Power of Plankton: Effects of Algal Biodiversity on Biocrude Production and Stability

Abstract

Algae-derived biocrude oil is a possible renewable energy alternative to fossil fuel based crude oil. Outdoor cultivation in raceway ponds is estimated to provide a better return on energy invested than closed photobioreactor systems. However, in these open systems, algal crops are subjected to environmental variation in temperature and irradiance, as well as biotic invasions which can cause costly crop instabilities. In this paper, we used an experimental approach to investigate the ability of species richness to maximize and stabilize biocrude production in the face of weekly temperature fluctuations between 17 and 27 °C, relative to a constant-temperature control. We hypothesized that species richness would lead to higher mean biocrude production and greater stability of biocrude production over time in the variable temperature environment. Counter to our hypothesis, species richness tended to cause a decline in mean biocrude production, regardless of environmental temperature variation. However, biodiversity did have stabilizing effects on biocrude production over time in the variable temperature environment and not in the constant temperature environment. Altogether, our results suggest that when the most productive and stable monoculture is unknown, inoculating raceway ponds with a diverse mixture of algae will tend to ensure stable harvests over time.

Biodiversity improves the ecological design of sustainable biofuel systems

“…Our experiment contradicts the prediction that algal diversity increases the production of biomass (H1). In our experiment, most of the polycultures actually produced less biomass than the average monoculture (Figure a)—results that are consistent with the recent laboratory experiment that used the same species pool (Narwani et al., ). The poor performance of polycultures in the laboratory experiment was attributable to competition among species of algae.…”

“…The species selected for this experiment were freshwater green microalgae that (a) were part of the Department of Energy's Aquatic Species Program, (b) are widespread throughout the United States (Environmental Protection Agency, ), and (c) are known to contribute to enhanced biomass production (Fritschie, Cardinale, Alexandrou, & Oakley, ), stability (Narwani et al., ), and feedstock quality (Hietala et al., ) in our own prior laboratory experiments. Based on these prior laboratory‐based experiments, we ranked each species and polyculture in terms of its mean biomass concentration, mean stability of biomass through time (mean divided by standard deviation), and the mean higher heating value (HHV) of biocrude produced from hydrothermal liquefaction (HTL) of biomass.…”

“…Based on this ranking, we selected four species for this experiment: Ankistrodesmus falcatus (A), Chlorella sorokiniana (B), Scenedesmus acuminatus (D), and Selenastrum capricornutum (F). We employ the same species codes as our previous work for consistency (Godwin et al., , ; Hietala et al., ; Narwani et al., ).…”

“…To concentrate biomass for HTL, we settled a 2.5 L sample in the dark for 7 days. The samples were further concentrated by decanting and centrifugation (Hietala et al., ; Narwani et al., ). The concentrated biomass was dried at 60°C until mass was constant.…”

“…To date, there have been few tests of the hypothesis that biodiversity can improve the cultivation of algal feedstocks, and nearly all have been constrained to laboratory‐scale experiments. The few laboratory experiments that have tested this hypothesis have shown that, compared to the average monoculture, diverse cultures of algae may (Liu, ; Shurin et al., ; Stockenreiter, Haupt, Seppälä, Tamminen, & Spilling, ; Stockenreiter et al., ) or may not (Narwani, Lashaway, Hietala, Savage, & Cardinale, ) exhibit higher total cell biovolume or lipid content, but do exhibit more stable production through time (Narwani et al., ). Although laboratory experiments suggest that biodiversity could improve multifunctionality in algal biofuel feedstock cultivation, it is unknown whether those findings are applicable to conditions in the field where conditions are often less favorable.…”