, but were replaced by strains with only the high-flux bicA gene when C i concentrations increased later in the season. Hence, our results provide both laboratory and field evidence that increasing carbon concentrations induce rapid adaptive changes in the genotype composition of harmful cyanobacterial blooms.climate change | harmful algal blooms | Microcystis aeruginosa | microevolution | natural selection