Carbon footprint and net energy yield.

The word sustainability has been used for the first time in the German language room in connection with a balanced wood economy more than 100 years ago: "Living wood biomass which can be harvested in a distinct time to be used elsewhere should not exceed the mass reproduced in the same period. By this operation formula the production potential is kept stationary and further user generations have the opportunity to yield equal amounts. Mass balance thinking may be the orientation line for a green economy (post growth economy) which is not based on fossil resources but is organized in production circuits combined with recovery techniques for materials/elements and energy under minimal entropy increase. Additional issues of sustainability introduced by the Brundtland commission as social and economic ones are finally only recommendations or guidelines for the integrated implementation of a paradigm change.

But this change is negated by the actual global market activity together with the turbo capitalism. In consequence global imbalances of essential compounds are increasing exponentially: By the elongated exploitation of fossil energy resources the concentration of CO2 in the atmosphere exceeds 400 ppm and is further mounting. In the same time the primary fixation of CO2 by photosynthesis is decreasing by infrastructure demands, by erosion and desertification and by burning tropical forest areas to gain arable land for monocultures. To cover the nutrition demand of the globally increasing number of human beings the expansion of arable land is bordered. The situation is worsened by the mounting demand of protein rich diet which will increase the demand for primary photosynthetic capacity additionally.

Regarding the 10fold higher productivity of microalgae per square meter compared to the productivity of traditional crops microalgae especially those in closed bioreactors can help to increase the primary fixation of CO2 to close the gap between global emission and absorption. The challenge hereby is that the energy content of algae dry matter must be higher than the energy needed to produce the solids. Nutrient allocation has to be included thus the use of waste CO2 (flue gas) and the use of technical growth media (aseptic waste water) are indispensable.

Finally the energy conservation by additional microalgae biomass will always be accomplished technically in a decentralized form and never must be done in competition to crop production because the technique is not dependent to arable land and therefore can really be an option to future sustainability in their Brundtland form too: The provision of employment for low and high qualified people.