The presence of living organisms in a fluid can affect a fluid’s properties. Organisms propel themselves through the fluid using specialised organelles that generate propulsion. This propulsion creates disturbances in the fluid, leading to variations in its properties. The overarching research question is: ”Can the properties of a fluid be modified with biologically active suspensions?”. Organisms respond to their environment for survival and thereby induce intriguing patterns which enhance flows. By using their natural instincts to react to external stimuli, fluid systems could generate flows or intrinsically modify their properties, which can be controlled by a user’s own will. It is thought that the local orientation of the organism influences the viscosity in a shear flow, because of the added stress induced by the activity of the organisms. This research investigates the influence of the orientation of the stress induced by the green alga Chlamydomonas reinhardtii on the viscosity using light-controlled rheology. This is done through two research questions: ”Is it possible to modulate the swimming orientation of the green algae using a light source?”, followed by the question: ”Can we thereby control the viscosity of the algal sample?”. Phototaxis is the natural response of the algae to swim in the direction of light and was the main asset used to control the swimming orientation. The first research question was explored through three-dimensional microscopy. Images were acquired from algae controlled by light entering at a specific angle of 45°. The second research question com bined light-control with a rheometer that imposed an external shear flow. The rheometer measures the viscosity of light-controlled algae using an on-and-off light cycle. Through the three-dimensional imaging, the position and velocity vectors of each par ticle was found and converted into the relevant spherical coordinates which in turn could present the trajectories of the organisms. These were found to be swimming at the tar geted angle at sufficient light intensity. The second experiment however showed ambiguous viscosity responses that had to be quantified using power analysis. The quantification of the data showed possibility of a viscosity change as a result of the reorientation towards light. The first experiment showed the possibility to control the orientation of the algae using phototaxis and thus positively answering the first research question. The second research question cannot be answered taking strong conclusions. The quantification of the data showed possibility of a viscosity change as a result of the reorientation towards light.