Estimating Quantities of Nitrogen and Phosphorus Needed to Support an Algal Biofuel Industry
Pate et al. (2011) estimated the quantities of nitrogen and phosphorus needed to produce at least 38 billion liters of algal biofuel. Their analysis was based on the assumption of Redfield stoichiometry determined in marine systems (C106:N16:P1). However, the canonical Redfield ratio recently has been called into question by Sterner et al. (2008), who reviewed more than 2,000 measurements of the chemical content of suspended particulate matter (seston) from freshwater and marine ecosystems worldwide and documented an enormous level of variability in nutrient use efficiency. They found that small freshwater lakes exhibited higher average ratios of C:P = 224 (standard deviation = 156) and C:N = 10.0 (standard deviation = 3.0) than the Redfield stoichiometry (C:P = 106 and C:N = 6.6 by moles). Across their entire dataset, a non-Redfield proportionality of C166:N20:P1 best described the elemental composition of algae.
These trends potentially imply a higher nitrogen- and phosphorus-use efficiency (a higher yield of algal biomass per unit nitrogen and phosphorus consumed by algal cells) in artificial algal biomass production systems than was assumed in the study by Pate et al. (2011). Stoichiometric data provided in a recent algal biofuel study by Sturm et al. (2012) do not support this conclusion for phosphorus, however. Sterner et al. (2008) suggest that algal stoichiometry varies significantly with habitat type: freshwater seston tends to have a greater nutrient use efficiency (higher C:P and C:N ratios) than marine seston, implying that the future nitrogen and phosphorus demands of freshwater- versus marine-based algal biomass cultivation systems potentially could differ.
Another key question revolves around the potential effects of CO2 enrichment on algal nutrient-use efficiency. The responses of both vascular plants and phytoplankton to enhanced CO2 are variable and often species-dependent (Sardans et al., 2011a), and the consistency of CO2 effects remains uncertain. Given the observed variation in algal C:N:P stoichiometry that has been reported in the literature, three key questions therefore arise:
• What expected values (or what ranges of expected values) of C:P and C:N would best be used to update the analyses of Pate et al. (2011)?
• At what CO2 levels will consistent and predictable effects of CO2 enrichment on algal nutrient-use efficiency occur?
• Will the net effects of CO2 enrichment differ in single-strain algal cultivation systems versus systems that contain mixed-species assemblages?
These three unanswered questions represent research needs that can be filled only by field-based measurements of algal biomass yield and C:N:P stoichiometry, using pilot-scale or commercial-scale large outdoor photobioreactor systems operating under a wide range of environmental conditions.
A major constraint on the future expansion of biofuel production is likely to be the limited amount of land suitable for producing bioenergy crops and for expanding related refinery and transportation infrastructure (Cai et al., 2011). Much greater efforts will be needed to develop a comprehensive picture of the ideal siting locations for algae cultivation facilities (Darzins et al., 2010). Careful land-use planning to create specific locations where all-important resource demands can be met can help to build capacity and allow algae to make a vital, even if only modest, contribution to the U.S. biofuels industry (Lundquist et al., 2010).