  Planktonic active matter represents an emergent system spanning different
scales: individual, population and community; and complexity arising from
sub-cellular and cellular to collective and ecosystem scale dynamics. This
cross-scale active matter system responds to a range of abiotic (temperature,
fluid flow and light conditions) and biotic factors (nutrients, pH, secondary
metabolites) characteristic to the relevant ecosystems they are part of. Active
modulation of cell phenotypes, including morphology, motility, and
intracellular organization enable planktonic microbes to dynamically interact
with other individuals and species; and adapt - often rapidly - to the changes
in their environment. In this chapter, I discuss both traditional and
contemporary approaches to study the dynamics of this multi-scale active matter
system from a mechanistic standpoint, with specific references to their local
settings and their ability to actively tune the behaviour and physiology, and
the emergent structures and functions they elicit under natural ecological
constraints as well as due to the shifting climatic trends.
