NOVELTY - Cultivating photosynthetic microorganisms in sealed photobioreactor to obtain volatile hydrocarbon of photosynthesis, involves: (A) culturing photosynthetic microorganism in sealed photobioreactor containing lower aqueous phase comprising photosynthetic microorganism, and upper gaseous phase having carbon dioxide concentration of greater than or equal to 10%; (B) introducing additional carbon dioxide into upper gaseous phase when carbon dioxide concentration drops below 10%; and (C) collecting volatile product that is sequestered into gaseous phase and displaced by additional carbon dioxide.    USE - For cultivating photosynthetic microorganisms in sealed photobioreactor to obtain volatile hydrocarbon of photosynthesis; where volatile hydrocarbon product is isoprene, alcohol (preferably ethanol, butanol or isobutanol), or aldehyde (preferably acetaldehyde, butyraldehyde, or isobutyraldehyde) (claimed). The photosynthetically generated compounds are useful for renewable generation of fuels and synthetic chemistry feedstock.    ADVANTAGE - The method provides improved method that allows for renewable production, sequestration and trapping of volatile hydrocarbon compounds such as isoprene, alcohols and aldehydes produced by photosynthetic microorganisms; where the method is diffusion-driven process carried out in gaseous/aqueous two-phase photobioreactors for carbon dioxide uptake, and hydrocarbon such as isoprene plus oxygen emission by photosynthetic microorganisms (preferably microalgae or cyanobacteria). The diffusion-driven exchange of CO2 with volatile hydrocarbon e.g. isoprene plus oxygen, can be scaled-up to mass culture conditions for commercial production and exploitation of volatile hydrocarbon, such as isoprene. The method involves diffusion of CO2 from gaseous headspace into microalgal or cyanobacterial-containing aqueous phase, followed by CO2 uptake, photosynthetic assimilation, and hydrocarbon production by these photosynthetic microorganisms, which are microalgae or cyanobacteria that are genetically engineered to produce volatile hydrocarbon. Volatile hydrocarbons are than emitted from aqueous phase and are sequestered into gaseous headspace. Periodic replacement (flushing) of volatile hydrocarbon e.g. isoprene, and oxygen content of gaseous headspace with CO2, and passage through cooled condenser and hydrophobic solvent enables trapping and retention of volatile hydrocarbon, while effecting separation of volatile hydrocarbon from O2 by-product of photosynthesis. The method thus allows for renewable generation of fuels and synthetic chemistry feedstock.