Competition for Light between Toxic and Nontoxic Strains of the Harmful Cyanobacterium Microcystis

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.     

Production of cyanopeptolins,anabaenopeptins, and microcystins by the harmful cyanobacteria Anabaena 90 and Microcystis PCC 7806

This study investigated the effects of light intensity, temperature, and phosphorus limitation on the peptide production of the cyanobacteria Microcystis PCC 7806 and Anabaena 90. Microcystis PCC 7806 produced two microcystin variants and three cyanopeptolins, whereas Anabaena 90 produced four microcystin variants, three anabaenopeptins, and two anabaenopeptilides. Microcystin and cyanopeptolin contents varied by a factor 2–3, whereas the anabaenopeptins and anabaenopeptilides of Anabaena varied more strongly. Under phosphorus limitation, peptide production rates increased with the specific growth rate. The response of peptide production to light intensity and temperature was more complex: in many cases peptide production decreased with specific growth rate. We observed compensatory changes of different peptide variants: decreased cyanopeptolin A and C contents were accompanied by increased cyanopeptolin 970 contents, and decreased anabaenopeptin A and C contents were accompanied by increased anabaenopeptilide 90B contents. Compensatory dynamics in peptide production may enable cyanobacteria to sustain stable peptide levels in a variable environment.     

Ethnic‐specific Correlations of Visfatin With Circulating Markers of Endothelial Inflammation and Function

Sub‐Saharan Africa is afflicted by high hypertension prevalence that is expected to rise even further along with increasing obesity rates. The present study aimed to investigate the role of visfatin in obesity and to explore associations of visfatin with markers of endothelial function and hemodynamics in African women compared to a well‐matched white sample. The present study involved urban African (n = 102) and white (n = 115) women from South Africa, individually matched for age and BMI. We measured blood pressure, cardiac output, and arterial compliance noninvasively, and analyzed visfatin as well as circulating markers of vascular function and inflammation in serum. Serum visfatin concentration did not differ between African and white women. Visfatin was unrelated to obesity in African women but positive associations for total and abdominal obesity were found in white women. Age‐ and obesity‐adjusted univariate and multivariate analyses revealed significant positive associations of visfatin with endothelin‐1 and fibrinogen in African women. Identical analyses in white women indicated a positive association of visfatin with C‐reactive protein and von Willebrand factor. Our findings suggest a possible role of visfatin in the cardiovascular system that seems to be independent of obesity in the African women.     

Successful bystander cardiopulmonary resuscitation complicated by liver rupture

A 40-year-old man, who had collapsed while running and was resuscitated successfully by bystanders, was referred. An automated external defibrillator had shown ventricular fibrillation before a single shock restored sinus rhythm. On arrival, the patient was alert and haemo-dynamically stable. He reported that he had recently suffered from chest pain during exercise but had not visited his general practitioner; before collapsing he had no complaints. Physical examination showed a moderately tender abdomen with normal peristalsis.     

Concerted changes in gene expression and cell physiology of the cyanobacterium Synechocystis sp. strain PCC 6803 during transitions between nitrogen and light-limited growth

Physiological adaptation and genome-wide expression profiles of the cyanobacterium Synechocystis sp. strain PCC 6803 in response to gradual transitions between nitrogen-limited and light-limited growth conditions were measured in continuous cultures. Transitions induced changes in pigment composition, light absorption coefficient, photosynthetic electron transport, and specific growth rate. Physiological changes were accompanied by reproducible changes in the expression of several hundred open reading frames, genes with functions in photosynthesis and respiration, carbon and nitrogen assimilation, protein synthesis, phosphorus metabolism, and overall regulation of cell function and proliferation. Cluster analysis of the nearly 1,600 regulated open reading frames identified eight clusters, each showing a different temporal response during the transitions. Two large clusters mirrored each other. One cluster included genes involved in photosynthesis, which were up-regulated during light-limited growth but down-regulated during nitrogen-limited growth. Conversely, genes in the other cluster were down-regulated during light-limited growth but up-regulated during nitrogen-limited growth; this cluster included several genes involved in nitrogen uptake and assimilation. These results demonstrate complementary regulation of gene expression for two major metabolic activities of cyanobacteria. Comparison with batch-culture experiments revealed interesting differences in gene expression between batch and continuous culture and illustrates that continuous-culture experiments can pick up subtle changes in cell physiology and gene expression.     

SYSTEMATIC REVISION OF THE GENERA LIAGORA AND IZZIELLA (LIAGORACEAE,RHODOPHYTA) FROM TAIWAN BASED ON MOLECULAR ANALYSES AND CARPOSPOROPHYTE DEVELOPMENT,WITH THE DESCRIPTION OF TWO NEW SPECIES1

Some Liagora and Izziella distributed in Taiwan display a wide range of morphological variation and can be difficult to distinguish. To clarify species concepts, we applied DNA sequence analyses and examined carposporophyte development in detail. These studies revealed two new species, which are described herein as Izziella hommersandii sp. nov. and Izziella kuroshioensis sp. nov. I. kuroshioensis superficially resembles Izziella formosana and Izziella orientalis in that its involucral filaments subtend rather than surround the lower portion of the gonimoblast mass (= Izziella type) and a fusion cell is formed from cells of the carpogonial branch, but it can be separated by differences in the cell numbers and branching pattern of the involucral filaments, as well as thallus morphology. In contrast to other species that also bear short lateral branchlets, I. hommersandii is unique in possessing a mixture of short and long involucral filaments, a phenomenon not reported before. The length of the involucral filaments is species specific among species of Izziella and contrasts to the behavior of the involucral filaments after fertilization in species such as “Liagora”setchellii [= Titanophycus setchellii comb. nov.], in which the filaments completely envelop the gonimoblast. In addition, the cells of the carpogonial branch in Titanophycus do not fuse after fertilization to form a fusion cell. Thus, a combination of characters with respect to the behavior of the carpogonial branch and the involucral filaments after fertilization is very useful for delineating species boundaries in Izziella and for separating Titanophycus from Izziella and Liagora.     

Nitrogen fixation rates in algal turf communities of a degraded versus less degraded coral reef

Algal turf communities are ubiquitous on coral reefs in the Caribbean and are often dominated by N₂-fixing cyanobacteria. However, it is largely unknown (1) how much N₂ is actually fixed by turf communities and (2) which factors affect their N₂ fixation rates. Therefore, we compared N₂ fixation activity by turf communities at different depths and during day and night-time on a degraded versus a less degraded coral reef site on the island of Curaçao. N₂ fixation rates measured with the acetylene reduction assay were slightly higher in shallow (5–10-m depth) than in deep turf communities (30-m depth), and N₂ fixation rates during the daytime significantly exceeded those during the night. N₂ fixation rates by the turf communities did not differ between the degraded and less degraded reef. Both our study and a literature survey of earlier studies indicated that turf communities tend to have lower N₂ fixation rates than cyanobacterial mats. However, at least in our study area, turf communities were more abundant than cyanobacterial mats. Our results therefore suggest that turf communities play an important role in the nitrogen cycle of coral reefs. N₂ fixation by turfs may contribute to an undesirable positive feedback that promotes the proliferation of algal turf communities while accelerating coral reef degradation.     

Comparison of predator–prey interactions with and without intraguild predation by manipulation of the nitrogen source

Theory predicts that intraguild predation leads to different community dynamics than the trophic cascades of a linear food chain. However, experimental comparisons of these two food‐web modules are rare. Mixotrophic plankton species combine photoautotrophic and heterotrophic nutrition by grazing upon other phytoplankton species. We found that the mixotrophic chrysophyte Ochromonas can grow autotrophically on ammonium, but not on nitrate. This offered a unique opportunity to compare predator–prey interactions in the presence and absence of intraguild predation, without changing the species composition of the community. With ammonium as nitrogen source, Ochromonas can compete with its autotrophic prey for nitrogen and therefore acts as intraguild predator. With nitrate, Ochromonas acts solely as predator, and is not in competition with its prey for nitrogen. We parameterized a simple intraguild predation model based on chemostat experiments with monocultures of Ochromonas and the toxic cyanobacterium Microcystis. Subsequently, we tested the model predictions by inoculating Ochromonas into the Microcystis monocultures, and vice versa. The results showed that Microcystis was a better competitor for ammonium than Ochromonas. In agreement with theoretical predictions, Microcystis was much more strongly suppressed by intraguild predation on ammonium than by top–down predation on nitrate. Yet, Microcystis persisted at very low population densities, because the type III functional response of Ochromonas implied that the grazing pressure upon Microcystis became low when Microcystis was rare. Our results provide experimental support for intraguild predation theory, and indicate that intraguild predation may enable biological control of microbial pest species.