Effects of spaceflight and simulated microgravity on microbial growth and secondary metabolism

Abstract

Spaceflight and ground-based microgravity analog experiments have suggested that microgravity can affect microbial growth and metabolism. Although the effects of microgravity and its analogs on microorganisms have been studied for more than 50 years, plausible conflicting and diverse results have frequently been reported in different experiments, especially regarding microbial growth and secondary metabolism. Until now, only the responses of a few typical microbes to microgravity have been investigated; systematic studies of the genetic and phenotypic responses of these microorganisms to microgravity in space are still insufficient due to technological and logistical hurdles. The use of different test strains and secondary metabolites in these studies appears to have caused diverse and conflicting results. Moreover, subtle changes in the extracellular microenvironments around microbial cells play a key role in the diverse responses of microbial growth and secondary metabolisms. Therefore, "indirect" effects represent a reasonable pathway to explain the occurrence of these phenomena in microorganisms. This review summarizes current knowledge on the changes in microbial growth and secondary metabolism in response to spaceflight and its analogs and discusses the diverse and conflicting results. In addition, recommendations are given for future studies on the effects of microgravity in space on microbial growth and secondary metabolism.

Keywords: Microbial growth; Microgravity; Microgravity analogs; Secondary metabolism; Simulated microgravity; Spaceflight.

Fig. 1

The clinostats used in the SMG experiments and their corresponding principle models. a The 2-D clinostats with a horizontal axis used to generate SMG condition on ground. b The 2-D clinostats with a vertical axis used to generate 1G control condition on ground. c The schematic diagram of mechanical principle of the different clinostats. SMG: Axis of rotation is perpendicular to the direction of the gravity vector, which is used to simulate microgravity effects on ground; NG: The samples fixed in the metal support are rotating, used as the dynamic control of SMG; 1G: The samples fixed in the metal support are static and are used as the static control of SMG

Fig. 2

Shenzhou-8 spaceflight experiments. Microbial samples cultured in solid (a) and liquid (b) media were loaded into Experiment Unique Equipments (EUEs) before launching. c The inside of SIMBOX. Red circles indicate the two EUEs in a static slot (microgravity position, μG) and a centrifuge slot (simulated 1G position, S-1G), respectively. d SIMBOX as an advanced space incubator

Fig. 3

Colony and cultural features of S. coelicolor after the 16.5-day spaceflight experiment [95]. a Colony features after the 16.5-day spaceflight experiment. “SCOA3(2)” framed in red indicates the pure S. coelicolor A3(2) culture, “SCOA3(2)*” framed in blue indicates the S. coelicolor A3(2) co-cultured with the indicator strain B. subtilis, and “SCOM145*” framed in yellow indicates S. coelicolor M145 co-cultured with B. subtilis. b Cultural features of S. coelicolor A3(2) grown in a JCM42 liquid medium after the 16.5-day spaceflight experiment. “SCOA3(2)” framed in red indicates the pure S. coelicolor A3(2) culture, and “SCOA3(2)*” framed in blue indicates S. coelicolor A3(2) co-cultured with B. subtilis