Ecological optimization of biomass and lipid production by microalgae

Microalgae have higher growth rates and higher lipid content than terrestrial plants and the yield per unit area is even higher by several orders of magnitude. Furthermore, the production of microalgae does not compete for fertile land for food production. Therefore, microalgae are in the focus of research for biodiesel production, nutritional supplements and aquaculture approaches. However, after almost half a century of research the full promise of microalgae as a feedstock for biofuel production has remained largely unfulfilled. My research was motivated by the obvious gaps in the application of ecological pros of microalgae. DIVERSITY-PRODUCTIVITY RELATIONSHIPS: THE ROLE OF DIVERSITY FOR MICROALGAL LIPID PRODUCTION The relationship between diversity and productivity within terrestrial and algal primary producers has been well documented in ecology. However, the importance of diversity for lipid production for biofuel remains limited. Hence, I set out to investigate, experimentally, whether diversity may also affect lipid production in microalgae. Microalgae from all major algal groups were grown in a large number of treatments differing in their diversity level. Additionally, I compared the growth and lipid production of laboratory communities with the lipid production of natural lake and pond phytoplankton communities along a diversity gradient. This comparison showed that the lipid production of selected laboratory monocultures was not significantly higher than that of natural phytoplankton communities. The lipid production in general increased with increasing diversity in both natural and laboratory microalgal communities. The underlying reason for the observed ‘diversity-productivity’ relationship seems to be resource use complementarity. Additionally, a very important observation was that diversity also influences the specific lipid production of each microalgae in the high diverse communities. DIVERSITY- LIGHT- LIPID RELATIONSHIPS: LIPID PRODUCTION IN THE RIGHT LIGHT The knowledge about the relationship between diversity and biomass/lipid production in primary producer communities for biofuel production is underestimated. However, basic ecological research studies on the growth of microalgal communities provide evidence of a positive relationship between diversity and biomass production and show that the observed positive diversity-productivity-relationships are related to an increase in the efficiency of light use by diverse microalgal communities. I cultivated microalgae from all major freshwater algal groups in treatments that differed in their species richness and functional group richness. Polycultures with high functional group richness showed higher light use and algal lipid content with increasing species richness. Additionally, I could show a clear correlation between light use and lipid production in functionally diverse communities. Therefore, a powerful and cost effective way to improve biofuel production might be accomplished by incorporating diversity related resource-use-dynamics into algal biomass production. DIVERSITY AND FOOD QUALITY: ADVANTAGES FOR AQUACULTURE FOOD WEBS Determining the factors that control the energy transfer at the plant-animal interface is a key issue in ecology, because this transfer is highly variable and despite its global importance it is still not well understood. Food quality of primary producers seems to be a crucial factor influencing the transfer efficiency towards higher trophic levels. One major aspect of food quality is the biomass fatty acid composition in terms of essential ω3-polyunsaturated fatty acids (ω3-PUFAs) of primary producers, because all animals are incapable to synthesize them de novo. However, the influence of diversity on phytoplankton food quality in terms of lipid composition (e.g. ω3-PUFAs) remains unclear. I tested via a series of experiments controlled for diversity how the diversity of microalgal communities influences their fatty acid composition. My study shows the significant influence of diversity of primary producer communities on their fatty acid composition; especially on essential ω3-PUFA content. MICROALGAL BIOMASS CONTROL VIA GRAZING: IMPACT OF MICROALGAL SIZE The direction and strength of phytoplankton community responses to zooplankton grazing most probably depend on the size of phytoplankton species. To examine the influence of migrating (diel vertical migration, DVM) and non migrating zooplankton communities on different sized phytoplankton communities, I designed an experiment where I manipulated the size distribution of a natural phytoplankton community a priori in field mesocosms. Comparison of “migration” and “no migration” zooplankton treatments showed that nutrient availability and total phytoplankton biovolume were higher in “no migration” treatments with phytoplankton communities comprising mainly small algae and in “migration” treatments with phytoplankton communities of a broader size spectrum of algae. Additionally my results showed experimentally that food size selection and migration behavior of Daphnia hyalina can cause a shift from small sized microalgae towards larger species. NEW CULTIVATION TECHNIQUES FOR BIOMASS AND LIPID YIELD OPTIMIZATION IN MICROALGAE For the installation of infrastructure for the large-scale production of biofuel from microalgae is essential to establish cultivation methods that maximize lipid production but which are also economically viable in terms of energy demand and resource supply. For this purpose, I compared different cultivation systems (semi-batch, continuous) to optimize simultaneously growth and biomass lipid content of Botryococcus braunii. To enhance both, biomass accumulation and lipid production at the same time I further investigated a two-stage cultivation method to replace one stage semi-batch cultivation systems. In the first step of this cultivation method a full growth medium allows an enhancement of biomass accumulation. In the next step, the culture was transferred into nitrogen limited growth medium, where a further accumulation of photosynthetic products in the form of lipids occurred. Two-stage cultivation cultures resulted in higher nutrient specific biomass production and lipid content of B. braunii compared to one stage cultivation. If a continuous cultivation of cultures with high biomass in stage one can be assured, an almost constant supply of huge amounts of algae with even high lipid content in the second step could be guaranteed. My results clearly show that a better understanding of general ecological principles for biomass and lipid production of microalgae provides a cost effective and environmental friendly way to cultivate high yielding microalgal communities for commercial approaches. The enhancement of the yield efficiency of lipid production in diverse microalgal communities would be difficult to do only by technical means such as increasing resource supply. In addition, increasing the supply of resources is usually correlated with high energy requirements and therefore cost intensive. It is therefore important for biomass production systems to utilize all possible ecological options to increase the efficiency of the use of the supplied resources by integrating basic ecological principles into the cultivation systems

Long-term acclimation might enhance the growth and competitive ability of Microcystis aeruginosa in warm environments

1. The positive effect of global warming on the growth of cyanobacteria has been widely predicted, but long-term studies targeting their adaptive potential to higher temperature have not been carried out so far. Predicting the magnitude and impact of cyanobacterial blooms in the future as a response to global warming requires an understanding of how cyanobacteria might change in the long term due to climate change. 2. Here we examined the effect of exposing three Microcystis aeruginosa strains isolated in Romania to ambient (22°C) and high (26°C) temperature for 6 months. Then, the competitive ability of the strains after heat acclimation was evaluated, by analysing their impact on plankton community composition. 3. One of the three strains displayed significantly higher growth rates after 6 months of cultivation at higher temperatures. Following inoculation into a natural plankton community, the overall cyanobacterial abundance significantly increased in the cultures inoculated with this heat-acclimated strain of M. aeruginosa as compared to the ambient-acclimated version. The structure of eukaryotic communities was impacted by both inoculated cyanobacteria and temperature during the experiments. 4. The results of this study emphasise the high potential of cyanobacteria to respond to stressors, and highlight the fact that previous acclimation to warming is a critical factor in shaping the overall structure of plankton communities. 5. Our study strongly advocates for including a step of culture acclimation to future experimental conditions in research programmes aiming to better understand the long-term impact of climate change on aquatic ecosystems

The combined impact of low temperatures and shifting phosphorus availability on the competitive ability of cyanobacteria

In freshwater systems, cyanobacteria are strong competitors under enhanced temperature and eutrophic conditions. Understanding their adaptive and evolutionary potential to multiple environmental states allows us to accurately predict their response to future conditions. To better understand if the combined impacts of temperature and nutrient limitation could suppress the cyanobacterial blooms, a single strain of Microcystis aeruginosa was inoculated into natural phytoplankton communities with different nutrient conditions: oligotrophic, eutrophic and eutrophic with the addition of bentophos. We found that the use of the bentophos treatment causes significant differences in prokaryotic and eukaryotic communities. This resulted in reduced biodiversity among the eukaryotes and a decline in cyanobacterial abundance suggesting phosphorus limitation had a strong impact on the community structure. The low temperature during the experiment lead to the disappearance of M. aeruginosa in all treatments and gave other phytoplankton groups a competitive advantage leading to the dominance of the eukaryotic families that have diverse morphologies and nutritional modes. These results show cyanobacteria have a reduced competitive advantage under certain temperature and nutrient limiting conditions and therefore, controlling phosphorus concentrations could be a possible mitigation strategy for managing harmful cyanobacterial blooms in a future warmer climate

Trophic switches in pelagic systems

Ecological studies need experimentation to test concepts and to disentangle causality in community dynamics. While simple models have given substantial insights into population and community dynamics, recent ecological concepts become increasingly complex. The globally important pelagic food web dynamics are well suited to test complex ecological concepts. For instance, trophic switches of individual organisms within pelagic food webs can elongate food webs or shift the balance between autotroph and heterotroph carbon fluxes. Here, we summarize results from mesocosm experiments demonstrating how environmental drivers result in trophic switches of marine phytoplankton and zooplankton communities. Such mesocosm experiments are useful to develop and test complex ecological concepts going beyond trophic level–based analyses, including diversity, individual behavior, and environmental stochasticity

Copepods Boost the Production but Reduce the Carbon Export Efficiency by Diatoms

The fraction of net primary production that is exported from the euphotic zone as sinking particulate organic carbon (POC) varies notably through time and from region to region. Phytoplankton containing biominerals, such as silicified diatoms have long been associated with high export fluxes. However, recent reviews point out that the magnitude of export is not controlled by diatoms alone, but determined by the whole plankton community structure. The combined effect of phytoplankton community composition and zooplankton abundance on export flux dynamics, were explored using a set of 12 large outdoor mesocosms. All mesocosms received a daily addition of minor amounts of nitrate and phosphate, while only 6 mesocosms received silicic acid (dSi). This resulted in a dominance of diatoms and dinoflagellate in the +Si mesocosms and a dominance of dinoflagellate in the -Si mesocosms. Simultaneously, half of the mesocosms had decreased mesozooplankton populations whereas the other half were supplemented with additional zooplankton. In all mesocosms, POC fluxes were positively correlated to Si/C ratios measured in the surface community and additions of dSi globally increased the export fluxes in all treatments highlighting the role of diatoms in C export. The presence of additional copepods resulted in higher standing stocks of POC, most probably through trophic cascades. However it only resulted in higher export fluxes for the +Si mesocosms. In the +Si with copepod addition (+Si +Cops) export was dominated by large diatoms with higher Si/C ratios in sinking material than in standing stocks. During non-bloom situations, the grazing activity of copepods decrease the export efficiency in diatom dominated systems by changing the structure of the phytoplankton community and/or preventing their aggregation. However, in flagellate-dominated system, the copepods increased phytoplankton growth, aggregation and fecal pellet production, with overall higher net export not always visible in term of export efficiency

Community shifts from eukaryote to cyanobacteria dominated phytoplankton: The role of mixing depth and light quality

Lake stratification strengthens with increasing surface water temperatures, thereby reducing the depth of the mixed layer. Phytoplankton communities are not only exposed to different nutrient availability within a mixed water column, but also to different light quality. We conducted controled laboratory and mesocosm experiments to investigate phytoplankton, especially cyanobacteria, responses to different light quality and mixing depths. Our mesocosm experiment allowed the manipulation of mixing depth in situ by a mesocosm approach and to follow the effects of changing mixing depth on the phytoplankton community composition. Our laboratory experiment allowed the control of temperature and light quantity. To investigate the effect of light quality on phytoplankton, we created a light gradient from full photosynthetic active radiation to a reduced blue spectrum. In both experiments, shifts in phytoplankton community composition from eukaryote to cyanobacteria occurred at shallow mixing depth with higher availability of photosynthetic active radiation. Our results from the mesocosm experiment support the idea that reduced mixing depth can promote cyanobacterial abundance. With our laboratory experiment, we were able to manipulate light quality independent of temperature, available nutrients and light intensity influencing phytoplankton abundance. Results from the laboratory experiments support our hypothesis that a shift in light spectrum alone is a driver, strong enough to enhance cyanobacteria occurrence. Most of the previous studies dealing with cyanobacterial blooms have investigated temperature and eutrophication effects. Certainly, these are major factors for the growth of phytoplankton, but our experiments show that other aspects, such as the quality of light, must be also taken into account to explain cyanobacterial blooms. Such shifts in the phytoplankton community from eukaryote to cyanobacteria dominated communities will have strong consequences for food web dynamics. Several cyanobacteria specific traits, (e.g., toxin production, lack of essential fatty acids, and inedibility through production of large colonies) reduce transfer efficiencies of energy and matter between phyto- and zooplankton and therefore can influence higher trophic levels such as fish

Initial size structure of natural phytoplankton communities determines the response to Daphnia diel vertical migration

Diel vertical migration (DVM) is a common behavior of many pelagic herbivorous zooplankton species in response to predation pressure. It is characterized by a twice daily habitat shift of the zooplankton species: staying in the epilimnion only during night time and migrating down in the crack of dawn in deeper water layers, staying there during the day time. This causes a discontinuous grazing regime and previous studies have shown that the direction and strength of phytoplankton community responses to zooplankton DVM most probably depends on the size of phytoplankton species. To examine the influence of zooplankton DVM on different sized phytoplankton communities, we designed an experiment where we manipulated the size distribution of a natural phytoplankton community a priori in field mesocosms. We investigated the influence of DVM of the cladoceran Daphnia hyalina on two different phytoplankton communities, by the use of deep (10 m) field enclosures. Epilimnetic lake water, containing a summer phytoplankton community, was filtered with two different mesh sizes (11 mm and 64 mm). The 11 mm phytoplankton community (“small”) contained mainly small algal species, while the 64 mm community (“large”) had a wider range of phytoplankton sizes. To simulate zooplankton DVM, D. hyalina were placed in mesh cages that were lowered or raised (“migration”) as dictated by the study design; a “no migration” (representing absence of DVM) treatment was also tested. Phytoplankton abundance was measured using chlorophyll-a and biovolume; size distribution of the algae and nutrient availability was also determined in each treatment. The results indicated that DVM had contrasting effects on the two evaluated phytoplankton communities. Comparison of “migration” and “no migration” zooplankton treatments showed that nutrient availability and total phytoplankton biovolume was higher in (1) “no migration” treatments with phytoplankton communities comprising mainly small algae and (2) “migration” treatments with phytoplankton communities of a broader size spectrum of algae. Hence our study showed two different mechanisms of how zooplankton DVM may influence the phytoplankton community dynamics. Nutrient cycling was an important factor in phytoplankton communities of mainly small algae, whereas the refuge effect was the main driver of phytoplankton dynamics in phytoplankton communities of a large size spectrum of algae

The effect of species diversity on lipid production by micro-algal communities

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