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
- NSF-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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Abstract Symbionts can have profound effects on host fitness, adaptations and range distributions. Stress‐induced evolution is difficult to show in obligate symbioses; however, adaptive evolution within an obligate symbiosis can be investigated experimentally or by correlating trait variation with stress along an ecological cline (i.e. temperature‐stress gradient).
We investigated the cold tolerance of the fungus‐growing ant
Trachymyrmex septentrionalis by performing cold tolerance assays comparing two populations collected from either the southernmost range of their distribution (Bastrop, Texas) or from a site that is approximately 600 km further north (Norman, Oklahoma). We first compared isolated fungal symbionts grown on artificial media to determine cold tolerance of fungus alone. Subsequently, we conducted cross‐fostering experiments between northern and southern host and symbionts to test for synergisms between the partners in generating adaptations of cold tolerance.Ants of the northern fungal populations were more cold adapted than southern fungal populations. Northern nests were deeper and northern colonies initially rejected fungi from the southern population. The cross‐fostering experiments demonstrated that only one partner must be cold tolerant to confer maximum cold tolerance to the ant–fungus symbiosis, because northern ants growing southern fungus under cold stress performed just as well as northern ants growing northern fungi.
Our results suggest that cold stress has been an important selective factor during the migration of this ant–fungus symbiosis into northern latitudes during the last 10,000 years, and that cold tolerance likely is an energetically demanding trait that may be traded off with other aspects of the symbiosis' life history. The symbiosis also appears to have evolved several additional adaptations that increase survival in cold environments, such as building deeper nests that insulate the fungi from cold surface.
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Arbuscular mycorrhizal (AM) fungi and rhizobium are likely important drivers of plant coexistence and grassland productivity due to complementary roles in supplying limiting nutrients. However, the interactive effects of mycorrhizal and rhizobial associations on plant community productivity and competitive dynamics remain unclear. To address this, we conducted a greenhouse experiment to determine the influences of these key microbial functional groups on communities comprising three plant species by comparing plant communities grown with or without each symbiont. We also utilized N-fertilization and clipping treatments to explore potential shifts in mycorrhizal and rhizobial benefits across abiotic and biotic conditions. Our research suggests AM fungi and rhizobium co-inoculation was strongly facilitative for plant community productivity and legume (Medicago sativa) growth and nodulation. Plant competitiveness shifted in the presence of AM fungi and rhizobium, favoring M. sativa over a neighboring C4 grass (Andropogon gerardii) and C 3 forb (Ratibida pinnata). This may be due to rhizobial symbiosis as well as the relatively greater mycorrhizal growth response of M. sativa, compared to the other model plants. Clipping and N-fertilization altered relative costs and benefits of both symbioses, presumably by altering host-plant nitrogen and carbon dynamics, leading to a relative decrease in mycorrhizal responsiveness and proportional biomass of M. sativa relative to the total biomass of the entire plant community, with a concomitant relative increase in A. gerardii and R. pinnata proportional biomass. Our results demonstrate a strong influence of both microbial symbioses on host-plant competitiveness and community dynamics across clipping and N-fertilization treatments, suggesting the symbiotic rhizosphere community is critical for legume establishment in grasslands.more » « less
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Kormas, Konstantinos Aristomenis (Ed.)ABSTRACT Corals owe their ecological success to their symbiotic relationship with dinoflagellate algae (family Symbiodiniaceae). While the negative effects of heat stress on this symbiosis are well studied, how heat stress affects the onset of symbiosis and symbiont specificity is less explored. In this work, we used the model sea anemone, Exaiptasia diaphana (commonly referred to as Aiptasia), and its native symbiont, Breviolum minutum , to study the effects of heat stress on the colonization of Aiptasia by algae and the algal cell-surface glycome. Heat stress caused a decrease in the colonization of Aiptasia by algae that were not due to confounding variables such as algal motility or oxidative stress. With mass spectrometric analysis and lectin staining, a thermally induced enrichment of glycans previously found to be associated with free-living strains of algae (high-mannoside glycans) and a concomitant reduction in glycans putatively associated with symbiotic strains of algae (galactosylated glycans) were identified. Differential enrichment of specific sialic acid glycans was also identified, although their role in this symbiosis remains unclear. We also discuss the methods used to analyze the cell-surface glycome of algae, evaluate current limitations, and provide suggestions for future work in algal-coral glycobiology. Overall, this study provided insight into how stress may affect the symbiosis between cnidarians and their algal symbionts by altering the glycome of the symbiodinian partner. IMPORTANCE Coral reefs are under threat from global climate change. Their decline is mainly caused by the fragility of their symbiotic relationship with dinoflagellate algae which they rely upon for their ecological success. To better understand coral biology, researchers used the sea anemone, Aiptasia, a model system for the study of coral-algal symbiosis, and characterized how heat stress can alter the algae's ability to communicate to the coral host. This study found that heat stress caused a decline in algal colonization success and impacted the cell surface molecules of the algae such that it became more like that of nonsymbiotic species of algae. This work adds to our understanding of the molecular signals involved in coral-algal symbiosis and how it breaks down during heat stress.more » « less
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Abstract Producer–decomposer interactions within aquatic biofilms can range from mutualistic associations to competition depending on available resources. The outcomes of such interactions have implications for biogeochemical cycling and, as such, may be especially important in northern peatlands, which are a global carbon sink and are expected to experience changes in resource availability with climate change. The purpose of this study was to evaluate the effects of nutrients and organic carbon on the relative proportion of primary producers (microalgae) and heterotrophic decomposers (bacteria and fungi) during aquatic biofilm development in a boreal peatland. Given that decomposers are often better competitors for nutrients than primary producers in aquatic ecosystems, we predicted that labile carbon subsidies would shift the biofilm composition towards heterotrophy owing to the ability of decomposers to outcompete primary producers for available nutrients in the absence of carbon limitation.
We manipulated nutrients (nitrate and phosphate) and organic carbon (glucose) in a full factorial design using nutrient‐diffusing substrates in an Alaskan fen.
Heterotrophic bacteria were limited by organic carbon and algae were limited by inorganic nutrients. However, the outcomes of competitive interactions depended on background nutrient levels. Heterotrophic bacteria were able to outcompete algae for available nutrients when organic carbon was elevated and nutrient levels remained low, but not when organic carbon and nutrients were both elevated through enrichment.
Fungal biomass was significantly lower in the presence of glucose alone, possibly owing to antagonistic interactions with heterotrophic bacteria. In contrast to bacteria, fungi were stimulated along with algae following nutrient enrichment.
The decoupling of algae and heterotrophic bacteria in the presence of glucose alone shifted the biofilm trophic status towards heterotrophy. This effect was overturned when nutrients were enriched along with glucose, owing to a subsequent increase in algal biomass in the absence of nutrient limitation.
By measuring individual components of the biofilm and obtaining data on the trophic status, we have begun to establish a link between resource availability and biofilm formation in northern peatlands. Our results show that labile carbon subsidies from outside sources have the potential to disrupt microbial coupling and shift the metabolic balance in favour of heterotrophy. The extent to which this occurs in the future will probably depend on the timing and composition of bioavailable nutrients delivered to surface waters with environmental change (e.g. permafrost thaw).
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.7554/eLife.47815
