I love these sorts of exchanges. Perhaps the romantic in me believes this is how science discuss was handled in days long gone. Scientists would converge at their respective Academies of Science and debate the pertinent issues of the day. You don't really see that nowadays. Occasionally you'll see a jab or two in a manuscript, but you hardly ever see two scientists butting heads publically. Or, maybe I'm not reading enough of the literature. Given the topic however, this shouldn't come as much of a surprise. Everyone is trying to fill the much needed "new fuel source" niche, so people are going to enter their horse into the fray when they can.
After Yusuf Chisti published the opinion piece mentioned earlier, Lucas Reijnders shot back a letter entitled "Do biofuels from microalgae beat biofuels from terrestrial plants?" to Trends in Biotechnology, the conclusion of which said basically that Chisti was wrong (see Reference #1).
According to Reijnders, Chisti forgot to consider some "hidden expenses". Reijnders writes:
However, Chisti did not consider fossil fuel inputs during the biofuel life cycle. Fossil fuels are currently used for building the facilities (bioreactor, pond) and for operational activities such as supplying nutrients, maintenance, mixing, the collection of microalgae and biomass processing.The letter goes on to say that while algae do produce more biomass, they take more fossil fuels to process ...
Empirical data show that, in practie, sugarcane and oil palm yield less biomass than the 100 Mg per hectare per year (dry weight) for Spirulina ... but these terrestrial plants are characterized by lower fossil fuel inputs into the biofuel life cycle for a specified amount of biofuel energy than in the case of the microalgal biofuels studies mentioned before.Doubt is also raised as to whether reported yields of microalgal biomass will be achievable in practice, and a citation is provided to support the fact that when compared to current commercial facilities, the reported values are well over what has currently been achieved.
Not so fast buddy
To Reijnders comments, Chisti responds back in the same issue in the letter entitled "Response to Reijnders: Do biofuels from microalgae beat biofuels from terrestrial plants?" (see Reference #2).
Chisti takes issue with the two algal reports cited by Reijnders, saying ...
However, both these studies show little understanding of large-scale algae culture and grossly overestimate the fossil energy required in producing algal biofuels.Chisti in the next couple of paragraphs cites a couple of instances of where the Hirano and Sawayama studies went wrong.
Chisti then takes a moment to point out that ~45% of the fossil energy input into algal biomass is linked to fertilizer. It is then pointed out that most of the added fertilizer re-emerges in the liquid effluent of the anaerobic digesters, which will be reused (remember the figure from Part I). This cuts down on the fertilizer costs by a third.
To close, if microalgal systems were in place, ~11% of cropping land would be needed to supply all US transport fuel needs. For bioethanol to do something similar, ~70% of all US cropping area will be needed.
While the discussion between Chisti and Reijnders may be over for now, this is probably not the end of the issue entirely. We still haven't picked another basket to put some, or all, of our eggs into. Meanwhile the price of food and fuel continue to rise.
1. REIJNDERS, L. (2008). Do biofuels from microalgae beat biofuels from terrestrial plants?. Trends in Biotechnology, 26(7), 349-350. DOI: 10.1016/j.tibtech.2008.04.001
2. CHISTI, Y. (2008). Response to Reijnders: Do biofuels from microalgae beat biofuels from terrestrial plants?. Trends in Biotechnology, 26(7), 351-352. DOI: 10.1016/j.tibtech.2008.04.002