INFORM January 2024

22 • inform January 2024, Vol. 35 (1)

TABLE 1. Biomass production, lipid content of some fungi that were cultured on various raw materials.

Lipid content (%, w/w) References

Biomass (g L-1)

Microorganism Mortierella alpina

Carbon source potato chips industry wastes

20.0

40.0

https://doi.org/10.1111/jam.14864

Umbelopsis isabellina

rice hull

5.6 7.2

64.0 44.1 33.3 37.5

https://doi.org/10.1016/j.biortech.2011.08.025 https://doi.org/10.1007/s12010-011-9411-z https://doi.org/10.1002/ejlt.201300358

Mortierella ramanniana glycerol

M. alpina

biodiesel derived waste

15.6 12.0

Aspergillus oryzae

oil palm empty fruit bunch

https://doi.org/10.1002/bbb.1645

Notably, in the past few decades researchers have discovered some yeast species that belong to the Basidiomycota are considered oleaginous, especially those that belong to Rhodotorula. In addition, they are used for the production of secondary metabolites such as carotenoids and fragrances. More importantly, these yeasts combine their ability to produce lipids in significant quantities with environmental remediation, due to their ability to grow on waste streams such as distillery wastewater and raw olive oil. OLEAGINOUS FUNGI AND MICROALGAE, A FRESH CONSORTIUM Fungi are known for being sociable in their ecosystems, a trait that research and industry have attempted to capitalize on through various applications. For instance, one of the most modern and advanced technologies draws upon the ability of fungi and microalgae to develop symbiotic relationships, including parasitism, commensalism, and mutualism. The fungi used in the production of SCO—as in many microbial pro cesses—burden the environment with greenhouse gases (CO 2 ), while their co-culture with microalgae somewhat exonerates them. Microalgae are photosynthetic organisms that convert solar energy and CO 2 to gain cell mass of high nutritional value and produce PUFA along with oleaginous fungi. The synergy of the two microbes is based on the microal gae’s ability to fix CO 2 produced by fungal respiration while fungi reduce complex organic compounds into simpler ones for microalgae to assimilate. Fungi also benefit from the pro duction of O 2 and microalgae extracellular secretions. They are useful for advanced wastewater treatment and biofilm for mation processes which could reduce the cost of harvesting microalgae in large scale applications, like aquaculture and cos metics, biofuels, nutraceuticals, and pharmaceuticals. More importantly, though, several investigations con cluded that co-cultures of fungi and microalgae can lead to increased production of biomass and lipids compared to mono-cultures of the same species. For example, during a co-cultivation of Aspergillus oryzae and Chlorella pyrenoidosa in starch wastewater the final biomass concentration was 1.58 and 2.04 times higher, respectively, and the lipid concentra tion was 2.41 and 1.41 times higher compared to the individual cultures of these microorganisms ( https://doi.org/10.1016/j. biortech.2022.127625) . In addition, researchers have isolated

a variety of high-value compounds from the biomass of fun gal and microalgal consortia, including polysaccharides, pro teins, pigments, and vitamins. A major barrier limiting the coexistence of these organisms is the preference of microal gae to grow at high pH values (typically above 8) while fungi like much lower pH values. This difficulty could be overcome by implementing adaptive evolution strategies (https://doi. org/10.1016/j.biotechadv.2021.107795 ). THE PRESENT IS JUST THE PRELUDE One of the greatest challenges remaining in this field is mak ing large-scale applications economically feasible. Researchers must optimize cell growth and lipid accumulation levels, while also increasing their PUFA content. Genetic engineering of spe cific strains has already been applied successfully. Although the improved strains are often not suitable for further exploitation as the conditions favoring lipid accumulation are not conducive to PUFA synthesis, and vice versa. An alternative option for strain improvement could be adaptive evolution strategies under con ditions that favor PUFA synthesis as a response to harsh con ditions, such as their cultivation at low temperatures or in the presence of toxic compounds. Moreover, such approaches could also reveal novel pathways relevant to PUFA production. Another recent, promising approach involves the co-cul ture of species belonging to the same or different kingdoms, like the consortia of fungi-microalgae previously mentioned. This collaboration has led to increased synthesis, while achiev ing bioremediation and reducing biomass harvesting costs. In addition, cross-species interactions and signaling affects metabolism and unveil valuable information leading to the design of specific protocols to improve their productivity. Lastly, despite the current clear preference to well-charac terized microorganisms for most research and industrial applica tions, it is necessary to expand our interest to unexplored species that might uncover promising candidates for the bioproduction of high-value metabolites, like SCO. In doing so, we could determine vital information for a comprehensive and improved understand ing of specific aspects of microbial physiology and the develop ment of more tools to study them further and benefit from them. Panagiotis Dritsas is a PhD candidate in the Biology Department of the University of Patras, Greece. He can be contacted at drit sas.p@ac.upatras.gr. He would like to thank George Aggelis, pro fessor of Microbiology, for his contribution to this article.