A single-cell polony method reveals low levels of infected Prochlorococcus in oligotrophic waters despite high cyanophage abundances

Long-term stability of picocyanobacteria in the open oceans is maintained by a balance between synchronous division and death on daily timescales. Viruses are considered a major source of microbial mortality, however, current methods to measure infection have significant methodological limitations. Here we describe a method that pairs flow-cytometric sorting with a PCR-based polony technique to simultaneously screen thousands of taxonomically resolved individual cells for intracellular virus DNA, enabling sensitive, high-throughput, and direct quantification of infection by different virus lineages. Under controlled conditions with picocyanobacteria-cyanophage models, the method detected infection throughout the lytic cycle and discriminated between varying infection levels. In North Pacific subtropical surface waters, the method revealed that only a small percentage of Prochlorococcus (0.35–1.6%) were infected, predominantly by T4-like cyanophages, and that infection oscillated 2-fold in phase with the diel cycle. This corresponds to 0.35–4.8% of Prochlorococcus mortality daily. Cyanophages were 2–4-fold more abundant than Prochlorococcus, indicating that most encounters did not result in infection and suggesting infection is mitigated via host resistance, reduced phage infectivity and inefficient adsorption. This method will enable quantification of infection for key microbial taxa across oceanic regimes and will help determine the extent that viruses shape microbial communities and ecosystem level processes.Subject terms: Microbial ecology, Microbial biooceanography, Molecular ecology, Microbial ecology, Microbial biooceanography     

Methylation of neutral pseudotetrahedral zinc thiolate complexes: model reactions for alkyl group transfer to sulfur by zinc-containing enzymes

Eight scorpionate-zinc thiolate complexes, [(L1O)ZnSPh], [(L1O)ZnSPhF5], [(L1O)ZnSBz], [(L1O)ZnSPh2,6-Me], [(L1O)ZnSPh2,4-Me], [(L1O)ZnSPh4-NO2], [(TpPh,Ph)ZnSPh], and [(L2S)ZnSPh], were reacted with methyl iodide in chloroform, liberating the corresponding methyl thioethers as determined by 1H NMR. Three of these complexes are new and their synthesis and structural characterization are reported here. Weak alkylating agents such as trimethyl phosphate failed to undergo methyl transfer to the zinc thiolates under these conditions. Analysis of kinetic data as a function of concentration, temperature, pKa of the exogenous thiolate, and donor atom of the tripodal ligand are consistent with a mechanism where the zinc-bound thiolate is the active nucleophile in an associative-type methyl transfer reaction. Our model studies also provide experimental evidence to support the hypothesis that some enzymes can use the charge of the metal coordination site to modulate catalytic activity.     

The relative importance of top-down and bottom-up control of phytoplankton in a shallow macrophyte-dominated lake

1. Mechanisms stabilizing the plant-dominated clear-water state were investigated in Little Mere, U.K. Replicated, factorial, mesocosm experiments, carried out in 1995 and 1996, were designed to investigate the relative importance of top-down (zooplankton grazing) and bottom-up (nitrogen-limitation) control in limiting algal growth, and the role of macrophytes in these processes. Treatments included increased salinity (1995) and sticklebacks (1996) to reduce zooplankton numbers, weekly nitrate additions and removal of macrophytes. 2. Contrary to the results of other studies, submerged plants did not reduce nitrate concentrations. Owing to the high stickleback density in the enclosures with fish, macrophytes did not provide a refuge for zooplankton during the experiment. In Little Mere, however, where fish densities are lower, macrophytes probably play a key role in maintaining clear water by providing refuge for pelagic zooplankton and habitat for attached Cladocera. 3. Phytoplankton in Little Mere was not nitrogen- (N) limited during the growing season. Although nitrogen availability sets a maximum potential phytoplankton biomass it was not realized owing to control by zooplankton grazing.     

Obesity in South Africa: The South African Demographic and Health Survey

Objectives: To ascertain the anthropometric profile and determinants of obesity in South Africans who participated in the Demographic and Health Survey in 1998. Research Methods and Procedures: A sample of 13,089 men and women (age, ≥15 years) were randomly selected and then stratified by province and urban and nonurban areas. Height, weight, mid-upper arm circumference, and waist and hip circumference were measured. Body mass index (BMI) was used as an indicator of obesity, and the waist/hip ratio (WHR) was used as an indicator of abdominal obesity. Multivariate regression identified sociodemographic predictors of BMI and waist circumference in the data. Results: Mean BMI values for men and women were 22.9 kg/m² and 27.1 kg/m², respectively. For men, 29.2% were overweight or obese (≥25 kg/m²) and 9.2% had abdominal obesity (WHR ≥1.0), whereas 56.6% of women were overweight or obese and 42% had abdominal obesity (WHR >0.85). Underweight (BMI <18.5 kg/m²) was found in 12.2% of men and 5.6% of women. For men, 19% of the variation of BMI and 34% of the variation in waist circumference could be explained by age, level of education, population group, and area of residence. For women, these variables explained 16% of the variation of BMI and 24% of the variation in waist circumference. Obesity increased with age, and higher levels of obesity were found in urban African women. Discussion: Overnutrition is prevalent among adult South Africans, particularly women. Determinants of overnutrition include age, level of education, ethnicity, and area of residence.