  Active particles with their characteristic feature of self-propulsion are
regarded as the simplest models for motility in living systems. The
accumulation of active particles in low activity regions has led to the general
belief that chemotaxis requires additional features and at least a minimal
ability to process information and to control motion. We show that
self-propelled particles display chemotaxis and move into regions of higher
activity, if the particles perform work on passive objects, or cargo, to which
they are bound. The origin of this cooperative chemotaxis is the exploration of
the activity gradient by the active particle when bound to a load, resulting in
an average excess force on the load in the direction of higher activity. Using
a minimalistic theoretical model, we capture the most relevant features of
these active-passive dimers and in particular we predict the crossover between
anti-chemotactic and chemotactic behaviour. Moreover we show that merely
connecting active particles to chains is sufficient to obtain the crossover
from anti-chemotaxis to chemotaxis with increasing chain length. Such an active
complex is capable of moving up a gradient of activity such as provided by a
gradient of fuel and to accumulate where the fuel concentration is at its
maximum. The observed transition is of significance to proto-forms of life
enabling them to locate a source of nutrients even in the absence of any
supporting sensomotoric apparatus.
