The Biosphere 2 project was a significant scientific endeavor, aiming to replicate Earth’s complex ecosystems within a sealed, artificial environment. Located north of Tucson, Arizona, the 3.14-acre facility was designed to test the viability of closed ecological systems for potential future space habitation. Its ambitious goal was to create a self-sustaining miniature world, complete with diverse biomes and an agricultural area to support a human crew. However, the experiment soon encountered a major challenge: an unexpected decline in energy storage molecules among its plant and animal inhabitants. This issue highlighted the intricate balance required for ecological stability in a confined system.
How Ecosystems Balance Energy
Energy flows through natural ecosystems, originating primarily from sunlight. Plants, as producers, capture this light energy through photosynthesis, converting light, carbon dioxide, and water into chemical energy stored in organic molecules like glucose and starch, while releasing oxygen. This chemical energy forms the base of the food web.
Both plants and animals release this stored energy through a process called respiration. Respiration breaks down organic molecules to fuel cellular activities. For an ecosystem to sustain itself, the amount of energy produced and stored by plants must exceed the energy consumed by all organisms, including the plants themselves, through respiration.
This balance is described by net primary productivity (NPP). NPP represents the total amount of organic matter produced by plants minus the energy they use for their own respiration. A healthy ecosystem maintains a positive NPP, meaning more energy is fixed and stored than expended. This surplus energy is then available to support consumers, such as animals and decomposers, and allows for the accumulation of biomass.
The Biodome’s Net Energy Deficit
The Biosphere 2 experiment revealed the sensitivity of ecological energy balance, as several unforeseen factors contributed to a net energy deficit. A significant problem was the reduced amount and quality of sunlight reaching the plants within the dome. The structure’s steel framework and glass panels, along with condensation, filtered out a substantial portion of incoming solar radiation, and cut photosynthetically active radiation (PAR) by about 55%. This limitation directly inhibited the plants’ ability to photosynthesize and produce sufficient chemical energy.
Compounding the issue was the excessive respiration carried out by soil microbes. The rich, organic soil introduced into the biodome contained a vast population of microorganisms. These microbes rapidly metabolized the organic material, consuming a disproportionately large amount of oxygen and releasing significant quantities of carbon dioxide. This microbial activity contributed to a net loss of energy within the system.
The imbalance in respiration and photosynthesis led to drops in oxygen levels and spikes in carbon dioxide. Oxygen levels inside Biosphere 2 steadily declined from an initial 20.9% to as low as 14.2% after 16 months. Concurrently, carbon dioxide levels rose dramatically, sometimes reaching 12 times that of the outside atmosphere. This altered atmospheric composition stressed the plants, inhibiting their growth and energy production, and negatively impacted the animal inhabitants.
Furthermore, the concrete used in the construction of Biosphere 2 inadvertently absorbed significant amounts of carbon dioxide. This chemical reaction with calcium hydroxide in the concrete formed calcium carbonate, removing CO2 from the atmosphere that would otherwise have been available for plant photosynthesis. This reduced the efficiency of the carbon cycle within the closed system, contributing to the energy imbalance.
Impacts and Insights
The energy deficit within Biosphere 2 had significant consequences for the organisms within its structure. Many plant species struggled to grow, failed to reproduce, or died off due to the insufficient light and unfavorable atmospheric conditions. The decline in plant biomass directly impacted the food resources available for animal inhabitants.
Animals experienced starvation, reproductive failures, and population crashes as a result of the diminished food supply and the inability to store enough energy. For example, the chickens produced very few eggs, and insect species, including important pollinators, largely went extinct. The ecosystem became unstable and unable to sustain itself, requiring intervention to restore oxygen levels for the human crew.
Despite these challenges, Biosphere 2 provided valuable insights for ecological science and the design of future closed-system environments, such as space habitats. The experiment underscored the sensitivity of ecological energy balance and the complex, interconnected nature of Earth’s systems. It highlighted the complexities of managing microbial respiration, which proved to be a larger factor than anticipated, and the importance of consistent light availability for photosynthetic processes. The lessons learned from Biosphere 2 continue to inform research in areas like life support systems for long-duration space missions and understanding global climate change.