Movement of Zoospores of Phytophthora citricola in Saturated Porous Media.

摘要

The genus Phytophthora comprises numerous plant pathogens in both natural and managed ecosystems. For Phytophthora spp. that infect roots, dispersal occurs in soil water through a combination of advection and swimming of specialized motile propagules (zoospores). Specific biological and physico-chemical processes, however, remain poorly understood, due to difficulties in studying phenomena in opaque media and lack of a theoretical framework for analyzing transport of motile microorganisms. The goal of this research was to elucidate the impacts of advection and swimming on zoospore movement in a saturated, ideal soil. The work was accomplished in two stages, (i) conceptualization of 3-dimensional topography and flow field heterogeneity at the subpore-scale, and (ii) observation of behavior of zoospore suspensions infiltrated into saturated media. Chapter 2 introduces a 3-dimensional particle tracking method and presents two studies investigating particle transport in simplified 'ideal pores'. The first study describes 'avoidance' by latex microspheres of a volume surrounding orthogonal grain contacts and the second describes 'capture', translation, and retention of microspheres under conditions unfavorable to deposition. Chapter 3 expands on the first study and demonstrates, with the aid of computational fluid dynamics, that low flow zones associated with orthogonal grain contacts are minimally connected to the main flow. Thus, probability of entry into these regions for large, non-Brownian particles by advection alone is low. In zoospore infiltration experiments, zoospore plumes 'converged' rather than dispersing as expected. To assess the possibility of zoospore auto-aggregation driving this 'convergence', Chapter 4 delves into the 'pattern swimming' observed in free-swimming zoospore suspensions, concluding that the concentrating is an example of bioconvection. Chapter 5 introduces a conceptual model to explain the anomalous zoospore plume behavior. Random walk simulations replicated plume convergence but were less successful at modeling anisotropic dispersion. At low infiltration rates (100 mum s-1), simulations predict that zoospores will remain at or near the soil surface, resulting in greater opportunity to find host tissues or to be transported with surface water. Further investigation is necessary to develop a robust theoretical framework with appropriate conceptualization of the subpore hydrodynamic environment for predicting transport of zoospores and other motile microorganisms in porous media.