Robin F. Harris

THE ECOLOGY AND BIOGEOGRAPHY OF MICROORGANISMS ON PLANT SURFACES.

The vast surface of the plant axis, stretching from root tips occasionally buried deeply in anoxic sediment, to apical meristems held far aloft, provides an extraordinarily diverse habitat for microorganisms. Each zone has to a greater or lesser extent its own cohort of microorganisms, in aggregate comprising representatives from all three primary domains of life-Bacteria, Archaea, and Eucarya. While the plant sets the stage for its microbial inhabitants, they, in turn, have established varied relationships with their large partner. These associations range from relatively inconsequential (transient epiphytic saprophytes) to substantial (epiphytic commensals, mutualistic symbionts, endophytes, or pathogens). Through recent technological breakthroughs, a much better perspective is beginning to emerge on the nature of these relationships, but still relatively little is known about the role of epiphytic microbial associations in the life of the plant.

r - and K -Selection and Microbial Ecology

The essence of the concept of r- and K-selection is that organisms strive to maximize their fitness for survival in either uncrowded (r-selection) or crowded (K-selection) environments. Fitness is defined following ecological convention as the proportion of genes left in the population gene pool (Pianka, 1983, p. 10). The terms r and K refer, respectively, to the maximum specific rate of increase (maximum specific growth rate minus minimum specific death rate) of an organism and to the density of individuals that a given environment can support at the population equilibrium. Since both r and K can vary within a species and are subject to modification, the division of natural selection into r- and K-selection is of considerable basic interest in evolutionary ecology.

Interpretation of sulfur and oxygen isotopes in biological and abiological sulfide oxidation

Sulfur and oxygen isotope ratios in sulfate cannot distinguish unambiguously between biological and abiological mechanisms of sulfide oxidation because similar isotope signatures have been observed or predicted for different mechanisms. However, field and laboratory data on sulfur and oxygen isotopes have demonstrated the importance of understanding environmental factors to limit the set of reasonable mechanisms. In this paper, we review sulfur and oxygen isotope compositions reported in both field and laboratory studies of sulfide oxidation. We develop a new classification scheme for sulfide oxidation based on whether the electron transfer involves incorporation of oxygen from H2O or O2 in the sulfate. Four mechanisms for H2O-oxygen incorporation and two for O2-oxygen incorporation are presented. We also point out several environmental factors such as the production of intermediate sulfoxyanions or the presence of certain Thiobacillus species that may determine whether sulfur is fractionated during oxidation. More information on environmental factors is needed.

Putative mechanism and dynamics of inhibition of the apple scab pathogen Venturia inaequalis by compost extracts

Abstract Clarified water extracts of slurries of spent mushroom substrate (SMS) inhibited in vitro germination of conidia of the apple scab pathogen Venturia inaequalis by up to 98% relative to germination in water controls. Inhibition of conidial germination increased with incubation time of slurries over 5 to 7 d and persisted for at least 14 d, at which time experiments were terminated. Compost slurries became anaerobic within 1 h when incubated without aeration. Aeration of slurries decreased efficacy of the resulting extracts compared to non-aerated controls. When aerated slurries were allowed to incubate without further aeration, extracts regained efficacy, becoming not significantly different from non-aerated controls. There was no difference in efficacy between filtered (0.1μm) and untreated extracts. Passage of filtrate through microconcentrators with molecular cut-off limits ranging from 100 to 3 kDa did not diminish activity of the extracts. Autoclaved extracts were less effective than mutreated extracts but retained most of their efficacy. Extracts produced from sterile SMS were virtually ineffective compared with those from non-sterile SMS. When small volumes of slurry from raw SMS were added to slurries of sterile SMS and incubated for an additional period, efficacy of the inoculated SMS was enhanced compared to uninoculated controls. We conclude that a major inhibitory principle of the SMS extract is a low molecular weight, heat-stable, non-protein metabolite produced by anaerobic microorganisms in the compost.

Sample size requirements to evaluate spore germination inhibition by compost extracts

We examined the effect of compost sample size (50 to 5000 g) on the precision of estimates of inhibition of Venturia inaequalis (Cke) Winter conidial germination induced by extracts of anaerobically-incubated composts. Composts were prepared from vegetable material; some were amended with manure. Variability due to sample size was considered as a function of extract efficacy (high, medium or low) and compost heterogeneity (high or low). Extract efficacy was characterized as ability to inhibit more than 75% of the conidia (high), 75 to 40% (medium), or fewer than 40% (low). Heterogeneity was characterized as visual or tactile presence (high) or absence (low) of parent material in the compost. To address the question of sample size-related variability we partitioned individual extract incubations (samples) into aliquots (sub-samples), and aliquots into microtiter plate wells (sub-sub-samples). For this nested design, the largest component of variation was consistently found to be that associated with wells; aliquots were a negligible source of variation. Sample size over the range examined was generally of small importance for extracts of medium and high efficacy, but not low efficacy, independent of compost heterogeneity. For previously untested composts, particularly those of high heterogeneity, statistical analyses of our data suggest that samples of at least 500 g circumvent potentially large errors and consequent difficulties to detect differences among composts or effects of experimental variables.

Pathogenicity of the fungus, Colletotrichum gloeosporioides (Penz.) Sacc., to Eurasian watermilfoil (Myriophyllum spicatum L.)

Abstract The pathogenicity to Eurasian watermilfoil (Myriophyllum spicatum L.) of an isolate of the fungus Colletorichum gloeosporioides (Penz.) Sacc. obtained from diseased milfoil plants was evaluated in laboratory culture experiments. Except in senescent plant tissue, the fungus was confined to the surface of the plant and rarely penetrated beyond the epidermis. Fungal infection prompted a hypersensitive plant response which resulted in the death of a few epidermal cells but evidently halted the internal spread of the fungus. The effect of the fungus was greatest in the commonly used Gerloffs plant culture medium at 25°C. Under these conditions, treated plants did not increase in biomass after inoculation and suffered extensive but superficial damage. At 15 and 20°C inoculation with the fungus did not produce a detectable reduction in yield. In nutrient-amended lakewater and in a carbonate-buffered artificial growth medium, inoculated plants exhibited little visible damage, but the fungus caused a significant reduction in yield. Because these media supported much greater plant growth than Gerloffs medium, the increase in biomass of inoculated plants was 60–70% that of control plants. We concluded that under realistic conditions the effect of this fungal isolate was too small to justify further consideration of it as a possible biological control agent for M. spicatum. Our results illustrate the importance of appropriate culture conditions and media in experiments designed to evaluate microbial pathogens of aquatic plants.