Elisabeth, N. H. | Search bioRxiv

bioRxiv2024

Label-free structural imaging of plant roots and microbes using third- harmonic generation microscopy

Root biology is pivotal in addressing global challenges including sustainable agriculture and climate change. However, roots have been relatively understudied among plant organs, partly due to the difficulties in imaging root structures in their natural environment. Here we used microfabricated ecosystems (EcoFABs) to establish growing environments with optical access and employed nonlinear multimodal microscopy of third-harmonic generation (THG) and three-photon fluorescence (3PF) to achieve label-free, in situ imaging of live roots and microbes at high spatiotemporal resolution. THG enabled us to observe key plant root structures including the vasculature, Casparian strips, dividing meristematic cells, and root cap cells, as well as subcellular features including nuclear envelopes, nucleoli, starch granules, and putative stress granules. THG from the cell walls of bacteria and fungi also provides label-free contrast for visualizing these microbes in the root rhizosphere. With simultaneously recorded 3PF fluorescence signal, we demonstrated our ability to investigate root-microbe interactions by achieving single-bacterium tracking and subcellular imaging of fungal spores and hyphae in the rhizosphere.

plant biology↗

bioRxiv2023

Geochemical constraints on bacteriophage infectivity in terrestrial environments

Lytic phages can be potent and selective inhibitors of microbial growth and can have profound impacts on microbiome composition and function. However, there is uncertainty about the biogeochemical conditions under which phage predation can proceed and modulate microbial ecosystem function, particularly in terrestrial systems. Ionic strength is known to be critical for infection of bacteria by many phages, but there is limited quantitative data on ion thresholds for phage infection that can be compared with environmental ion concentrations. Similarly, while carbon composition varies in terrestrial environments, we know little of which carbon sources favor or disfavor phage infection and how these higher order interactions impact microbiome function. Here, we measured the half-maximal effective concentrations (EC50) of 80 different inorganic ions for the infection of E. coli with two canonical dsDNA and ssRNA phages, T4 and MS2, respectively. We found that many alkaline earth metals and alkali metals enabled successful lytic infection but that the ionic strength thresholds varied for different ions between phages. Additionally, using a freshwater nitrate reducing microbiome, we found that the ability of lytic phage to influence nitrate reduction end-products was dependent on the carbon source as well as the ion concentration. For all phage:host pairs we tested, the ion EC50s for phage infection we measured exceed the ion concentrations found in many terrestrial freshwater systems. Thus, our findings support a model where the influence of phages on terrestrial microbial functional ecology is greatest in hot spots and hot moments such as metazoan guts, drought influenced soils, or biofilms where ion concentration is locally or transiently elevated and carbon source composition is of a sufficiently low complexity to enrich for a dominant phage susceptible population. SignificanceViral-prokaryote dynamics greatly influence microbial ecology and the earths biogeochemical cycles. Thus, identifying the key environmental controls on phage predation is critical for predictive microbial ecology. Here we conduct laboratory experiments that implicate ionic strength and carbon composition as major controls on phage interactions with bacterial hosts in terrestrial microbiomes. We propose a model in which terrestrial phage predation is most favored in drought impacted soils and in higher ionic strength environments such as metazoan guts or between adjacent cells in biofilms.

microbiology↗