Bacterial metabolism of algal extracellular carbon

Measurements of microbial utilization of extracellular organic carbon (EOC) released by phytoplankton commonly consider only EOC fractions subject to rapid uptake. Questions remain whether other EOC fractions are metabolized, what portion is labile, and with what assimilation efficiency this carbon substrate is utilized. 14 C-EOC was prepared by incubation of the natural mixed planktonic community from an oligotrophic lake with H 14 CO 3 in the light. 14 C-EOC which was not rapidly removed by heterotrophs remained in solution and was isolated by filtration. This residual EOC was inoculated with lake microheterotrophs in laboratory microcosms, and utilization kinetics were determined through long-term assays of cumulative 14 CO 2 production. Time-courses for 14 CO 2 production were consistent for all assays and were well described by a deterministic mixed-order degradation model. On twelve sampling occasions, from 29% to 76% of residual 14 C-EOC was labile to further metabolism by lake heterotrophs. First-order rate constants for EOC utilization showed a mode of 0.05 to 0.15 per day. From 33% to 78% of gross 14 C-EOC uptake was respired (mean 50%), indicating appreciable return of algal EOC to the pelagic food web as microbial biomass.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/42876/1/10750_2004_Article_BF00015524.pd

A method for determining enzymatically hydrolyzable phosphate (EHP) in natural waters

A method for determining enzymatically hydrolyzable phosphate (EHP) in natural waters is described, based on the determination of released inorganic phosphate after the hydrolysis of organophosphoric esters by free, dissolved phosphohydrolases (mainly phosphatase) produced by the biota. The method gives higher values in highly eutrophic waters than the classical procedure of Strickland and Parsons; in samples from less eutrophic habitats the two methods give similar results. The new method is simple, accurate, and can be used in both freshwater and marine studies. It is particularly recommended for determination in hypereutrophic waters, when Pi concentrations exceed 25 µg P liter−1. Representative field data from the euphotic zone of seven lakes and two marine habitats are presented

Temporal and Stoichiometric Patterns of Algal Stimulation of Litter-Associated Heterotrophic Microbial Activity

Periphyton communities associated with submerged plant detritus contain interacting autotrophic and heterotrophic microbes, and are sites of extracellular enzymatic activity. The strength and nature of these interactions might be expected to change over time as microbial communities develop on plant litter. Microbial interactions and enzymatic activity can be altered by nutrient availability, suggesting that litter stoichiometry could also affect these phenomena.We grew wetland plants under ambient and nutrient‐enriched conditions to generate plant litter of differing nutrient content. In two experiments, we investigated: (1) the influence of algal photosynthesis on fungal and bacterial production and the activities of four extracellular enzymes throughout a 54‐day period of microbial colonisation and growth; and (2) the influence of litter stoichiometry on these relationships.Ambient and nutrient‐enriched standing‐dead plant litter was collected and then submerged in wetland pools to allow for natural microbial colonisation and growth. Litter samples were periodically retrieved and transported to the laboratory for experiments manipulating photosynthesis using the photosystem II inhibitor DCMU (which effectively prevents algal photosynthetic activity). Algal (14C‐bicarbonate), bacterial (3H‐leucine), and fungal (14C‐acetate) production, and β‐glucosidase, β‐xylosidase, leucine aminopeptidase, and phosphatase activities (MUF‐ or AMC‐labelled fluorogenic substrates) were measured under conditions of active and inhibited algal photosynthesis.Photosynthesis stimulated overall fungal and bacterial production in both experiments, although the strength of stimulation varied amongst sampling dates. Phosphatase activity was stimulated by photosynthesis during the first, but not the second, experiment. No other enzymatic responses to short‐term photosynthesis manipulations were observed.Microbial communities on high‐nutrient litter occasionally showed increased extracellular enzyme activity, fungal growth rates, and bacterial production compared to communities on non‐enriched litter, but algal and fungal production were not affected. Litter stoichiometry had no effects on fungal, bacterial, or enzymatic responses to photosynthesis, but the mean enzyme vector analysis angle (a measure of P‐ versus N‐acquiring enzyme activity) was positively correlated to litter N:P, suggesting that elevated litter N:P led to an increase in the relative activity of P‐acquiring enzymes.These results supported the hypothesis that algal photosynthesis strongly influences heterotrophic microbial activity on macrophyte leaf litter, especially that of fungi, throughout microbial community development. However, the strength of this photosynthetic stimulation does not generally depend on small differences in litter nutrient content.Stimulation of microbial heterotrophs by algal photosynthesis could drive diurnal shifts in periphyton community and aquatic ecosystem function, as well as linking green (photoautotroph‐based) and brown (detrital‐based) food webs

Structural determination for H₂O adsorption on Si(001)2 × 1 using scanned-energy mode photoelectron diffraction

Using scanned-energy-mode photoelectron diffraction we have determined the local adsorption geometry of the OH fragments adsorbed on Si(100)(2 × 1) surface. On this substrate water is known to adsorb dissociatively even at low temperature (90 K), which gives rise to a surface layer comprising coadsorbed OH and H species. The OH fragments are found to be adsorbed in off-atop sites at a dimerised surface Si atom with Osingle bondSi bond-lengths of 1.7 ± 0.1Å and bond-angles relative to the surface normal of 22 ± 5°