Contribution of dinitrogen fixation to bacterial and primary productivity in the Gulf of Aqaba (Red Sea)

We evaluated the seasonal contribution of heterotrophic and autotrophic diazotrophy to the total dinitrogen (N2) fixation in a representative pelagic station in the northern Gulf of Aqaba in early spring when the water column was mixed and during summer under full thermal stratification. N2 fixation rates were low during the mixed period (∼ 0.1 nmol N L−1 d−1) and were significantly coupled with both primary and bacterial productivity. During the stratified period N2 fixation rates were four-fold higher (∼ 0.4 nmol N L−1 d−1) and were significantly correlated solely with bacterial productivity. Furthermore, while experimental enrichment of seawater by phosphorus (P) enhanced bacterial productivity and N2 fixation rates during both seasons primary productivity was stimulated by P only in the early spring. Metatranscriptomic analyses from the stratified period identified the major diazotrophic contributors as related to heterotrophic prokaryotes from the Euryarchaeota and Desulfobacterales (Deltaproteobacteria) or Chlorobiales (Chlorobia). Moreover, during this season, experimental amendments to seawater applying a combination of the photosynthetic inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and a mixture of amino acids increased both bacterial productivity and N2 fixation rates. Our findings from the northern Gulf of Aqaba indicate a~shift in the diazotrophic community from phototrophic and heterotrophic populations, including small blooms of the cyanobacterium Trichodesmium, in winter/early spring, to predominantly heterotrophic diazotrophs in summer that may be both P and carbon limited as the additions of P and amino acids illustrated.

Comparison of Escherichia coli O157:H7 Enrichment in Spiked Produce Samples†

Two strains of Escherichia coli O157:H7 were spiked into six varieties of produce at approximately 0.5 CFU g−1. Samples were enriched by using the U.S. Food and Drug Administration (FDA) Bacteriological Analytical Manual (BAM) method and by using an experimental method incorporating acid shock. Target colonies were detectable on selective agars after 30 of 48 analyses with BAM enrichment and 48 of 48 analyses with acid enrichment. Real-time PCR screening of 24-h enrichment broths revealed the presence of the diagnostic stx1 or stx2 genes after 27 of 48 analyses with BAM enrichment and 42 of 48 analyses with acid enrichment. The efficiency of the enrichment varied with strain and type of produce spiked but overall was better with the experimental enrichment method. Modifications of both the acid enrichment and BAM enrichment methods also were tested. The acid method with a modified incubation temperature consistently yielded high rates of recovery (>108 CFU ml−1), with no instances in which target cells could not be detected. Modification of the BAM procedure did not reproducibly improve enrichment efficiency.

The Development of Play During Childhood: Forms and Possible Functions

In this review we consider the nature and possible developmental functions of three forms of play: pretend, social, and locomotor. Play is defined in terms of dispositional and contextual criteria. First, the frequency of occurrence of each form of play across the period of childhood is documented. Developmental function of play is conceptualised in terms of immediate or deferred beneficial consequences. Four strategies for examining developmental function are reviewed: arguments from design, correlational analyses, experimental enrichment and deprivation, and cost-benefit analyses. Whereas most theories of play implicitly assume that during childhood it occurs frequently and has benefits deferred until adulthood, we suggest that some benefits of play are immediate.

Effects of Daphnia on the response of mesotrophic lakes to experimental enrichment

We are currently involved in a whole-lake experiment designed to assess how Daphnia affect the response of mesotrophic lakes to increased nutrient loading. In the first year of the experiment, we wished to demonstrate that there are P loading rates which eutrophy lakes without Daphnia but not lakes with Daphnia. In order to do this, we needed to choose an appropriate level and schedule for the experimental enrichment. This paper describes how we made that choice, then evaluates whether our choice was successful. Based on the literature, a simulation model, and a mesocosm experiment, we decided that enriching at a rate of 1 μg P L−1 d−1 would create the desired contrast between lakes with and without Daphnia. Model simulations indicated that mean algal response to enrichment would be comparable under monthly, weekly, or continuous additions. We chose to add nutrients continuously at ambient N:P ratios. Experimental results from 1993 suggest that we achieved a nutrient loading rate consistent with our goal: chlorophyll responded less to enrichment in the lake with Daphnia than in the lake without Daphnia. The modeling, mesocosm, and whole-lake studies summarized here support the idea that Daphnia reduce chlorophyll at P loads ≤1 μg P L−1 d−1. However, cyanobacteria may escape control by all grazers at relatively low P loading rates.