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Supplementation of carbon is critical for high productivity cultivation of most microalgae. Moreover, using microalgae for atmospheric CO2 mitigation to combat climate change is promising as waste sources and atmospheric CO2 can be utilized to produce useful products. The challenge is developing technologies, processes, and strategies that utilize carbon efficiently such that the overall system is sustainable. Through engineering system modeling combined with techno-economic and life-cycle assessments, this study examined the implications of various delivery methods of carbon to a production-scale algal biorefinery. Five primary carbon sources were considered: atmospheric CO2; CO2 from direct chemical or power plant waste emissions; CO2 that has been concentrated from waste sources and compressed; inorganic carbon in the form of hydrogen carbonate; and organic carbon in the form of cellulosic sugars derived from corn stover. Each source was evaluated assuming co-location as well as pipeline transportation up to 100 km. Sustainability results indicate that economics are more prohibitive than energy and emissions. Of the scenarios evaluated, only two met both the economic and environmental criteria of contributing less than 0.50 US-$ GGE-1 and 20 g CO2-eq MJ(-1) to the overall system, respectively: uncompressed, pure sources of gaseous CO2 with pipeline transportation of 40 km or less; and compressed, supercritical CO2 from pure sources for pipeline transportation up to 100 km. A first order scalability assessment of algal biofuels based on these results shows carbon to be a limiting nutrient in an algal biorefinery with a total US production capability of 360 million gallons of fuel per year.