A Promising Approach to Enhance Microalgae Productivity by Exogenous Supply of Vitamins Puja Tandon, Qiang Jin* and Limin Huang

Tandon et al. Microb Cell Fact (2017) 16:219 DOI 10.1186/s12934-017-0834-2 Microbial Cell Factories REVIEW Open Access A promising approach to enhance microalgae productivity by exogenous supply of vitamins Puja Tandon, Qiang Jin* and Limin Huang Abstract In order to reduce the consumption of traditional fossil fuels and their impact on the environment, strategies to mitigate greenhouse gas emissions especially carbon dioxide needs exploration. Microalgae-based biofuels can be the best-ft plant based feed-stocks for diminishing a majority of the Universe’s energy problems. Interestingly, the eukaryotic microalgae aid in fxation of almost 50% of the global carbon in the environment. Thus, determination of param- eters that will enhance microalgal growth and productivity is crucial, if they are to be used as future renewable energy sources. A large percentage of phytoplankton species are auxotroph for one or more vitamins. These species, in turn, are also dependent upon the vitamin biosynthetic pathways for processing of these vitamins. The present study serves as a base to discuss the prevalence of vitamin auxotrophy in microalgae and the methods of its acquirement from external sources such as heterotrophic bacteria. The next section of the paper sheds light on possible species- specifc symbiotic interactions among microalgae and bacteria. Lastly is the discussion on how heterotrophic bacteria can act as a vitamin prototroph for an explicit microalgal vitamin auxotroph. The overall focus is placed upon harness- ing these symbiotic interactions with intentions to obtain enhancements in microalgal biomass, lipid productivity, and focculation rates. Moreover, the growth and distribution of a microalgal cell that thrives on a specifc vitamin is perhaps met by growing it with the bacterial communities that nourish it. Thus, possibly by ecologically engineering a potential species-specifc microalgal–bacterial consortium, it could tremendously contribute to the acceleration of photosynthetic activity, microalgal productivity, exchange of primary metabolites and other biogeochemical nutrients within the mini ecosystem. Keywords: Renewable energy, Microalgae, Bacteria, Vitamin, Auxotroph, Symbiosis Background exhausted by the frst half of twenty-frst century, leading Environmental concerns and diminishing fossil fuels to the global energy crises [2]. Nevertheless, in the con- Te accelerating population, impact of greenhouse gases sumption and depletion of these fnite energy resources, on global warming, and depletion of fossil fuels have led we have challenged our environment and health. In order to the exploration of other alternatives for renewable to reduce the consumption of traditional fossil fuel and energy sources. Te consumption of non-renewable fos- as well as their impact on the environment, strategies sil fuel energy has increased by three times over the last to mitigate greenhouse gas emissions, especially carbon 50 years because of the intense development of civiliza- dioxide (CO2), require exploration of viable sustainable tion [1]. Te economic engine of any civilization depends alternatives to fossil fuels [3, 4]. upon the production of everlasting energy resources. However, reserves of the fossil fuels are predicted to be Microalgae as a best‑ft for future renewable energy source Renewable and sustainable energy resources such as wind, solar, water, tidal, bioenergy (biofuels) and geo- *Correspondence: [email protected] thermal are being extensively generated with intentions School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China of transiting to a clean, green, domestic and resilient © The Author(s) 2017. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/ publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Tandon et al. Microb Cell Fact (2017) 16:219 Page 2 of 13 renewable energy resource. Of the above resources, bio- phytohormones) are exchanged between each other. fuels can be the best-ft plant based feed-stock for dimin- Moreover, both the species alter their metabolism to suit ishing a majority of the Universe’s energy problems [2, 4]. each partner’s needs [15]. A complex succession of the Among the three generations of feed-stock (food crops, endosymbiotic associations has led to the formation of non-food crops, and microalgae) for biofuels, microalgae the present dominant microalgal lineages. can be regarded as the best source of feed-stocks in terms Vitamins play a vital part in the development of cel- of high photosynthetic activity, reducing food insecurity lular biochemistry of microalgae. However, very little and the harmful impacts on the environment [5, 6]. Te information is available concerning the infuence of these initial use of microalgae by Homo sapiens dates back to micronutrients on the microalgal growth, diversity, and 2000 years when the Chinese used to cultivate Nostoc in productivity [16]. Recent research study have proven that order to preserve food during years of famine [7]. Lately, microalgal growth can be enriched by certain growth microalgae have received greater attention in the feld of stimulating co-factors synthesized by bacteria such as research as they have vast advantages over crop plants in phytohormones (indole-3-acetic acid: IAA, auxin), vibri- terms of their ability to produce almost 300 times more oferrin, antibiotics, vitamins, and siderophores [12]. For renewable oil. In addition, they can better adapt to the example, in the case of mutualistic symbiotic associa- environment, can reduce negative ecological impacts, tion between the two, bacterial species in reciprocation can ofer positive role in bioremediation, are commer- of fxed carbon, provide vitamin B 12 (cobalamin) to the cially competitive and can be considered as an ideal microalgae [17]. Some microalgae require diferent com- organism [8]. Microalgae consist of simple unicellular binations of vitamins (biotin, cobalamin, and thiamine) and multicellular photosynthetic autotrophs. Tey can as a growth factor, however; they cannot produce them. be either a prokaryote (cyanobacteria) or a eukaryote, As only prokaryotes have the ability to produce some of with growing in diferent ecological environments and these vitamins there has to be some defnitive source of producing diverse metabolites [9]. Microalgae can easily the vitamins for the microalgae. grow via photoautotrophic mode by using sunlight as a Te present study begins with a review of the preva- solitary energy source and carbon dioxide as the carbon lence of vitamin auxotrophy in microalgae and the source through the photosynthetic reactions occurring at methods of its acquirement from other micro-organ- an optimum temperature of 25 °C [10]. Eukaryotic micro- isms (heterotrophic bacteria) or by exogenous supply of algae using photosynthetic modes can aid in fxation of it from the environment. Te next section of the paper almost 50% of the global carbon [11]. sheds light on possible symbiotic interactions among microalgae and bacteria. Lastly is the discussion on how Microalgal–bacterial interactions and vitamin auxotrophy heterotrophic bacteria can act as vitamin prototroph for Microalgae and bacteria have existed together from the microalgal vitamin auxotroph is discussed. Tus, possibly early days of evolution [12, 13]. All existing earth’s eco- by ecologically engineering a potential species-specifc systems covering the aquatic and terrestrial biomes are microalgal–bacterial consortium, it could tremendously composed of them. Additionally, their coevolution has contribute to the acceleration of microalgal productiv- transformed life on earth extending from deep seas (sea ity, photosynthetic ability, exchange of primary metabo- sponges) to mycorrhizal fungus/lichens in all feasible lites, and other biogeochemical nutrients within this mini modes of symbiotic associations, encompassing from environment. mutualism to parasitism [4, 13]. Microbial associations contribute towards industrial microbiology by playing How do vitamins auxotrophy in microalgae work? an integral part in environmental ecosystems. Some of Vitamins can be defned as an organic compound and a the well-known symbiotic associations have been found metabolite that is crucial for the life of an organism but between mycorrhizal fungus-plant, fungus-microalga, cannot be synthesized by them [18]. Tese compounds termite-enterobacterium, and between rhizobia–legume are helpful in overcoming the majority of defciency dis- [14]. A greater insight into the microalgal–bacterial asso- eases in many living entities. Te organisms that cannot ciations could be useful for unfolding their evolutionary produce these crucial compounds but require it to be and ecological roles. Te knowledge of species-specifc supplied to them through exogenous sources are called microalgal–bacterial symbiotic associations is manda- vitamin auxotrophs [19]. However, in many cases, these tory to harness their biotechnological potential as the organic compounds are not required as a nutritional

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