  In this study, we aim to explore the behavior of microorganisms in response
to natural lighting conditions, considering the off-normal angles at which the
sun strikes the Earth's surface. To achieve this, we investigate the effect of
oblique irradiation on a rotating medium, as this combination represents a more
realistic scenario in the natural environment. Our primary focus is on
understanding the phototactic behavior of microorganisms, which refers to their
movement towards or away from light. Under conditions of low light,
microorganisms tend to exhibit positive phototaxis, moving towards the light
source, while in intense light, they display negative phototaxis, moving away
from the light source. By studying a suspension that is illuminated by oblique
collimated flux with a constant radiation intensity applied to the top surface,
we can gain insights into how microorganisms respond to varying light
conditions and rotation. The stability analysis is conducted using linear
perturbation theory, which allows us to predict both the stationary and
oscillatory characteristics of the bio-convective instability at the onset of
bioconvection. Through this analysis, we observe that rotation plays a
significant stabilizing role in the system, while oblique irradiation has a
destabilizing effect on the suspension.
