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Abstract
BackgroundAlthough microalgal biofuels have potential advantages over conventional fossil fuels, high production costs limit their application in the market. We developed bio-flocculation and incubation methods for the marine alga, Nannochloropsis oceanica CCMP1779, and the oleaginous fungus, Mortierella elongata AG77, resulting in increased oil productivity.ResultsBy growing separately and then combining the cells, the M. elongata mycelium could efficiently capture N. oceanica due to an intricate cellular interaction between the two species leading to bio-flocculation. Use of a high-salt culture medium induced accumulation of triacylglycerol (TAG) and enhanced the contents of polyunsaturated fatty acids (PUFAs) including arachidonic acid and docosahexaenoic acid in M. elongata. To increase TAG productivity in the alga, we developed an effective, reduced nitrogen-supply regime based on ammonium in environmental photobioreactors. Under optimized conditions, N. oceanica produced high levels of TAG that could be indirectly monitored by following chlorophyll content. Combining N. oceanica and M. elongata to initiate bio-flocculation yielded high levels of TAG and total fatty acids, with ~ 15 and 22% of total dry weight (DW), respectively, as well as high levels of PUFAs. Genetic engineering of N. oceanica for higher TAG content in nutrient-replete medium was accomplished by overexpressing DGTT5, a gene encoding the type II acyl-CoA:diacylglycerol acyltransferase 5. Combined with bio-flocculation, this approach led to increased production of TAG under nutrient-replete conditions (~ 10% of DW) compared to the wild type (~ 6% of DW).ConclusionsThe combined use of M. elongata and N. oceanica with available genomes and genetic engineering tools for both species opens up new avenues to improve biofuel productivity and allows for the engineering of polyunsaturated fatty acids.
Highlights
Microalgal biofuels have potential advantages over conventional fossil fuels, high produc‐ tion costs limit their application in the market
We discovered that N. oceanica could be efficiently flocculated by M. elongata AG77, a soil fungus that accumulates high levels of TAG and arachidonic acid (ARA), and for which a sequenced genome is available [32]
The interaction of N. oceanica with filamentous fungi appeared specific to M. elongata, as it was not observed in co-culture with Morchella americana 3668S (Fig. 1)
Summary
Microalgal biofuels have potential advantages over conventional fossil fuels, high produc‐ tion costs limit their application in the market. Advantages of microalgae over other biofuel feedstocks include high oil yield, short generation times, low agricultural land requirements, reduced fresh water needs, and reduced greenhouse gas emissions during algal cultivation [1,2,3]. The genomes of many species and strains of Nannochloropsis including N. gaditana and N. oceanica have been sequenced [13,14,15], and genetic engineering methods have been developed for gene disruption, i.e. CRISPR–Cas9 [8, 16, 17], and to generate overproduction systems for triacylglycerols (TAGs) and other target molecules [18, 19]
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