Does algae diversity help algae biofuel production?

How do you maximize algae production? Researchers have posed this question countless times, under numerous variations, aiming to reach cost-effective algal biofuel production in this century.

Ecologists proposed an answer.

Val Smith spearheaded the idea that growing multiple different algae species together would produce more biomass than when growing a single algae species alone. This idea rests on the theory that biodiversity can increase productivity, because a diverse community will be better able to take advantage of all resources an environment has to offer. Since different species have different resource preferences (or “niches”), this complementarity allows more resources to be utilized overall to help produce biomass.

Algae biomass from three different cultures collected on filters.

Another feature of mixed communities is that they are often more stable than a single species (also called a monoculture). In a mixed community there will likely be some species that are resistant to a particular environmental stressor, but the same stressor might cause a monoculture to collapse if that species is sensitive to the stressor. Furthermore, a monoculture could be more easily invaded by a predator or pathogen since there will be under-utilized resources available to help these organisms take over.

These ideas were mostly verified during experiments in some prominent studies on the effect of algal diversity on productivity (see full references at end of this post). However, just last month a new study published results with the opposite conclusion: the more algae species in a culture, the lower the amount of fuel produced from that algae culture. This was very exciting news because it offers a case where diversity doesn’t always lead to higher productivity, and supports the need to replicate experiments with different species or conditions.

Some of the recent studies that showed evidence of higher productivities in mixed algal communities were by Stockenreiter et al and Shurin et al. Stockenreiter found that increasing the number of algae species in a culture tended to increase the total lipids produced from the culture (lipids are used to make biofuels). Lipid production in the mixed communities was higher than the sum of lipid contributions of each species in the community when they grew alone.

If mixed communities are to be used for algae cultivation, then they have to be more productive than their most productive monoculture: this is called overyielding. Shurin (2014) found evidence of overyielding when growing pairs of algae species together, though most of the pairs tested did not lead to overyielding.

Now, Narwani et al has published results showing that not only is there a lack of overyielding in mixed algae communities, but average biofuel production actually decreases with increasing diversity (number of algae species in the culture). No mixed community was able to outperform the monoculture Selenastrum capricornutum. Furthermore, the most stable mixed community culture was less stable than the most stable monoculture (also Selenastrum capricornutum), refuting the idea that mixed communities are better able to withstand environmental stressors (in this case temperature). Why is there such a difference in results between this study and previous ones?

Narwani et al explain that perhaps the species used in their experiments were too similar (they were all freshwater green algae species that can be found in Michigan lakes) and therefore their niches were too similar. Experimenting with more diverse combinations of different taxonomic and functional groups could lead to the overyielding results observed in previous studies, since species would be able to take advantage of more resources in their environment.

Using flow cytometry in the lab, we can differentiate between two algae species in a mixed culture based on their cell size.

I’d been waiting for a couple years to see the results of this new study, and was very surprised to read about their drastically different results compared to previous research. However, in support of the new study, it is probably more realistic that algae diversity experiments require the study species to be found in the same geographic regions. This helps ensure that the species are suited to outdoor cultivation in a location similar to their native environment. While cultivating two very different algae species may lead to overyielding in the lab, if the species can’t grow together in outdoor ponds at a commericial facility, the benefit of overyielding is no longer apparent.