  A central need in the field of astrobiology is generalized perspectives on
life that make it possible to differentiate abiotic and biotic chemical
systems. 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. 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
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.
