Nutrient stoichiometry and biological dynamics

Project members: Anshuman Swain and William F Fagan

Status: Published; Swain, A. and Fagan, W.F., 2018. A mathematical model of the Warburg Effect: Effects of cell size, shape and substrate availability on growth and metabolism in bacteria. Mathematical biosciences and engineering: MBE, 16(1), pp.168-186.

https://doi.org/10.3934/mbe.2019009

The Warburg effect refers to a curious behavior observed in many organisms and cell types including cancer cells, yeast and bacteria, wherein both the efficient aerobic pathway and the inefficient fermentation pathway are utilized for respiration, despite the presence of ample oxygen. Also termed as overflow metabolism in bacteria, this phenomena has remained an enigmatic and poorly understood phenomenon despite years of experimental work. Here, we focus on bacterial cells and build a model of three trade offs involved in the utilization of aerobic and anaerobic respiration pathways (rate versus yield, surface area versus volume, and fast versus slow biomass production) to explain the observed behavior in cellular systems. The model so constructed also predicts changes in the relative usage of both pathways in terms of size and shape constraints of the cell, and identifies how substrate availability influences growth rate. Additionally, we use the model to explain certain complex phenomena in modern-and paleo-ecosystems, via the concept of overflow metabolism.

Project members: Ze Ren*, Nicolas Martyniuk*, Isabella A. Oleksy*, Anshuman Swain* & Scott Hotaling; 

Status: Published; Ren, Z., Martyniuk, N., Oleksy, I.A., Swain, A. and Hotaling, S., 2019. Ecological stoichiometry of the mountain cryosphere. Frontiers in Ecology and Evolution, 7, p.360.

https://doi.org/10.3389/fevo.2019.00360

Roughly 10% of the Earth's surface is permanently covered by glaciers and ice sheets and in mountain ecosystems, this proportion of ice cover is often even higher. From an ecological perspective, ice-dominated ecosystems place harsh controls on life including cold temperature, limited nutrient availability, and often prolonged darkness due to snow cover for much of the year. Despite these limitations, glaciers, and perennial snowfields support diverse, primarily microbial communities, though macroinvertebrates and vertebrates are also present. The availability and mass balance of key elements [(carbon (C), nitrogen (N), phosphorous (P)] are known to influence the population dynamics of organisms, and ultimately shape the structure and function of ecosystems worldwide. While considerable attention has been devoted to patterns of biodiversity in mountain cryosphere-influenced ecosystems, the ecological stoichiometry of these habitats has received much less attention. Understanding this emerging research arena is particularly pressing in light of the rapid recession of glaciers and perennial snowfields worldwide. In this review, we synthesize existing knowledge of ecological stoichiometry, nutrient availability, and food webs in the mountain cryosphere (specifically glaciers and perennial snowfields). We use this synthesis to develop more general understanding of nutrient origins, distributions, and trophic interactions in these imperiled ecosystems. We focus our efforts on three major habitats: glacier surfaces (supraglacial), the area beneath glaciers (subglacial), and adjacent downstream habitats (i.e., glacier-fed streams and lakes). We compare nutrient availability in these habitats to comparable habitats on continental ice sheets (e.g., Greenland and Antarctica) and show that, in general, nutrient levels are substantially different between the two. We also discuss how ongoing climate warming will alter nutrient and trophic dynamics in mountain glacier-influenced ecosystems. We conclude by highlighting the pressing need for studies to understand spatial and temporal stoichiometric variation in the mountain cryosphere, ideally with direct comparisons to continental ice sheets, before these imperiled habitats vanish completely.

Project members: A Swain, A J Kaufman, M Kalinowski and W F Fagan 

Status: In Review; preprint link here 

In this work, we introduce a microbial-species-interaction based ordinary differential equation model for understanding the conditions under which methane is preferentially released. Methane was the principal greenhouse gas in the Archean and early Proterozoic eons, where its fluctuation in the atmosphere would have played an important role in regulating the climate and the flux of ultraviolet radiation. Exploring certain first-order environmental controls (such as O2 or resource concentration) in the biological methane cycle, and their interactions with various microbial communities, might allow us to deduce and explain the biological, atmospheric and climatic events preserved in the sedimentary rock record between 2.8 and 2.0 billion years ago. Environmental controls on the dynamics of methane cycling may further explain other repetitious events in deep time, as well as the present-day increase in the methane flux to the atmosphere from wetland environments.

We report in this manuscript that the interplay of resource and O2 availability can cause complex cyclic patterns in methane dynamics irrespective of the size any of the (explored) microbial communities and their associated metabolic processes. Although the exact dynamics depend on certain metabolic parameters, but the system behavior in being cyclic or cyclic depend solely on the interplay of resources and oxygen concentration. Based on these model results, we propose that the cyclicity of methane haze events and glacial episodes in the late Archean and early Proterozoic may have been linked to the progressive increase in oceanic and atmospheric O2.