This is the sixth post in our series The Impact of Gravity on Life, a paper written by Dr. Emily R. Morey-Holton, NASA Ames Research Center, Moffett Field, California. Read the series in its entirety here in blog posts tagged Impact of Gravity on Life.
Algal mats and protists are fascinating! If orientation and stratification are weight- dependent, then the microgravity of space could significantly alter interactions between organisms. How these organisms would fare without gravity is unknown, but changes from Earth-based mats would be predicted. For example, microbes migrating during the day using gravity as an environmental cue to minimize exposure to solar radiation would not be able to migrate and could have greater radiation damage. Such damage would tend to select species with radiation resistance. Protists appear to detect gravity at about 0.1-G. Thus, the general principle of mechanoreceptors in metazoa is represented in unicellular organisms (Hemmersbach, et al., 1999) suggesting that the ability to detect and use Earth’s gravity must have occurred very early in the evolutionary history. In fact, the Hemmersbach et al., 1999 review article suggests that these organisms have evolved structures, such as mechano- or stretch- sensitive ion channels, cytoskeletal elements and second messengers, to amplify the gravity signal rather than evolving intracellular gravity sensors. Interestingly, the genes for light sensing and gravity sensing occur very early in evolution. Some organisms seem to be able to use either or both as directional cues. Space, with very low gravity levels, provides an unique laboratory to sort out the importance of light in a relatively gravity-free environment. Or, perhaps these physical environmental factors have created redundant biological sensing systems.
Decreased gravity causes very complex changes in the environment. For example, gaseous boundary layers build up due to lack of convective mixing in the atmosphere and these boundary layers expose plants and simple ecosystems to stratified environments not present on Earth. Soil substrates in space have a different shape, do not pack like Earth soils, and wet in a very different way than on Earth.In space, water does not drain through
In space, water does not drain through soil, as that process requires gravity. Such changes make the management of simple ecosystems in space rather difficult. Plants may be the most difficult species to evolve efficiently in space, as they must adapt simultaneously to two environments (above and below the ground) that change during spaceflight. Atmospheric issues include pollination and mixing of gases. During spaceflight, insects, gravity, and wind may be missing in the plant habitat. In addition, the gaseous environment above the ground may stratify due to lack of convective mixing and expose plant shoots to boundary layers that are new and novel. Such stratification may be responsible for the uneven ripening of seedpods noted during flight, i.e., ripening begins at the tip and is not uniform. Levels of potentially dwarfing compounds (e.g., ethylene and CO2) might cling to the plants creating shorter plants rather than the taller ones that one might expect with lower gravity levels. Not only will plant shoots have to adapt to novel environments in flight, but also water management for the roots may be problematic.
Soil issues during spaceflight focus on root-zone management. The team from Utah State University has grappled with water-management issues while studying evolution of plant systems in space by attempting to grow multiple generations of wheat on the Russian space station MIR (Jones and Or, 1999). Their results show that we have a lot to learn before we can achieve successful and repeatable plant growth under reduced gravity. Root media wet differently in space. On Earth, water drains vertically through a soil column where each particle is held in contact by gravity. In space, the wetting front is free to move in all directions. The wetting front bulges out to reach a neighbor particle that may be floating a small distance away, “gulps” the particle, and then opens an air space on the other side of it. Where particles are touching, water wicks along the particle surfaces partially filling the channels around the soil particulates. This tends to trap air between the particles rather than forming the saturated slurries that one finds on Earth. In a partially hydrated system, water wicks between particles as it does on Earth, but in all directions rather than just flowing “down”. This wicking, due to capillary attraction, inhibits exchange of nutrients and, by bridging between the particle contact points, can suffocate plants at lower water contents than would be observed on Earth. Soil-based systems in space can be managed to resemble hydroponic (i.e., water-based) systems on Earth. It is impossible to simulate space-substrate-water-content conditions using the same soil substrate on Earth. In space, when water is forced into a substrate, the water does not drain and can fill up to 90% of the soil matrix. Until the water is wicked out of the substrate, the flooded volume will stay in the substrate, creating an oxygen free zone that is not useable by most plants. Proper root-zone management in space is an active process that requires sensors for continuous monitoring of both water and oxygen content in the soil matrix during spaceflight. Such monitoring is essential for effectively managing plant systems and for learning how the decreased gravity experienced during spaceflight alters the environment. Understanding and controlling the environment is often a prelude to survival and adaptation in unique environmental niches.
Many challenges remain for plant growth and crop management in space, including understanding of boundary layers above the ground and water and oxygen management in the root media. All physical environmental factors must be considered when predicting the evolutionary fate of a species in an unique environment.