There is currently no known organism with an absolute requirement for Li. Li has been suggested to be essential for mammalian nutrition based on studies using Li-deficient diets conducted with rats and goats, but the effects were largely limited to decreased fecundity (Schrauzer, 2002). The current consensus seems to indicate a beneficial, although probably not essential, role in eukaryotes (Szklarska & Rzymski, 2019). Some of the most notable effects of Li are on mood. Li salts are used in the treatment of manic-depressive disorders and several studies have established inverse correlations between dietary Li and suicide, for example (Del Matto et al., 2020). 

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Li is beneficial for certain bacteria as part of a Na-sparing response. Vibrio parahaemolyticus is a halophilic bacterium that requires high NaCl for growth. At low concentrations of NaCl, it has been found that Li, which resembles Na in many physical properties, is the most effective monovalent cation in restoring growth (Morishita & Takada, 1976). In a medium of 0.05 M NaCl, which is significantly below the minimum requirement, addition of LiCl, KCl, or sucrose can support growth by providing osmotic pressure. The functional substitution of Na for Li allows growth at concentrations as low as 0.003 M Na. Li can also functionally substitute for at least some Na in several species of Salinispora, a marine actinomycete, to maintain maximal growth and yield (Tsueng & Lam, 2010). 

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Perhaps the best candidates for organisms with a Li requirement are those that grow in extremely Li-rich environments. For example, in the Andes mountains there are extensive salars (salt flats generated by solar evaporation) with very high Li content. Two Bacillus spp.  isolates obtained from these salars were able to grow in Li levels of 1.44 M, but also grew well in the absence of Li salts (Cubillos et al., 2019).Thus, Li is best viewed as a generally toxic element, but with an ability to benefit some select microbes under very specific conditions of Na limitation.