Editing Symbiosis (section)

=== Obligate versus facultative ===

Relationships can be obligate, meaning that one or both of the symbionts entirely depend on each other for survival. For example, in [[lichen]]s, which consist of fungal and [[photosynthetic]] symbionts, the fungal partners cannot live on their own.<ref name="Wilkinson-2001"/><ref>{{Harvnb|Isaac|1992|p=266}}</ref><ref>{{Harvnb|Saffo|1993}}</ref><ref name="Douglas-2010-2">{{harvnb|Douglas|2010|p=4}}</ref> The algal or cyanobacterial symbionts in lichens, such as ''[[Trentepohlia (alga)|Trentepohlia]]'', can generally live independently, and their part of the relationship is therefore described as facultative (optional), or non-obligate.<ref name="Muggia-2013">{{cite journal |last1=Muggia |first1=Lucia |last2=Vancurova |first2=Lucie |last3=Škaloud |first3=Pavel |last4=Peksa |first4=Ondrej |last5=Wedin |first5=Mats |last6=Grube |first6=Martin  |title=The symbiotic playground of lichen thalli--a highly flexible photobiont association in rock-inhabiting lichens |journal=FEMS Microbiology Ecology |volume=85 |issue=2 |pages=313–323 |date=August 2013 |pmid=23530593 |doi=10.1111/1574-6941.12120 |bibcode=2013FEMME..85..313M |doi-access=free }}</ref> When one of the participants in a symbiotic relationship is capable of photosynthesis, as with lichens, it is called photosymbiosis.<ref>{{cite web |title=photosymbiosis |website=Oxford Reference |url=https://www.oxfordreference.com/display/10.1093/oi/authority.20110803100324758;jsessionid=FFCC5585B976D5CF87DDD905A035A006 }}</ref><ref>{{cite journal |last1=Gault |first1=Jordan A. |last2=Bentlage |first2=Bastian |last3=Huang |first3=Danwei |last4=Kerr |first4=Alexander M. |title=Lineage-specific variation in the evolutionary stability of coral photosymbiosis |journal=Science Advances |date=2021 |volume=7 |issue=39 |pages=eabh4243 |doi=10.1126/sciadv.abh4243 |pmid=34550731 |pmc=8457658 |bibcode=2021SciA....7.4243G }}</ref>