  The capacity to sense gradients efficiently and acquire information about the
ambient environment confers many advantages like facilitating movement toward
nutrient sources or away from toxic chemicals. The amplified dispersal evinced
by organisms endowed with motility is possibly beneficial in related contexts.
Hence, the connections between information acquisition, motility, and microbial
size are explored from an explicitly astrobiological standpoint. By using prior
theoretical models, the constraints on organism size imposed by gradient
detection and motility are elucidated in the form of simple heuristic scaling
relations. It is argued that environments such as alkaline hydrothermal vents,
which are distinguished by the presence of steep gradients, might be conducive
to the existence of "small" microbes (with radii of $\gtrsim 0.1$ $\mu$m) in
principle, when only the above two factors are considered; other biological
functions (e.g., metabolism and genetic exchange) could, however, regulate the
lower bound on microbial size and elevate it. The derived expressions are
potentially applicable to a diverse array of settings, including those
entailing solvents other than water; for example, the lakes and seas of Titan.
The paper concludes with a brief exposition of how this formalism may be of
practical and theoretical value to astrobiology.
