Hunt, D. E. | Search bioRxiv

bioRxiv2024

Microbial Response to Natural Disturbances: Rare Biosphere often plays a role

Understanding how microbial populations respond to disturbances represents a major goal for microbial ecology. While several theories have been advanced to explain microbial community compositional changes in response to disturbances, appropriate data to test these theories is scarce, especially when considering the challenges to define rare vs. abundant taxa and generalists vs. specialists, a prerequisite for testing the theories. Here, we define these two key concepts by employing the patterns of coverage of a (target) genome by a metagenome to define rare populations, and by borrowing concepts from macroecology, the proportional similarity index (PS index), to define generalists. Using these concepts, we found that coastal microbial communities are resilient to major perturbations such as tropical cyclones and (uncommon) cold or warm weather events snaps-in part-due to the response of rare populations, providing support for the insurance hypothesis (i.e., the rare biosphere has the buffering capacity to mitigate the effects of disturbances). Generalists appear to contribute proportionally more than specialists to community adaptation to perturbations like warming, supporting the disturbance-specialization hypothesis, i.e., disturbance favors generalists. Taken together, our results advance understanding of the mechanisms governing microbial populations dynamics under changing environmental conditions and have potential applications for ecosystem management.

ecology↗

bioRxiv2023

Investigating a Gulf Stream Frontal Eddy North of Cape Hatteras: Questioning Overall Impacts on Ecology

Ocean physics and biology can interact in myriad and complex ways. Eddies, features found at many scales in the ocean, can drive substantial changes in physical and biogeochemical fields with major implications for marine ecosystems. Mesoscale eddies are challenging to model and difficult to observe synoptically at sea due to their fine-scale variability yet broad extent. In this work we observed a frontal eddy just north of Cape Hatteras via an intensive hydrographic, biogeochemical, and optical sampling campaign. Frontal eddies occur in western boundary currents around the globe and there are major gaps in our understanding of their ecosystem impacts. In the Gulf Stream, frontal eddies have been studied in the South Atlantic Bight, where they are generally assumed to shear apart passing Cape Hatteras. However, we found that the observed frontal eddy had different physical properties and phytoplankton community composition from adjacent water masses, in addition to continued cyclonic rotation. In this work we first synthesize the overall ecological impacts of frontal eddies in a simple conceptual model. This conceptual model led to the hypothesis that frontal eddies could be well timed to supply zooplankton to secondary consumers off Cape Hatteras where there is a notably high concentration and diversity of top predators. Towards testing this hypothesis and our conceptual model we report on the biogeochemical state of this particular eddy connecting physical and biological dynamics, analyze how it differs from Gulf Stream and shelf waters even in "death", and refine our initial model with this new data. Key PointsO_LIIn-depth investigation of a frontal eddy in the Gulf Stream off Cape Hatteras, North Carolina C_LIO_LIContinued physical and biogeochemical differences are observed between the eddy and adjacent water masses even as it begins to shear apart C_LIO_LIWe share a conceptual model of the ecological impact of frontal eddies with a hypothesis that they supply zooplankton to secondary consumers C_LI Plain Language SummaryFrontal eddies are spinning masses of water (~30km in diameter) that move along western boundary currents like the Gulf Stream. When they form they carry productive coastal water into the Gulf Stream and drive upwelling within their cores. Together this leads to an increase in the amount of phytoplankton within them - much higher compared to surrounding nutrient-limited Gulf Stream water. On the east coast of the United States one common area of frontal eddy formation is just off Charleston, SC. Eddies then travel up the coast and dissipate near Cape Hatteras, NC. In this work we measured a wide range of physical and biological properties of a frontal eddy just north of Cape Hatteras. We compared these properties within the eddy to the coastal water on one side and the Gulf Stream water on the other, finding clear differences in phytoplankton community composition and other physical and chemical properties. Using the results of these observations together with previous studies we share a simple model for how frontal eddies may impact phytoplankton, zooplankton, and fish - hypothesizing that they may contribute to the high diversity and density of top predators off Cape Hatteras.

ecology↗