Enhancing oil production and harvest by combining the marine alga Nannochloropsis oceanica and the oleaginous fungus Mortierella elongata

Zhi-Yan Du; Jonathan Alvaro; Brennan Hyden; Krzysztof Zienkiewicz; Nils Benning; Agnieszka Zienkiewicz; Gregory Bonito; Christoph Benning

doi:10.1186/s13068-018-1172-2

Enhancing oil production and harvest by combining the marine alga Nannochloropsis oceanica and the oleaginous fungus Mortierella elongata

Biotechnol Biofuels. 2018 Jun 22:11:174. doi: 10.1186/s13068-018-1172-2. eCollection 2018.

Authors

Zhi-Yan Du^{1

2}, Jonathan Alvaro², Brennan Hyden¹, Krzysztof Zienkiewicz^{2

3}, Nils Benning², Agnieszka Zienkiewicz^{2

4}, Gregory Bonito^{5

4}, Christoph Benning^{1

2

4

6}

Affiliations

¹ 1Department of Energy-Plant Research Laboratory, Michigan State University, East Lansing, MI 48824 USA.
² 2Department of Biochemistry and Molecular Biology, Michigan State University, East Lansing, MI 48824 USA.
³ 3Department of Plant Biochemistry, Albrecht-von-Haller-Institute for Plant Sciences, Georg-August-University, 37073 Goettingen, Germany.
⁴ 5Great Lakes Bioenergy Research Center, Michigan State University, East Lansing, MI 48824 USA.
⁵ 4Department of Plant, Soil and Microbial Sciences, Michigan State University, East Lansing, MI 48824 USA.
⁶ 6Department of Plant Biology, Michigan State University, East Lansing, MI 48824 USA.

Abstract

Background: Although 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.

Results: By 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).

Conclusions: The 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.

Keywords: Bio-flocculation; Biofuel; Cell–wall interaction; Filamentous fungi; Microalgae; Mortierella; Nannochloropsis; Nitrogen starvation; Photobioreactor; Polyunsaturated fatty acid; Triacylglycerol.