Raffel, T. | Search bioRxiv

bioRxiv2020

Pesticide-induced Alterations to Phytoplankton Abundance and Community Structure Alter Ecosystem Respiration: Implications for the Carbon Cycle?

AO_SCPLOWBSTRACTC_SCPLOWCurrent predictions of the effects of synthetic chemicals on freshwater ecosystems are hampered by the sheer number of chemical contaminants entering aquatic systems, the diversity of organisms inhabiting these systems, and uncertainties about how contaminants alter ecosystem metabolism. We conducted a mesocosm experiment that elucidated the responses of ponds composed of phytoplankton and zooplankton to standardized concentrations of 12 pesticides, nested within four pesticide classes and two pesticide types. We show that the effects of the pesticides on algae were consistent within herbicides and insecticides and responses of over 70 phytoplankton species and genera were consistent within broad taxonomic groups. Insecticides generated top-down effects on phytoplankton community composition and abundance, which were associated with persistent increases in ecosystem respiration. Herbicides reduced phytoplankton abundance, which was associated with decreases in primary productivity and ecosystem respiration. These results suggest that widespread pesticide use could have underexplored implications for the global carbon cycle. While these effects on ecosystem respiration were mediated through complex effects on communities, taxonomic groups of organisms responded similarly to pesticide types, suggesting opportunities to simplify ecological risk assessment.

ecology↗

bioRxiv2020

Consistent Effects of Pesticides on Community Structure and Ecosystem Function in Freshwater Systems

AO_SCPLOWBSTRACTC_SCPLOWPredicting ecological effects of contaminants remains challenging because of the sheer number of chemicals and their ambiguous role in biodiversity-ecosystem function relationships. We evaluated responses of pond ecosystems to standardized concentrations of 12 pesticides, nested in four pesticide classes and two pesticide types. We show consistent effects of herbicides and insecticides on ecosystem function, but slightly less consistent effects on community composition. Effects of pesticides on ecosystem functions were often mediated by changes to biodiversity, and our analyses show that consistency in effects of pesticide types on functions was driven by functional redundancy among species. These results suggest that risk assessment of the thousands of registered chemicals on ecosystem responses could be simplified to a smaller number of chemical groups and to groups of functionally redundant taxa.

ecology↗

bioRxiv2019

Caldicellulosiruptor bescii regulates pilus expression in response to the polysaccharide, xylan

Biological hydrolysis of cellulose above 70{degrees}C involves microorganisms that secrete free enzymes, and deploy separate protein systems to adhere to their substrate. Strongly cellulolytic Caldicellulosiruptor bescii is one such extreme thermophile, which deploys modular, multi-functional carbohydrate acting enzymes to deconstruct plant biomass. Additionally, C. bescii also encodes for non-catalytic carbohydrate binding proteins, which likely evolved as a mechanism to compete against other heterotrophs in carbon limited biotopes that these bacteria inhabit. Analysis of the Caldicellulosiruptor pangenome identified a type IV pilus (T4P) locus encoded upstream of the t[a]pirins, that is encoded for by all Caldicellulosiruptor species. In this study, we sought to determine if the C. bescii T4P plays a role in attachment to plant polysaccharides. The major C. bescii pilin (CbPilA) was identified by the presence of pilin-like protein domains, paired with transcriptomics and proteomics data. Using immuno-dot blots, we determined that the plant polysaccharide, xylan, induced production of CbPilA 10 to 14-fold higher than glucomannan or xylose. Furthermore, we are able to demonstrate that recombinant CbPilA directly interacts with xylan, and cellulose at elevated temperatures. Localization of CbPilA at the cell surface was confirmed by immunofluorescence microscopy. Lastly, a direct role for CbPilA in cell adhesion was demonstrated using recombinant CbPilA or anti-CbPilA antibodies to reduce C. bescii cell adhesion to xylan and crystalline cellulose up to 4.5 and 2-fold, respectively. Based on these observations, we propose that CbPilA and by extension, the T4P, plays a role in Caldicellulosiruptor cell attachment to plant biomass. IMPORTANCEMost microorganisms are capable of attaching to surfaces in part to persist in their environment. Here, we describe that the thermophilic, plant degrading bacterium, Caldicellulosiruptor bescii, uses type IV pili to attach to carbohydrates found in plant biomass. This ability is likely key to survival in environments where carbon sources are limiting, allowing C. bescii to compete against other plant degrading microorganisms. Interestingly, the carbohydrate that induced the highest expression of pilin protein was xylan, a hemicellulose that is not the majority polysaccharide in plant biomass. Not only do we demonstrate a direct interaction of the pilin with the polysaccharides, but also that cell attachment to polysaccharides can be disrupted by the addition of recombinant pilin and notably by antibody neutralization of the native pilin. This mechanism mirrors those recently described in pathogenic Gram-positive bacteria, and further supports the ancient origins of type IV pilus systems.

microbiology↗