Potential biofilm control strategies for extended spaceflight missions

Abstract

Biofilms, surface-adherent microbial communities, are associated with microbial fouling and corrosion in terrestrial water-distribution systems. Biofilms are also present in human spaceflight, particularly in the Water Recovery System (WRS) on the International Space Station (ISS). The WRS is comprised of the Urine Processor Assembly (UPA) and the Water Processor Assembly (WPA) which together recycles wastewater from human urine and recovered humidity from the ISS atmosphere. These wastewaters and various process streams are continually inoculated with microorganisms primarily arising from the space crew microbiome. Biofilm-related fouling has been encountered and addressed in spacecraft in low Earth orbit, including ISS and the Russian Mir Space Station. However, planned future missions beyond low Earth orbit to the Moon and Mars present additional challenges, as resupplying spare parts or support materials would be impractical and the mission timeline would be in the order of years in the case of a mission to Mars. In addition, future missions are expected to include a period of dormancy in which the WRS would be unused for an extended duration. The concepts developed in this review arose from a workshop including NASA personnel and representatives with biofilm expertise from a wide range of industrial and academic backgrounds. Here, we address current strategies that are employed on Earth for biofilm control, including antifouling coatings and biocides and mechanisms for mitigating biofilm growth and damage. These ideas are presented in the context of their applicability to spaceflight and identify proposed new topics of biofilm control that need to be addressed in order to facilitate future extended, crewed, spaceflight missions.

Keywords: Biofilm; Life-support; Mars; Moon; Spacecraft; Water.

Fig. 1

Cluster analysis of publications on biofilm control and/or prevention on surfaces by quid software (https://quid.com/) from 1968 to 2019, showing the relative density of publications per application. This analysis indicates that studies of coatings for biofilm control within spaceflight systems are not strongly aligned to other applications.

Fig. 2

Number of studies on the use of surface coatings to control biofilms showing a rapid increase in publications in the last two decades. Safeflight systems-related studies were published between 1998 and 2010.

Fig. 3

Schematic and concept of operations of a proposed ‘two-flexible bags’ approach to the wastewater tank for future ECLSS. Instead of one hard-shell tank, this approach uses two flexible bags, where only one is completely full at any given time. Bag 1 is used for the Earth-to-Mars and Mars-orbit phases of mission (A) similarly to how the current wastewater tank is currently utilized. Before return to Earth, the contents of Bag 1 are transferred through a filter into an unused Bag 2 (B), resulting in the emptying of Bag 1 and the filling of Bag 2 (C), and the collection of biofilm in a filter that will no longer be used (vertical filter in schematic). The Mars-to-Earth mission phase uses the new Bag 2 as wastewater tank (D).