NOVELTY - The photobioreactor comprises reactor chambers (110) for enclosing a phototrophic microorganism and culture medium (140), heat energy systems that is a heat exchange chamber (120) to contain a heat exchange fluid (150), and an inflatable thermal control layer that provides a controllable thermal contact. A part of the reactor chamber is transparent for light of a wavelength that is photosynthetically active in the phototrophic microorganism, and is a plastic layer that is part of the enclosure of the control layer. The chamber and energy system are present in controllable thermal contact.    USE - The photobioreactor is useful in a solar energy gathering system for thermal control of a culture medium containing a phototrophic organism (all claimed), and for the production of the carbon-based products of interest including: biofuel such as ethanol; hydrocarbons and alkanes such as jet propellant 8; polymers such as terephthalate; commodity chemicals such as lactate and docosahexaenoic acid; specialty chemicals such as carotenoids; and pharmaceuticals and pharmaceutical intermediates such as 7-aminodeacetoxycephalosporanic acid/cephalosporin.    ADVANTAGE - The thin-film photobioreactor allows temperature control in a cost effective manner, reduces the energy required for temperature control of a culture medium containing phototrophic microorganisms, and can be economically manufactured with large dimensions.    DETAILED DESCRIPTION - The photobioreactor comprises reactor chambers (110) for enclosing a phototrophic microorganism and culture medium (140), heat energy systems that is a heat exchange chamber (120) to contain a heat exchange fluid (150), and an inflatable thermal control layer that provides a controllable thermal contact. A part of the reactor chamber is transparent for light of a wavelength that is photosynthetically active in the phototrophic microorganism, and is a plastic layer that is part of the enclosure of the control layer. The chamber and energy system are present in controllable thermal contact through a separating layer (130) that is polymer sheet, are an integral part of the photobioreactor, are structurally coupled in close proximity, are separated by the thermal control layer, are designed for passive thermal regulation, and are made of thin-film material. The heat energy system present in controlled thermal contact abuts the reactor chamber. The heat energy system present in reduced thermal contact is spatially distanced from the reactor chamber. The thermal control layer is adapted to control the spatial distance between the reactor chamber and the heat energy system, comprises an enclosure for containing fluid that is a gas or liquid, where the gas comprises air, abuts the reactor chamber on a first side of the thermal control layer, and abuts the heat exchange volume on a second side of the thermal control layer. The enclosure is flexible to allow significant structural and thermal contact between the reactor chamber and the heat energy system in the absence of the gas and reduced or no structural and thermal contact in the presence of the gas. The reactor chamber is made of a flexible material in an area that is present in contact with the thermal control layer, is channels positioned parallel to each other and providing a first side of the photobioreactor, and is flexible to inflation of the inflatable thermal control layer. The thermal contact layer is provided between the reactor chamber and the heat energy system through the enclosure and liquid. The heat energy system is made of a flexible material in an area that is present in contact with the thermal control layer, is channels positioned parallel to each other and providing a second side opposite to the first side, and is adapted to be partially embedded in the ground. The average daily cooling power requirement for controlling the temperature of the culture medium is less than 2.5 W/m. The volumes are adapted to be to different extents with liquid to provide a tilt of the reactor chambers. The photobioreactor is structurally adapted for horizontal placement on ground and/or for floating on water, and for sterilization with vaporized hydrogen peroxide, ozone, sodium hydroxide or radiation. INDEPENDENT CLAIMS are included for:    (1) a solar energy gathering system; and    (2) a method for thermal control of a culture medium containing a phototrophic organism in a photobioreactor.    DESCRIPTION OF DRAWING(S) - The diagram shows a cross-sectional view of channels of a photobioreactor panel.    Reactor chamber (110)    Heat exchange chamber (120)    Separating layer (130)    Culture medium (140)    Headspace (145)    Heat exchange liquid. (150)