Symbioses – fungus with a cyanobacterium What is the term for a stable mutualistic association composed of a fungal partner and a cyanobacterial partner living as a single thallus?

Introduction / Context:
Symbiotic partnerships drive ecosystem processes. Lichens are classic examples where a fungus teams with a photosynthetic partner to colonize harsh habitats. In many cases the photobiont is a green alga, but some lichens partner with cyanobacteria, which adds nitrogen fixation to the relationship.

Given Data / Assumptions:

• The association is stable and mutually beneficial.
• One partner is a fungus (the mycobiont).
• The other partner is a cyanobacterium (the photobiont).

Concept / Approach:
Lichen refers to the integrated thallus made by a fungus and a photosynthetic partner. When the photobiont is a cyanobacterium such as Nostoc, the lichen can fix atmospheric nitrogen, enriching poor substrates. Saprobes are decomposers, not symbiotic consortia. A gamete is a sexual cell, not a symbiosis. Dimorphic refers to a fungus that can grow as yeast or mold depending on conditions. Mycorrhiza denotes a mutualism between a fungus and plant roots, not cyanobacteria. Therefore lichen is the correct term for a fungus plus cyanobacterium partnership.

Step-by-Step Solution:
Identify the two partners: fungus and cyanobacterium.Match this combination to known named symbioses.Select “Lichen.”

Verification / Alternative check:
Ecology manuals describe cyanolichens where Nostoc bearing cephalodia are present and contribute nitrogen fixation to the lichen system, confirming the fungus cyanobacterium pairing.

Why Other Options Are Wrong:

• Saprobe: lifestyle, not a symbiotic unit.
• Gamete: reproductive cell, not a mutualism.
• Dimorphic: growth form of a single fungus, not a partnership.
• Mycorrhiza: fungus plus plant roots, not cyanobacterium.

Common Pitfalls:
Assuming lichens always contain green algae. Many lichens include cyanobacterial photobionts and therefore have different physiology and habitat preferences.

Final Answer:
Lichen.