Alien Registration- Smith, Harriet E. (Lewiston, Androscoggin County)

https://digitalmaine.com/alien_docs/29521/thumbnail.jp

Auxotrophic interactions: A stabilizing attribute of aquatic microbial communities?

© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Johnson, W. M., Alexander, H., Bier, R. L., Miller, D. R., Muscarella, M. E., Pitz, K. J., & Smith, H. Auxotrophic interactions: A stabilizing attribute of aquatic microbial communities? FEMS Microbiology Ecology, (2020): fiaa115, doi: 10.1093/femsec/fiaa115.Auxotrophy, or an organism's requirement for an exogenous source of an organic molecule, is widespread throughout species and ecosystems. Auxotrophy can result in obligate interactions between organisms, influencing ecosystem structure and community composition. We explore how auxotrophy-induced interactions between aquatic microorganisms affect microbial community structure and stability. While some studies have documented auxotrophy in aquatic microorganisms, these studies are not widespread, and we therefore do not know the full extent of auxotrophic interactions in aquatic environments. Current theoretical and experimental work suggests that auxotrophy links microbial community members through a complex web of metabolic dependencies. We discuss the proposed ways in which auxotrophy may enhance or undermine the stability of aquatic microbial communities, highlighting areas where our limited understanding of these interactions prevents us from being able to predict the ecological implications of auxotrophy. Finally, we examine an example of auxotrophy in harmful algal blooms to place this often theoretical discussion in a field context where auxotrophy may have implications for the development and robustness of algal bloom communities. We seek to draw attention to the relationship between auxotrophy and community stability in an effort to encourage further field and theoretical work that explores the underlying principles of microbial interactions.This work was supported by the National Science Foundation [OCE-1356192]

Are quasars accreting at super-Eddington rates?

In a previous paper, Collin & Hur\'e (2001), using a sample of Active Galactic Nuclei (AGN) where the mass has been determined by reverberation studies (Kaspi et al. 2000), have shown that if the optical luminosity is emitted by a steady accretion disc, about half of the objects are accreting close to or higher than the Eddington rate. We conclude here that this result is unavoidable, unless the masses are strongly underestimated by reverberation studies, which does not seem to be the case. There are three issues to the problem: 1. Accretion proceeds at Eddington or super-Eddington rates through thick discs. Several consequences follow: an anti-correlation between the line widths of the lines and the Eddington ratios, and a decrease of the Eddington ratio with an increasing black hole mass. Extrapolated to all quasars, these results imply that the amount of mass locked in massive black holes should be larger than presently thought. 2. The optical luminosity is not produced directly by the gravitational release of energy, and super-Eddington rates are not required. The optical luminosity has to be emitted by a dense and thick medium located at large distances from the center (10$^3$ to $10^4$ gravitational radii). It can be due to reprocessing of the X-ray photons from the central source in a geometrically thin warped disc, or in dense "blobs" forming a geometrically thick system, which can be a part of the accretion flow or the basis of an outflow. 3. Accretion discs are completely "non standard". Presently neither the predictions of models nor the observed spectral distributions are sufficient to help choosing between these solutions.Comment: 16 pages, 11 figures, accepted in A&