The underwater tubular photobioreactor is a fully controlled outdoor system to study photosynthetic bacteria. Before growing bacteria cells outdoors, two modified van Niel medium (vN-A, vN-B) were tested under artificial light. During exponential growth, the specific growth rates were 0.0416 and 0.0434 h(-1), respectively; vN-B was chosen for outdoor experiments. The growth behavior of Rhodopseudomonas palustris was investigated under a natural light-dark cycle (sunrise-sunset, 15L/9D) and a forced light-dark cycle (9:00-19:00, 10L/14D). Relationships between solar radiations, daily growth rates, and biomass output rates were also investigated. After determining the elemental biomass molar composition and its combustion heat, some trends of photosynthetic efficiency (PE) were obtained over daylight. The PE trends were always of the oscillatory type, with the exception of that achieved at low biomass concentration. Under a natural light/dark cycle, the maximum PE (11.2%) was attained at sunset, while under a forced light/dark cycle, the highest PE (8.5%) was achieved in the morning. Three initial biomass concentrations were investigated (0.65, 1.01, and 1.54 g l(-1)). The stoichiometric equation for bacteria cells indicated that 87.7% of the carbon of acetic acid was converted to biomass and only 12.3% was lost as CO2.