International audienceCurrent research investigating the importance of diversity for biofuel lipid production remains limited. In contrast, the relationship between diversity and productivity within terrestrial and algal primary producers has been well documented in ecology. Hence, we set out to investigate, experimentally, whether diversity may also affect lipid production in micro-algae. We investigated the growth and lipid production of micro-algae using species from all major algal groups. Algae were grown in a large number of treatments differing in their diversity level. Additionally, we compared the growth and lipid production of laboratory communities to natural lake and pond phytoplankton communities of different diversity. Our results show that lipid production increased with increasing diversity in both natural and laboratory micro-algal communities. The underlying reason for the observed 'diversity-productivity' relationship seems to be resource use complementarity. We observed higher lipid production of highly diverse algal communities under the same growth and resource supply conditions compared to monocultures. Hence, the incorporation of the ecological advantages of diversity-related resource-use dynamics into algal biomass production may provide a powerful and cost effective way to improve biofuel production

Currently, very few studies address the relationship between diversity and biomass/lipid production in primary producer communities for biofuel production. Basic studies on the growth of microalgal communities, however, provide evidence of a positive relationship between diversity and biomass production. Recent studies have also shown that positive diversity-productivity relationships are related to an increase in the efficiency of light use by diverse microalgal communities. Here, we hypothesize that there is a relationship between diversity, light use, and microalgal lipid production in phytoplankton communities. Microalgae from all major freshwater algal groups were cultivated in treatments that differed in species richness and functional group richness. Polycultures with high functional group richness showed more efficient light use and higher algal lipid content with increasing species richness. There was a clear correlation between light use and lipid production in functionally diverse communities. Hence, a powerful and cost-effective way to improve biofuel production might be accomplished by incorporating diversity related, resource-use-dynamics into algal biomass production.

Diel vertical migration (DVM) is a common behaviour of many pelagic zooplankton species. While the causes (mostly predator avoidance) and ecophysiological consequences of DVM for zooplankton have been well studied, little is known about the consequences of DVM for the pelagic food web. DVM creates a temporal and spatial grazer-free niche for edible phytoplankton, and theoretical models predict that parts of the phytoplankton community should use this niche. Conceivably, DVM could also cause nutrient transport between separated water layers. We experimentally investigated the influence of DVM of the zooplankton species, Daphnia hyalina, on the nutrient and phytoplankton dynamics of an oligotrophic lake. We used 10-m deep field enclosures with a 4-m deep, well-mixed surface layer. The enclosures contained either migrating or nonmigrating Daphnia populations; temperature was kept nearly constant across the entire enclosure depth. Our results show that DVM had significant quantitative and qualitative effects on the phytoplankton community. There was no measurable net nutrient transport between hypolimnion and epilimnion. The gelatinous green alga Planktosphaeria gelatinosa, was the dominant algal species in our experiment. Its abundance decreased in DVM treatments, and thus also influenced the total biomass and diversity of phytoplankton communities.

Although open outdoor pond systems are the most economically viable option for mass cultivation of algae as a biofuel source, such systems face a number of limitations. Open ponds experience environmental fluctuations (i.e., light levels, nutrient ratios, and temperature), invasion pressure by undesired algal species, pathogen infections, and herbivory by invading zooplankton, all of which may negatively influence the system's overall harvestable yield. Using ecological principles to address the limitations of open-pond cultivation is a promising direction in algal biofuel research. This review surveys the growing body of work on these topics and offers a mechanistic framework for optimizing algal biofuel production while minimizing the negative effects of invasion, infection, and herbivory. High levels of productivity (in terms of biomass and lipids) are crucial for viable biofuel production and can be achieved by increasing algal diversity and assembling communities based on species' eco-physiological traits. Herbivory can be significantly reduced by choosing algal species resistant to grazing or by introducing biotic controls on herbivores. Diverse assemblages of algal species can be constructed to fill in the available ecological niche space, leading not only to high productivity but also reduced invasibility by undesired strains and potentially reduced susceptibility to algal diseases. Optimization of the mass cultivation of algae requires an interdisciplinary approach that includes using ecological principles for designing productive, resistant, and resilient algal communities.

No abstract