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.