School of Botany - Theses

Permanent URI for this collection

http://hdl.handle.net/11343/307

Filters

1999 2
2 2
2
Reset filters

Settings
Statistics
Citations
Author: Foard, Helen Judith. × End date: 1999 × Start date: 1980 ×

Search Results

Now showing 1 - 2 of 2
  • Item
    No Preview Available
    Feeding behaviour in mixotrophic chrysophytes : qualitative and quantitative observations using video microscopy
    Foard, Helen Judith. (University of Melbourne, 1999)
    Complexity in the natural history and feeding behaviour of mixotrophic chrysophytes is investigated using video microscopy and digital image-enhanced observations of living cells. An introductory survey of eight species of freshwater chrysophytes demonstrates variability in characteristics of the natural history of cells in culture and suggests a diversity of mixotrophic strategies, or balance between photosynthesis and phagotrophy for nutrition, in the species examined. Four levels of phagotrophic dependence are suggested based on variability in polymorphic life cycles, feeding mechanisms, continuous or bimodal (on/off) feeding in the trophic stage, associations of chrysophytes with bacteria and general growth and pigment characters. Two feeding-types are reported: (i) the Ochromonas-like stage that feeds by a direct interception, or raptorial, mechanism occurs in seven out of the eight species examined (Ochromonas spp., Poterioochromonas spp., Chromulina nebulosa, Lepochromulina bursa, Epipyxis pulchra), and (ii) Chrysamoeba mikrokonta is a rhizopodial chrysophyte that feeds by diffusion. Qualitative multi-step feeding models, which include active (post-encounter) and passive stages, are proposed for both mechanisms and are compared to the classical direct interception and diffusion concepts. Feeding behaviour in a sessile Ochromonas-like chrysophyte, Epipyxis pulchra, is investigated in more detail using a flow-chamber system Size-selective feeding is studied by offering Epipyxis pulchra cells beads of various sizes and describing the observed responses in the context of the multi-step feeding model. The observations demonstrate that both passive and active mechanisms delineate the ingestable size-range, and the preferred sizes within this range. Subsequently, feeding-history- related-selection (FHRS), or active selection behaviour that is triggered by factors that are related to the recent feeding history of cells, is investigated. In Epipyxis pulchra, cells that are photrophically grown and never previously exposed to beads, the retention efficiency on preferred bead sizes (0.87 ?m) is high (c. 100%), but cells tend to reject beads as their recent feeding history expands. The outcome of encounters with beads in cells with variable recent feeding histories is analysed using logistic regression modelling and hypotheses that could explain the underlying biological mechanism(s) of FHRS are discussed. A further complexity in the feeding strategy of Epipyxis pulchra, is its bimodal feeding behaviour. The available evidence demonstrates that the behavioural strategies, or the combined effects of active particle rejection and non-continuous feeding, can vary when Epipyxis pulchra cells are offered different bacteria species and when different prey concentrations are available. Overall, there is a tendency for cells to spend more time switched-off when offered bacteria in the flow-chamber than when cells are offered beads in the same conditions (P < 0.001). The behavioural complexity demonstrated in Epipyxis pulchra provides a perspective for explaining variable feeding strategies among direct interception feeding chrysophytes. Some overall ecological implications of flexible behaviour and life cycles in mixotrophic chrysophytes are discussed in relation to the role of mixotrophic chrysophytes in microbial food webs and for the interpretation of grazing experiments that are dependent on simple models of predation.
  • Item
    No Preview Available
    Feeding behaviour in mixotrophic chrysophytes : qualitative and quantitative observations using video microscopy
    Foard, Helen Judith. (University of Melbourne, 1999)
    Complexity in the natural history and feeding behaviour of mixotrophic chrysophytes is investigated using video microscopy and digital image-enhanced observations of living cells. An introductory survey of eight species of freshwater chrysophytes demonstrates variability in characteristics of the natural history of cells in culture and suggests a diversity of mixotrophic strategies, or balance between photosynthesis and phagotrophy for nutrition, in the species examined. Four levels of phagotrophic dependence are suggested based on variability in polymorphic life cycles, feeding mechanisms, continuous or bimodal (on/off) feeding in the trophic stage, associations of chrysophytes with bacteria and general growth and pigment characters. Two feeding-types are reported: (i) the Ochromonas-like stage that feeds by a direct interception, or raptorial, mechanism occurs in seven out of the eight species examined (Ochromonas spp., Poterioochromonas spp., Chromulina nebulosa, Lepochromulina bursa, Epipyxis pulchra), and (ii) Chrysamoeba mikrokonta is a rhizopodial chrysophyte that feeds by diffusion. Qualitative multi-step feeding models, which include active (post-encounter) and passive stages, are proposed for both mechanisms and are compared to the classical direct interception and diffusion concepts. Feeding behaviour in a sessile Ochromonas-like chrysophyte, Epipyxis pulchra, is investigated in more detail using a flow-chamber system Size-selective feeding is studied by offering Epipyxis pulchra cells beads of various sizes and describing the observed responses in the context of the multi-step feeding model. The observations demonstrate that both passive and active mechanisms delineate the ingestable size-range, and the preferred sizes within this range. Subsequently, feeding-history- related-selection (FHRS), or active selection behaviour that is triggered by factors that are related to the recent feeding history of cells, is investigated. In Epipyxis pulchra, cells that are photrophically grown and never previously exposed to beads, the retention efficiency on preferred bead sizes (0.87 ?m) is high (c. 100%), but cells tend to reject beads as their recent feeding history expands. The outcome of encounters with beads in cells with variable recent feeding histories is analysed using logistic regression modelling and hypotheses that could explain the underlying biological mechanism(s) of FHRS are discussed. A further complexity in the feeding strategy of Epipyxis pulchra, is its bimodal feeding behaviour. The available evidence demonstrates that the behavioural strategies, or the combined effects of active particle rejection and non-continuous feeding, can vary when Epipyxis pulchra cells are offered different bacteria species and when different prey concentrations are available. Overall, there is a tendency for cells to spend more time switched-off when offered bacteria in the flow-chamber than when cells are offered beads in the same conditions (P < 0.001). The behavioural complexity demonstrated in Epipyxis pulchra provides a perspective for explaining variable feeding strategies among direct interception feeding chrysophytes. Some overall ecological implications of flexible behaviour and life cycles in mixotrophic chrysophytes are discussed in relation to the role of mixotrophic chrysophytes in microbial food webs and for the interpretation of grazing experiments that are dependent on simple models of predation.