Optimal iron concentrations for growth-associated polyhydroxyalkanoate biosynthesis in the marine photosynthetic purple bacteriumRhodovulum sulfidophilum

Abstract

26 Polyhydroxyalkanoates (PHAs) are a group of natural biopolyesters that resemble petroleum-27 derived plastics in terms of physical properties but are less harmful biologically to the 28 environment and humans. Most of the current PHA producers are heterotrophs, which require 29 expensive feeding materials and thus contribute to the high price of PHAs. Marine 30 photosynthetic bacteria are promising alternative microbial cell factories for cost-effective, 31 carbon neutral and sustainable production of PHAs. In this study, Rhodovulum sulfidophilum, 32 a marine photosynthetic purple nonsulfur bacterium with a high metabolic versatility, was 33 evaluated for cell growth and PHA production under the influence of various media 34 components found in previous studies. We evaluated iron, using ferric citrate, as another 35 essential factor for cell growth and efficient PHA production and confirmed that PHA 36 production in R. sulfidophilum was growth-associated under microaerobic and 37 photoheterotrophic conditions. In fact, a subtle amount of iron (1 to 2 µM) was sufficient to 38 promote rapid cell growth and biomass accumulation, as well as a high PHA yield during the 39 logarithmic phase. However, an excess amount of iron did not enhance the growth rate or 40 PHA productivity. Thus, we successfully confirmed that an optimum concentration of iron, 41 an essential nutrient, promotes cell growth in R. sulfidophilum and also enhances PHA 42 utilization. 43 44 Introduction 45 Polyhydroxyalkanoates (PHAs) are a family of biopolyesters that are produced by a wide 46 variety of microorganisms for the purpose of surviving in unfavorable growth and stress 47 conditions; these molecules act as carbon and energy storage, redox regulators and 48 cryoprotectants [1-3]. PHA is one of the well-known bioplastics (biobased and 49 biodegradable) that has been extensively developed with the aims to overcome problems such 3 50 as plastic solid wastes, harmful chemical substance leaching and dependence on 51 nonrenewable fossil fuels. These are the main disadvantages of petroleum-derived synthetic 52 plastics [4, 5]. 53 54 However, the high price of PHAs has made them less competitive compared to conventional 55 synthetic plastics due to the high cost of raw materials used in fermentation and downstream 56 purification steps [6-8]. Heterotroph bacteria such as wild-type or engineered strains of 57 Cupriavidus necator H16, Alcaligenes latus, Pseudomonas putida and Escherichia coli are 58 the main workhorses for large-scale production of PHAs [9-11]. These heterotroph bacteria 59 lack photosynthesis ability and thus require expensive carbon supplies to sustain their growth 60 and PHA biosynthesis. On the other hand, photosynthetic microorganisms could produce 61 their own foods by utilizing inexpensive and abundantly available resources such as sunlight,62 carbon dioxide and nitrogen and are thus potential next-generation microbial cell factories 63 [12, 13]. Several studies have reported successful PHA production by photosynthetic bact...