Primary succession and weathering: Which microbial group plays a key role in the transformation of bare rock to soil during early ecosystem development?

Introduction / Context:
Soil formation on fresh volcanic rock or glacial retreat surfaces begins with primary colonizers that can tolerate desiccation, radiation, and nutrient scarcity. Microbial pioneers initiate weathering and organic matter accrual.

Given Data / Assumptions:

• Bare rock lacks organic carbon, nitrogen, and structured soil.
• Photosynthetic and nitrogen-fixing pioneers are advantageous.
• Lichens (fungi + photobionts such as cyanobacteria) are classic early colonizers.

Concept / Approach:
Cyanobacteria, frequently as lichen photobionts, fix CO2 via photosynthesis and often fix N2, seeding organic matter and nutrients. Their biofilms and lichen acids chemically weather minerals, while physical expansion/contraction contributes to microfracturing, accelerating pedogenesis (soil formation).

Step-by-Step Solution:
Identify the needs of primary succession: energy capture and nutrient introduction. Choose microbes capable of photosynthesis and sometimes N2 fixation. Recognize the lichen consortium’s ability to adhere to rock and produce weathering agents. Select cyanobacteria (often in lichens) as key pioneers.

Verification / Alternative check:
Field studies document lichen crusts increasing organic carbon and nitrogen in early soils; mineral assays show enhanced weathering beneath lichen thalli.

Why Other Options Are Wrong:
Pectin-decomposers – require existing plant polymers.

Nitrifiers/denitrifiers – rely on prior nitrogen compounds and developed soil; they are important later but not primary pioneers.

Enteric bacteria – lack persistence on bare rock and typically arrive with animals/humans.

Common Pitfalls:
Assuming nitrifiers start the process; without initial organic inputs and bio-weathering, nitrifiers have little substrate or habitat.

Final Answer:
Cyanobacteria (often in lichens with fungi).