Which statement about global photosynthesis by primary producers is correct?

Introduction / Context:
Primary producers are organisms that convert inorganic carbon, usually carbon dioxide, into organic matter using an external energy source such as light. This question focuses on which group of primary producers is especially important for photosynthesis in aquatic ecosystems like oceans and lakes. It tests your understanding of the relative contributions of land plants, protists, and prokaryotes to global photosynthesis.

Given Data / Assumptions:

Primary producers include land plants, photosynthetic protists (such as algae), and photosynthetic prokaryotes (such as cyanobacteria).
Aquatic communities rely heavily on microscopic and small photosynthetic organisms.
We consider general patterns rather than exact numerical percentages.

Concept / Approach:
In aquatic ecosystems, large land plants are largely absent, so most photosynthesis is carried out by microscopic or small photosynthetic organisms. These include various protists (like diatoms and dinoflagellates) and photosynthetic prokaryotes such as cyanobacteria. Together, they form the base of the aquatic food web. Although land plants are extremely important globally, it is not correct to say that they produce over 80 percent of all photosynthesis on Earth; estimates suggest that land and aquatic producers contribute in roughly comparable proportions. Cyanobacteria are significant, but they do not individually surpass all protists in total global photosynthesis. Therefore, the only accurate statement is the one highlighting photosynthetic protists and prokaryotes as the main producers in aquatic communities.

Step-by-Step Solution:
Step 1: Focus on the phrase "in aquatic communities" in the correct statement. Step 2: Recall that oceans and lakes are dominated by phytoplankton, which are mostly protists and cyanobacteria. Step 3: Recognize that land plants cannot be the major photosynthesizers in aquatic environments. Step 4: Conclude that the statement about photosynthetic protists and prokaryotes is correct.

Verification / Alternative check:
Another way to verify is to remember food chains. In marine ecosystems, the primary producers are mostly microscopic phytoplankton, not large seaweeds alone. Phytoplankton include both protists and prokaryotes. Because these organisms occupy the sunlit upper layers of oceans worldwide, their combined photosynthetic activity is huge, confirming the statement about their importance in aquatic photosynthesis.

Why Other Options Are Wrong:
Land plants perform over 80 percent: This exaggerates their share of global photosynthesis and ignores the major role of marine producers.
Cyanobacteria alone conduct more than protists: While important, cyanobacteria do not single handedly exceed all protists in photosynthetic output.
All of the above: Since at least two statements are incorrect, this combined option is also incorrect.

Common Pitfalls:
A frequent mistake is to overestimate the role of land plants simply because they are more visible to us. Another error is failing to distinguish between contributions in aquatic communities and global totals. Carefully reading qualifiers like "in aquatic communities" is essential for answering such questions correctly.

Final Answer:
The correct statement is that photosynthetic protists and prokaryotes carry out most of the photosynthesis in aquatic communities across the world.