2) What is the goal of bioremediation? A) To improve human health with the help of living organisms such as bacteria B) To clean up areas polluted with toxic compounds by using bacteria C) To improve soil quality for plant growth by using bacteria D) To improve bacteria for production of useful chemicals E) To kill pathogenic bacteria with the use of antibiotics 33) An organic solvent widely used in industrial processes, 1,1,1-trichloroethane (TCA), is a major environmental pollutant affecting human health and damaging the ozone layer. Recently, researchers isolated the first bacteria able to degrade TCA. They found that the bacteria use hydrogen as an electron donor, TCA as an electron acceptor, and acetate as a carbon source. Based on the preceding information, deduce the method used to isolate these bacteria. A) Gram staining B) Enrichment culture C) Direct sequencing D) Seeding 34) Multicellularity and large body size of eukaryotic organisms require high metabolic rates and efficient ATP production by aerobic respiration. How did bacteria change Earth’s atmosphere to enable aerobic respiration? A) Oxygenic photosynthesis by cyanobacteria significantly increased the amount of oxygen in the atmosphere. B) Splitting of water during anaerobic respiration by cyanobacteria dramatically increased the amount of oxygen in the atmosphere. C) Nitrogen fixation by cyanobacteria decreased the amount of nitrogen in the atmosphere, leading to the simultaneous increase of oxygen. D) Anaerobic respiration by cyanobacteria increased the amount of oxygen in the atmosphere by reducing the amount of iron oxides that were able to react instantly with oxygen.
The Correct Answer and Explanation is:
Here are the correct answers to the questions followed by a comprehensive explanation:
2) What is the goal of bioremediation?
Correct Answer: B) To clean up areas polluted with toxic compounds by using bacteria
33) Based on the preceding information, deduce the method used to isolate these bacteria.
Correct Answer: B) Enrichment culture
34) How did bacteria change Earth’s atmosphere to enable aerobic respiration?
Correct Answer: A) Oxygenic photosynthesis by cyanobacteria significantly increased the amount of oxygen in the atmosphere.
Explanation
Bioremediation refers to the process of using living organisms—mainly microorganisms like bacteria, fungi, or plants—to degrade or neutralize pollutants from a contaminated site. The main objective is to clean up environments polluted with hazardous substances, including heavy metals, petroleum products, or industrial solvents like TCA (1,1,1-trichloroethane). Therefore, Option B is correct because bioremediation exploits the natural metabolic capabilities of bacteria to transform toxic compounds into less harmful ones or completely mineralize them into harmless products like water and carbon dioxide.
In question 33, the bacteria described are capable of using TCA as an electron acceptor and hydrogen as an electron donor, which are very specific metabolic requirements. This indicates that researchers likely used an enrichment culture method—growing the microbes in a selective medium designed to favor the growth of organisms with those exact metabolic traits. By providing hydrogen and TCA under anaerobic conditions and using acetate as the carbon source, researchers enriched for bacteria that could survive and thrive under those specific conditions. Thus, Option B is the correct answer.
For question 34, aerobic respiration requires molecular oxygen (O₂), which was almost absent in Earth’s early atmosphere. Cyanobacteria were among the first organisms to perform oxygenic photosynthesis, a process that uses water as an electron donor and produces oxygen as a byproduct. This marked the beginning of the Great Oxygenation Event about 2.4 billion years ago, dramatically increasing atmospheric oxygen and making aerobic respiration possible for other organisms. Hence, Option A is correct. This was a key evolutionary step that enabled the rise of complex, multicellular life forms that require high levels of energy (ATP).
