The energy input into closed photobioreactors (PBRs) for aeration and/or agitation by pneumatic and mechanical devices impacts their material, operational and production costs, accounting for up to circa 80% of the total costs. This work describes the developmental details of a photobioreactor based on the thermosiphon effect, where passive fluid circulation arises from temperature-induced density differences due to light absorption. The prototype reactor's thermal and hydrodynamic performance was investigated using Computational Fluid Dynamics (CFD) and validated with experimentation. The non-uniform volumetric sensible heating from light absorption, primarily mediated by microbial cells in the reactor, was modeled using a radiative transport equation which incorporates experimentally obtained spectral irradiance and attenuation parameters of Rhodopseudomonas palustris, a photosynthetic bacterium of industrial interest. The thermosiphon photobioreactor's (TPBR's) buoyancy driven convection was characterized by the boussinesq approximation as well as experimental and theoretical heat transfer coefficients. Experimental data from thermocouple sensors, light meters, and marker image tracking in the TPBR containing a biomass loading of 0.5 kg/m(3) R. palustris were used to validate the CFD model at the same operating conditions. The CFD simulation results were generally in very good agreement with experimental data and have indicated that the TPBR demonstrated excellent liquid circulation and turbulence properties, capable of circulating up to 88% of the microbial cells in free suspension, using only passive circulation. The reactor demonstrates an anaerobic photobioreactor system entirely agitated using passive circulation, which has the potential to be used as a low energy-input reactor for photosynthetic organism cultivation.