Through photosynthesis, at least about half of the total global carbon dioxide fixation on Earth is carried out by which major group of photosynthetic organisms?

Introduction / Context:
Photosynthesis is the process by which carbon dioxide is fixed into organic compounds using light energy, and it is performed by many different groups of organisms. While land plants are very visible, a significant portion of global photosynthesis occurs in aquatic environments. This question asks you to identify which major group of photosynthetic organisms is responsible for at least about half of the total carbon dioxide fixation on Earth.

Given Data / Assumptions:

• The process in focus is photosynthetic carbon dioxide fixation.

• The options include pteridophytes, bryophytes, algae, gymnosperms, and lichens.

• We assume a broad view of global photosynthesis, including both land and aquatic environments.

• Only one group in the list represents the dominant contributors to marine and freshwater primary productivity.

Concept / Approach:
Algae, including microscopic phytoplankton in oceans and larger forms such as seaweeds, contribute massively to global photosynthesis. Oceanic phytoplankton fix vast amounts of carbon dioxide due to their enormous numbers and the large surface area of water bodies. Estimates suggest that aquatic photosynthetic organisms, especially algae, account for at least half of the Earth's total carbon fixation. Pteridophytes, bryophytes, and gymnosperms are land plants with important ecological roles but do not dominate global carbon fixation in the same way. Lichens contribute locally but are minor on a global scale. Therefore, algae are the correct group.

Step-by-Step Solution:
Step 1: Recall that algae include diverse organisms from microscopic phytoplankton to large marine seaweeds. Step 2: Recognise that phytoplankton are extremely abundant in the upper layers of oceans and freshwater bodies and carry out photosynthesis on a global scale. Step 3: Understand that because the oceans cover most of Earth surface, even small photosynthetic organisms there can collectively fix huge amounts of carbon dioxide. Step 4: Consider pteridophytes (ferns and their relatives), which are land plants restricted to certain habitats and do not dominate total planetary photosynthesis. Step 5: Bryophytes (mosses and liverworts) are usually small and confined to moist areas, contributing only a minor share to global carbon fixation. Step 6: Gymnosperms are important forest trees like pines and spruces, but they represent only part of terrestrial plant biomass. Step 7: Lichens are symbiotic associations that can colonise harsh environments, but their total biomass is small compared to global algae. Step 8: Therefore, algae as a group are responsible for at least half of the global carbon dioxide fixation through photosynthesis.

Verification / Alternative check:
Ecology and environmental science sources describe marine phytoplankton as the foundation of ocean food webs and major contributors to global oxygen production and carbon dioxide fixation. Charts of primary productivity often show that oceans and freshwater systems, dominated by algae, contribute approximately as much or more carbon fixation as all terrestrial plants combined. In contrast, pteridophytes, bryophytes, and gymnosperms are treated as subsets of land vegetation. These data confirm that algae are key players in global photosynthetic carbon fixation.

Why Other Options Are Wrong:

Pteridophytes: Important land plants but limited in distribution compared with all algae combined.

Bryophytes: Small, non vascular land plants with relatively low total biomass and contribution.

Gymnosperms: Large trees that contribute to terrestrial carbon fixation but not to the majority of global carbon fixation by themselves.

Lichens: Local pioneers in harsh environments but collectively a small component of global photosynthetic activity.

Common Pitfalls:
A common mistake is to think only of land plants when imagining photosynthesis, leading students to choose gymnosperms or other terrestrial groups. Another pitfall is underestimating the importance of microscopic organisms such as phytoplankton. Remember that the sheer surface area of the oceans and the high turnover of phytoplankton make algae central to Earth carbon cycle.

Final Answer:
At least half of the total global carbon dioxide fixation on Earth is carried out by Algae.