  In this article, the effect of diffuse flux in the absence of direct
collimated flux on the onset of phototactic bioconvection is investigated. The
main effect of diffuse flux in the absence of collimated flux is on the
swimming behaviour of microorganisms in the suspension. At higher diffuse flux,
the horizontal component of swimming orientation exhibits a higher magnitude,
which slows the rate of pattern formation in the suspension. Also, the linear
stability of the suspension predicts that the most unstable mode of disturbance
transits from stationary (oscillatory) to oscillatory (stationary) at the
variation in the magnitude of diffuse flux for some fixed parameters. The
overstable nature of disturbance is mostly observed at the high value of
swimming speed and extinction coefficient. Moreover, the suspension shows a
more stable behaviour at the higher magnitude of diffuse flux.
