Question

The first photoautotroph organisms were........

• A. Bryophytes
• B. Algae
• C. Cyanobacteria
• D. Bacteria

Answer 1

The correct option is D Bacteria

Chemosynthesis led to accumulation of organic compounds in primitive sea. These organic molecules served as source of carbon and energy source for chemoheterotrophs which are supposed to be the first organism on the earth. Due to limited supply of organic compounds, organisms evolved photosystem to capture sunlight to be used as source of energy thereby leading to origin of photoautotrophs. The first photoautotrophs used reduced inorganic compounds (hydrogen sulphide, ammonia etc) as source of energy and carbon dioxide as their chief carbon source. Since the source of electrons is an inorganic molecule such as hydrogen sulphide; oxygen is not formed and the process is termed as non oxygenic photosynthesis. Thus, the first photoautotrophs were non oxygenic bacteria i.e. the purple bacteria. Option D is correct. Deletion of inorganic compounds leads to evolution of oxygenic photoautotrophs that use light as source of energy and carbon dioxide as chief carbon source. These photoautotrophs use water as electron donor during photophosphorylation that results in production of oxygen; hence the name oxygenic bacteria which in turn added oxygen to otherwise primitive reducing atmosphere. The first oxygenic photoautotrophs were cyanobacteria. Option C is incorrect. Accumulation of oxygen in earth’s atmosphere led to origin of first photosynthetic eukaryotes, the algae. It appeared around 3.4 billion yrs ago. Option B is incorrect. Bryophytes appeared much later during Ordovician period which makes option A incorrect.