Generalized Stoichiometry and Biogeochemistry for Astrobiological Applications

Abstract

A central need in the field of astrobiology is generalized perspectives on life that make it possible to differentiate abiotic and biotic chemical systems McKay (2008). A key component of many past and future astrobiological measurements is the elemental ratio of various samples. Classic work on Earth’s oceans has shown that life displays a striking regularity in the ratio of elements as originally characterized by Redfield (Redfield 1958; Geider and La Roche 2002; Eighty years of Redfield 2014). The body of work since the original observations has connected this ratio with basic ecological dynamics and cell physiology, while also documenting the range of elemental ratios found in a variety of environments. Several key questions remain in considering how to best apply this knowledge to astrobiological contexts: How can the observed variation of the elemental ratios be more formally systematized using basic biological physiology and ecological or environmental dynamics? How can these elemental ratios be generalized beyond the life that we have observed on our own planet? Here, we expand recently developed generalized physiological models (Kempes et al. 2012, 2016, 2017, 2019) to create a simple framework for predicting the variation of elemental ratios found in various environments. We then discuss further generalizing the physiology for astrobiological applications. Much of our theoretical treatment is designed for in situ measurements applicable to future planetary missions. We imagine scenarios where three measurements can be made—particle/cell sizes, particle/cell stoichiometry, and fluid or environmental stoichiometry—and develop our theory in connection with these often deployed measurements.

Searching for Life, Mindful of Lyfe’s Possibilities

“…Such general descriptions for lyfe may not be detailed enough to predict specific biomolecules, but they may still predict more general classes of biosignatures—all of which can still be effective for the detection of Earthly life—that can be used to motivate the detection of truly alien lyfe. These agnostic approaches include homochirality, chemical disequilibria e.g., [ 5 , 6 ], elemental distributions [ 7 ], molecular complexity [ 8 ], and planetary complexity [ 9 ].…”

Observations of Elemental Composition of Enceladus Consistent with Generalized Models of Theoretical Ecosystems

“…One the most consistent patterns associated with life on Earth is the Redfield ratio of elements [19, 28, 58]. Originally discovered in particulate organic matter in seawater [58], this ratio describes the abundances of three of life’s essential elements, relating carbon to nitrogen to phosphorus in a ratio of 106:16:1.…”

“…We implemented a single-nutrient chemostat model similar to the model presented in [28] (see Methods). Briefly, our model assumes nitrogen to be replete (due to the high reported concentrations [42, 72]).…”

“…The most general version of this framework to tally biomass elemental totals is to combine the distribution of organism sizes with allometric scaling relationships for macromolecular components. While both cell size distributions and macromolecular allometries are general across life on Earth [9, 10, 29], they can be easily generalized to consider alternative physiologies and ecosystem structures [28]. We deploy a size-based framework to first generate elemental ratio expectations for Enceladus if the ecosystem were comprised entirely of methanogenic Archaea as known on Earth, we then generalize the perspective to life that radically departs from Earth-like parameters.…”

Special Collection: Celebrating J.D. Murray’s Contributions to Mathematical Biology

“…He showed that low order systems of ordinary and partial differential equations can capture the essence of processes such as embryonic development, tumour growth and disease spread. Kempes et al (2021) applies this philosophy to the field of astrobiology. The paper uses a simple ordinary differential equation model coupled with previously established scaling relationships of the main macromolecular components of cells.…”