In the present study, microorganisms whose stimulus is influenced by gravity and viscosity is taken for consideration in Casson fluid saturated porous medium. The behaviour of microorganisms in Casson fluid due to gravitational modulation and internal heat generated in the system is studied analytically. Darcy-Brinkman model is implemented to analyze thermo bioconvection in horizontal flow of the fluid. Stationary mode of convection produces both linear and nonlinear stability. The threshold Rayleigh number, which addresses the initiation of bioconvection, is determined by linear stability analysis. This comprises parameters for which marginal stability curves are plotted to understand the stability and onset of convection of the system against wavenumber . The equation for Nusselt number is also used to describe nonlinear stability and to graphically illustrate how various parameters affect heat transfer in the system. In heat transfer analysis, the Ginzburg-Landau equation which characterizes the modulation amplitude is significant. Here, the behaviour of gyrotactic microorganisms in Casson fluid due to various parameters under the influence of gravity modulation and internal heat is picturised. From this study, we find that the Casson parameter decreases the critical Rayleigh number leads to the advancement of onset of convection and reduces heat transfer in the system.