Problem 13 - Brock Biology of Microorganisms

Question

Many pelagic prokaryotes can use light energy but are not considered "phototrophs" in the same sense as cyanobacteria or purple bacteria. Explain. (Section 19.11)

Step-by-Step Solution

Answer

Pelagic prokaryotes use proteorhodopsin to capture light energy for cellular functions but do not perform photosynthesis like cyanobacteria or purple bacteria.

Step 1: Define Key Terms

Identify and define the key terms in the question. 'Pelagic prokaryotes' refers to microorganisms living in open ocean waters. 'Phototrophs' are organisms that capture light energy to carry out photosynthesis.

Step 2: Understand Phototrophs

Explain what makes cyanobacteria and purple bacteria true phototrophs. Cyanobacteria use photosynthesis to convert light energy into chemical energy, producing oxygen as a byproduct. Purple bacteria perform a similar function but do not produce oxygen.

Step 3: Differentiate Light Utilization

Describe how these pelagic prokaryotes utilize light energy in a different way. Many pelagic prokaryotes use a pigment called proteorhodopsin to capture light energy, which is used to move protons across the cell membrane to generate a proton gradient, aiding in ATP production. However, they do not undergo photosynthesis like cyanobacteria or purple bacteria.

Step 4: Summarize the Distinction

Conclude why pelagic prokaryotes are not considered 'phototrophs' in the traditional sense. They capture light energy for cellular functions but do not use it for photosynthesis to produce organic compounds.

Key Concepts

pelagic prokaryotes

Pelagic prokaryotes are microorganisms that live in the open ocean waters. They are a fascinating part of the marine ecosystem due to their ability to survive in such vast and nutrient-scarce environments.
Some common examples include bacteria and archaea, which float freely in the water column. These prokaryotes have adapted various mechanisms to thrive in such conditions, including nutrient uptake and light-harvesting capabilities.
Although they can use light energy, they are not typically considered traditional phototrophs. This distinction comes down to how they utilize light, which we'll delve into in the following sections.

phototrophs

Phototrophs are organisms that capture light energy to carry out photosynthesis. They play a crucial role in converting solar energy into chemical energy, which is essential for the survival of many ecosystems.
Cyanobacteria and purple bacteria are classic examples of true phototrophs. Cyanobacteria are well-known for their ability to produce oxygen through the process of photosynthesis. On the other hand, purple bacteria perform photosynthesis without producing oxygen.
These organisms use special pigments like chlorophyll or bacteriochlorophyll to absorb light, which then drives the synthesis of organic compounds from carbon dioxide and water.
This intricate process is different from the light utilization methods of some pelagic prokaryotes, which will be explored next.

proteorhodopsin

Proteorhodopsin is a unique pigment found in many pelagic prokaryotes. Unlike chlorophyll or bacteriochlorophyll, proteorhodopsin is used to capture light energy without driving the photosynthetic process.
This pigment operates similarly to the well-known rhodopsin found in the human eye, which helps us see light.
In these marine microorganisms, proteorhodopsin helps generate a proton gradient across the cell membrane when exposed to light. This proton gradient is essential for cellular energy production.
Instead of photosynthesis, proteorhodopsin converts light energy directly into a form that can be used to power the cell’s ATP production, distinguishing these prokaryotes from traditional phototrophs.

photosynthesis

Photosynthesis is a complex and vital process used by phototrophs to convert light energy into chemical energy.
During photosynthesis, organisms like cyanobacteria use light to turn carbon dioxide and water into glucose and oxygen. Purple bacteria also perform photosynthesis but do not produce oxygen as a byproduct.
This conversion is facilitated by pigments like chlorophyll, which absorb light and drive the reactions needed to form organic compounds.
Photosynthesis provides the base of the food chain for many ecosystems, producing energy-rich compounds that other organisms can consume and use. However, many pelagic prokaryotes do not use this method, relying instead on other ways to utilize light energy.

ATP production

ATP (adenosine triphosphate) is the primary energy currency of the cell. It is essential for many cellular processes, including muscle contraction, protein synthesis, and cell division.
Both phototrophs and pelagic prokaryotes need ATP to survive, but they produce it differently.
In true phototrophs, ATP is generated as a part of the photosynthetic process. Light energy helps to drive the formation of ATP from ADP and inorganic phosphate via a process called photophosphorylation.
Pelagic prokaryotes with proteorhodopsin, however, generate ATP differently. The light captured by proteorhodopsin helps create a proton gradient, which then drives ATP synthase to produce ATP.
This way, even though they are not performing full photosynthesis, these microorganisms can still harness light to generate the energy they need.