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symbiosis (noun, pl. symbioses;ecology term) – any form of cohabitation where two organisms form a close association, thus living together either to the benefit (mutualism) or detriment of one another (parasitism) or without any apparent effect (commensalism). Some definitions of symbiosis restrict the term to a mutualistic, i. e., beneficial relationship only, this concept is, however, much too narrow because it is often impossible to decide the true nature of a symbiotic relationship. A benefit or detriment for one of the symbionts is often not very obvious and may change over time. For example, orchid seeds lack endosperm and their germination is dependent on a symbiotic fungus, that provides the nutrients. The fungus attacks the seed and, in some cases, digests the seed entirely. Thus germination can fail and the fungus would be a parasitic symbiont. In most cases, however, the orchid is capable to withstand the fungal attack and on the contrary even deprives the fungus of nutrients. Although thee fungal cells aggressively invade the cells of the germinating orchid, fungal hyphae are eventually digested and thus destroyed. Thus, in turn, the orchid becomes a parasite. It can be assumed that almost all symbioses are that dynamic. In mycorrhizae a transfer of sugars from vascular plants to the mycorrhizal as well as the transfer of mineral salts from the fungus to the vascular plant have both been confirmed. It can be assumed that most of the time the nature of this symbiosis is balanced and indeed beneficial to both of the partners. Subject to environmental variation this balance may, however, shift in favour of one of the partners. For example, on very rich soils, roots will benefit little from additional increased mineral supply. Thus, if minerals are already abundantly present and accessible, mycorrhizal fungi may act mainly as a sugar drain for the plant. In lichen symbioses the transfer of sugars from photobiont to mycobiont has been confirmed. An increased mineral supply for the photobiont might also be assumed, but could so far not be experimentally confirmed. Instead, some researchers postulate another, rather vague advantage for the photobiont, arguing that the mycobiont provides shelter from harsh ecological conditions. Thus it is assumed that the mycobiont expands the ecological niche of the photobiont. Other researchers doubt whether this hypothesis will ever be substantiated and instead interpret the lichen as a form of controlled parasitism.

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