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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: Grown 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 content of polyunsaturated fatty acids (PUFAs) including arachidonic acid (ARA) and docosahexaenoic acid (DHA) in M. elongata. To increase TAG productivity in the alga, we developed an effective reduced nitrogen supply regime based on ammonium in environmental photobioreactors (ePBRs). 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, ~15% and 22% of total dry weight (DW), respectively, as well as high levels of PUFAs. Genetic engineering 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 bioflocculation 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 bio-fuel productivity and allows the engineering of polyunsaturated fatty acids. Keywords: Microalgae, Filamentous fungi, Flocculation, Cell wall interaction, Biofuel, Nitrogen starvation, Polyunsaturated fatty acid, Triacylglycerol
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Algal-fungal symbiosis leads to photosynthetic mycelium
Yeast, molds and other fungi are found in most environments across the world. Many of the fungi that live on land today form relationships called symbioses with other microbes. Some of these relationships, like those formed with green algae, are beneficial and involve the exchange carbon, nitrogen and other important nutrients. Algae first evolved in the sea and it has been suggested that symbioses with fungi may have helped some algae to leave the water and to colonize the land more than 500 million years ago. A fungus called Mortierella elongata grows as a network of filaments in soils and produces large quantities of oils that have various industrial uses. While the details of Mortierella’s life in the wild are still not certain, the fungus is thought to survive by gaining nutrients from decaying matter and it is not known to form any symbioses with algae. In 2018, however, a team of researchers reported that, when M. elongata was grown in the laboratory with a marine alga known as Nannochloropsis oceanica, the two organisms appeared to form a symbiosis. Both the alga and fungus produce oil, and when grown together the two organisms produced more oil than when the fungus or algal cells were grown alone. However, it was not clear whether the fungus and alga actually benefit from the symbiosis, for example by exchanging nutrients and helping each other to resist stress. Du et al. – including many of the researchers involved in the earlier work – have now used biochemical techniques to study this relationship in more detail. The experiments found that there was a net flow of carbon from algal cells to the fungus, and a net flow of nitrogen in the opposite direction. When nutrients were scarce, algae and fungi grown in the same containers grew better than algae and fungi grown separately. Further, Mortierella only obtained carbon from living algae that attached to the fungal filaments and not from dead algae. Unexpectedly, further experiments found that when grown together over a period of several weeks or more some of the algal cells entered and lived within the filaments of the fungus. Previously, no algae had ever been seen to inhabit the living filaments of a fungus. These findings may help researchers to develop improved methods to produce oil from M. elongata and N. oceanica. Furthermore, this partnership provides a convenient new system to study how one organism can live within another and to understand how symbioses between algae and fungi may have first evolved.
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- Award ID(s):
- 1737898
- PAR ID:
- 10105943
- Date Published:
- Journal Name:
- eLife
- Volume:
- 8
- ISSN:
- 2050-084X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.7554/eLife.47815
