Low-Gravity Bioreactor: Requirements + Design

The low gravity simulator is responsible for exploring bacteria behaviour and growth under low-gravity conditions found on Mars and in space. A key component of our research and conceptualization was considering how the project would be validated as it is difficult to measure acceleration in dynamics environment without having biased data. Our final concept is based off an RPM machine that spins in two axes.

Background and Motivation

Terrestrial research on cyanobacteria has revealed much about their physiology, growth dynamics, photosynthetic behavior, and bioproduct accumulation under 1 g. However, when using cyanobacteria in extraterrestrial settings (e.g. for in situ resource utilization on Mars), the gravitational difference introduces several uncertainties. Mars has about 0.38 g (roughly one-third Earth gravity), and this reduction is expected to modulate many transport and fluid phenomena, e.g. sedimentation, diffusion, convection, buoyant flows, and boundary layer formation.

Because cyanobacteria are being considered for production of living building materials and regolith cementation, it becomes essential to understand how reduced gravity could affect both their biological performance and the mechanical / material quality of their outputs. For example:

• Lower gravity could slow sedimentation of cells, altering how culture density gradients form.
• The thickness and stability of diffusion boundary layers around filaments or aggregates may increase, reducing mass transfer rates.
• Gas bubbles (e.g. O2) may detach differently or remain entrained longer, impacting gas exchange.
• Convection and mixing driven by buoyant forces are weaker, so nutrient and gas distribution (e.g. CO2 supply, O2 removal) may become diffusion-limited in new ways.
• The mechanical structure of cell aggregates or extracellular polymeric substances (EPS) might differ under lower mechanical load, potentially influencing porosity, tensile strength, or other material properties. Because of these uncertainties, a ground-based low-gravity simulator offers a way to explore these phenomena in microbial systems before sending it to Mars. The low-gravity bioreactor is a platform for hypothesis testing and design exploration, enabling intermediate steps between pure 1 g lab experiments and real Martian experiments.

Expanded Objectives & What We Can Learn

Below is a more detailed articulation of the objectives and possible insights from such a system.

1. Growth kinetics and biomass yield

• Measure growth curves (lag, exponential, stationary phases) under reduced-gravity simulation and compare with Earth controls.
• Compute doubling times, specific growth rates, and biomass productivity per unit volume or light input.
• Evaluate whether the carrying capacity or maximum cell density shifts under low-gravity effects.

1. Pigment and photosynthetic metrics

• Quantify chlorophyll, phycobiliproteins, carotenoids, and other accessory pigments to see whether light harvesting strategies shift.
• Monitor quantum yields (e.g. Fv/Fm), PS I / PS II balance, and nonphotochemical quenching (NPQ) responses.
• Detect whether changes in internal gas (e.g. O₂ accumulation) or CO₂ availability under reduced mixing alter photosynthetic efficiency.

1. Mass transfer, nutrient diffusion, and gas exchange

• Characterize nutrient uptake rates (e.g. nitrates, phosphates, trace metals) under diffusion-limited or boundary-layer constrained conditions.
• Monitor dissolved gas composition (O₂, CO₂) spatially and temporally to detect gradients, supersaturation, or relief limitations.
• Assess whether gas bubble formation, detachment, or coalescence dynamics differ, possibly affecting gas removal or mixing.

1. Physical structure, aggregation, and morphology

• Observe whether cells tend to cluster differently, form looser or denser aggregates, or produce more EPS under low-gravity stress.
• Assess how hydrodynamic shear, sedimentation, or settling behavior changes, especially in filamentous or colonial species.
• Investigate boundary layer thickness changes around filaments or clusters, which may limit diffusion of nutrients or wastes.

1. Material / mechanical properties of the biomass output

• If the biomass is to be processed into bio-bricks or structural composites, test porosity, compressive strength, tensile strength, adhesion to regolith, etc.
• Compare mechanical modulus (Young’s modulus, flexural stiffness, etc.) of biomass grown under Earth gravity vs simulated low gravity to detect microstructural differences.
• Evaluate how residual moisture content or internal microstructure (voids, channels) differ, which could affect drying or binding steps.

1. Engineering / design feedback

• Use the experimental results to guide vessel geometry, mixing designs, internal baffling, aeration strategies, or flow regimes optimized for partial gravity.
• Test hybrid designs (e.g. internal stirring, periodic agitation) to offset gravity-related mass transport limitations.
• Refine scale-up strategies by factoring in changed transport coefficients, and thus estimating performance under Martian gravity.

1. Uncertainty quantification and risk mitigation

• The simulator helps bound worst-case vs best-case behavior, highlighting potential failure modes (e.g. stalling growth, gas stagnation, nutrient limitation).
• It allows parametric sweeps (e.g. varying mixing rate, vessel size, cell density) to find robust operating regimes.
• It provides data to calibrate models (e.g. computational fluid dynamics, mass transfer, growth models) with partial gravity parameters, improving predictive confidence for space deployment.

Though the simulator does not perfectly replicate all aspects of Martian gravity (e.g. a direct 0.38 g field), it allows one to probe mechanistic trends and trade-offs in a controlled, repeatable, and economical way. The insights can reduce design risk and guide the roadmap toward actual extraterrestrial bioreactor deployment.

Approaches to Simulating Low Gravity on Earth

To achieve this, various technologies and strategies have been developed in the literature to simulate reduced or microgravity conditions. Some of these are:

• Rotating wall vessels / rotating bioreactors (e.g. NASA’s rotating bioreactor, rotating wall vessel systems) simulate microgravity by balancing centrifugal forces and drag to hold cells in suspension with minimal net shear. [1]
• Random Positioning Machines (RPMs), which continuously rotate specimens along multiple axes, averaging gravitational vector directions over time and reducing net directional gravity effects. For instance, Ellena et al. used an RPM with cyanobacteria and observed slower growth and shifts in proteome expression (e.g. downregulation of ribosomal proteins and nitrate uptake transporters). [2]
• Partial gravity drop tower + linear acceleration platforms: For instance, a “controlled partial gravity platform” uses a drop tower plus internal linear acceleration to sustain a target partial g for a short duration (on the order of seconds) within microgravity. [3]
• Lab-on-a-chip microfluidics and miniaturized systems can simulate altered gravitational effects on micro-scale fluidic systems, especially for diffusion and convection studies. [4] Each method has advantages and drawbacks (e.g. trade-offs in duration, sample size, shear forces, g-jitter, disturbance during transitions). A bioreactor combining some of these approaches (e.g. miniaturized rotating vessel + periodic repositioning) might offer a pragmatic compromise.

For example, Ketteler et al. developed a simulated microgravity biofilm reactor (SMBR) with integrated microfabricated sensors to explore biofilm formation under low gravity—like conditions. [5]

In the cyanobacteria field, Ellena et al. used a ground analog (RPM) for oxygenic cyanobacteria, finding slower growth under low-shear simulated microgravity, increased thickness of diffusion boundary layers, and proteomic differences. [2]

Additionally, general reviews of simulated microgravity devices note that numerous stem cell and microbial studies have used clinostats, RPMs, and rotating walls to study altered gravity responses. [6]

Connecting to Mars / Low-Pressure / Extraterrestrial Cultivation Constraints

Simulating reduced gravity is one dimension, but real Martian cultivation also implies other non-terrestrial constraints (low ambient pressure, atmospheric composition, CO₂ / N₂ ratios, radiation, temperature swings, etc.). Some relevant observations:

• Cyanobacteria have been tested under low pressure (e.g. 100 hPa, using Mars-like gas mixtures) and still show vigorous growth in certain strains, though growth rates and productivity may shift. [7]
• The partial pressures of metabolizable gases (e.g. pCO₂, pN₂) are intertwined with total pressure and can limit growth if too low. [7]
• In closed-loop life support and bioregenerative systems (BLSS), photobioreactors in space must balance CO₂ removal, O₂ production, nutrient recycling, and mass constraints. [8]
• Engineering design (e.g. wall thickness, reactor pressure vessel constraints) must consider pressure differentials, mass penalty, and structural rigidity, which becomes more critical at lower operating pressures. [7] Thus, a low-gravity simulator used in parallel or in sequence with low-pressure or Martian atmospheres enables more realistic, holistic experiments before actual Martian deployment.