Biology is an inherently messy process. Organisms are comprised of a plethora of carefully balanced interdependent bio-systems. These individual bio-systems are themselves comprised of many biomolecules that utilize complicated chemical reactions. Living cells and their internal systems are engaged in an unending chaotic dance, and all biological processes, from food chains down to DNA replication, are fraught with noise. This is problematic because while the processes that drive life might be chaotic organisms require consistent energy and a stable internal environment to live. Therefore, a principle design challenge in nature is transforming noisy environmental inputs into quiet energy flows that an organism can make use of.

While biological systems are profoundly complex their overarching architecture can be expressed by simple design principles. An important and well-studied biological process is photosynthesis--the conversion of light from the sun into energy in plants, bacteria, and other phototrophs. In this case, a critical design principle is to quiet noise in a photosynthetic antenna; that is to maintain a consistent flow of energy into the organism despite rapidly changing light conditions. To see how a complex biological system can be modeled with a simple design principle, we consider the light harvesting complex of green plants, which capture the energy of the sun. Figure 1a shows the LH2 light harvesting complex which consists of multiple interconnected pigments that capture and transfer energy through a long chain of electro-chemical reactions. Rather than attempt to develop a detailed model of each chemical reaction and biomolecule we describe this system as a network of nodes and links (figure 1b) that transfer energy from varied inputs, A and B, to a single output, O. We then ask a simple question. What overarching network architecture transforms noisy environmental input to a quiet output?

Understanding the design principles of a biological system can lead to testable predictions about real biological organisms. In the case of photosynthesis, we take the abstraction of a simple two-input network and analyze a given light environment to mathematically determine the ideal architecture to quiet noise. This generates a prediction we can test against real organisms. We considered the solar spectrum that three different photosynthetic organisms thrive in. Green terrestrial plants see sunlight filtered only through atmosphere, purple bacteria live on the forest floor where sunlight is filtered through a leafy canopy, and green sulfur bacteria thrive deep in the ocean where saltwater filters the solar spectrum. In each case, we calculate the ideal absorption parameters to satisfy our design principle of softening the environmental noise within the photosynthetic energy network. Figure 1 compares the absorption spectra (top row) of these three prototypical photosynthetic organisms to the ideal network predicted by our model (bottom row). Remarkably, we find that our calculations reproduce the absorption peaks of the real photosynthetic organisms with a high degree of accuracy. This may indicate that this simple design principle, quieting a noisy antenna, is a major driving force in the evolution of these organisms.