20.109(F12): Pre-proposal WFGreen

Investigators

Mahesh Thapa

Shulin Ye

WF Green

Title of Proposed Project

20.109(F12) Pre-Proposal: Optimizing the Size of Algal Chloroplast Antennea for Bioreactors

Project Summary

Modern American society depends on fossil fuels such as petroleum; however, our reliance on these fuels puts both our economy and national security at risk and the CO₂ released is destabilizing our planet’s climate. Because of these issues, many are researching renewable energy sources such as solar panels, hydrogen power, and biological fuels. Although biodiesel is the most petroleum-like fuel among these options, it is less efficient, in part due to the fact that algae tend to be optimized for low-light conditions; we propose a genetic screen in order to find a strain of algae more optimized for direct sunlight.

Introduction

Currently, the United States uses nearly 8.5 million barrels of gasoline per day in order to power our automobiles, buses, and other methods of transportation. Gasoline is well suited to this task; it is extremely energy-dense (~46 MJ/kg, ~36 MJ/liter), allowing it to be easily transported in order to supply a non-stationary engine. Of all the biological fuels, biodiesel comes the closest: ~42 MJ/kg, ~33 MJ/liter. (In comparison, lithium ion batteries are much less energy dense - 0.7 MJ/kg, ~2.3 MJ/liter.) However, the process of creating biofuels is extremely inefficient. Algae, which are the most efficient, can currently produce about 1200 galloons per acre year with an efficiency of around 0.6%. (Vasudevan 2008) There are, however, various inefficiencies that can probably be reduced. For example, algae chloroplasts contain large light-gathering complex; an adaptation that helps them live in low-light (200-400 micromol photon/m²) conditions but often photosaturates under direct sunlight (>2000 micromol photon/m²). Up to 60% of the light energy is wasted this way (Ort et. al. 2009), and we seek to find the antenna size that is optimal under sunlight that is also economical for biodiesel production.

Your idea

Our experimental subject is Chlorella protothecoides, a microalga that is well suited for biodiesel production as it can attain a lipid content of over 22%. For an initial screen, we will first create a library of mutants of Chlorella in antennae size. We will then set up outdoor closed bioreactors with these mutants in both nutrient-deprived and nutrient-permissive conditions, as well as a quality-control null of a chlorophyll-knockout strain and a wild-type strain for comparison. (In order to prevent contamination, we will add antibiotics at this stage). For a period of seven days, we will twice daily (8AM, 5PM) measure the concentration of algae and the lipid content of the algae. The 8AM measurement is to see how much lipid the algae consumes overnight; the 5PM measurement is to see how productive the algae are during the day. We will also monitor the oxygen generated by these reactors as a measure of photosynthetic effiency. At the end of these seven days, we will analyze the fatty acid composition and select a handful of mutants that have the highest lipid content and generate the most oxygen. For longer-term results, we will allow these to continue running, and we will continue to take daily measurements, in part to see if the algae declines over time.

Because keeping a bioreactor contamination-free is quite expensive, we will then test the evolutionary fitness of our most promising mutants Afterwards, we will pick a handful of promising mutants and re-innoculate them into new bioreactors without antibiotics, in both nutrient-deprived and nutrient-permissive conditions. We will then innoculate those bioreactors with a mixture of algae found naturally, let them sit unattended outdoors (again for seven days) and then analyze the populations in the bioreactors. A bioreactor that contains a high amount of our mutant algaes as compared to the natural algae probably contains a mutant that is capable of outcompeting natural algaes. We will also overnight the mutants, in dark and also under high-light conditions (2000 micromol photon/m²), at both 5°C and 35°C, in order to test for temperature stress.

A sketch

Caption:"Figure 1: Top - Chlorophyll reaction centers can become saturated and less effective, a process known as photosaturation. Bottom - Experiment overview"

Citations

Ort, Donald R., Xinguang Zhu, and Anastasios Melis. "Optimizing Antenna Size to Maximize Photosynthetic Efficiency." Plant Physiology 155.1 (2011): 79-85. PubMed Central. Web. 29 Nov. 2012.

Vasudevan, Palligarnai T., and Michael Briggs. "Biodiesel Production--Current State of the Art and Challenges." Journal of industrial microbiology & biotechnology 35.5 (2008): 421-30. ABI/INFORM Complete; ProQuest Research Library. 29 Nov. 2012 .