Calcium oxalate mineralisation in the algae

The morphology, anatomical and cellular localisation, abundance, and taxonomic distribution of calcium oxalate deposits in algae are described. Comparisons are drawn to the same features of calcium oxalate crystals in embryophytes. A review of the early history of work on these mineral deposits in algae reveals a wider distribution of taxa and greater research activity than has generally been recognised in modern literature. Documentation of the presence of calcium oxalate in Vaucheria (Vaucheriales), the sole genus of Ochrophyta known to deposit this mineral, is provided for the first time. Additional species of Dasycladales are reported to be calcium oxalate mineralisers. Although calcium oxalate crystals are a consistent feature of some species, they appear only sporadically in others. It is proposed that three different mechanisms of calcium oxalate deposition operate amongst the algae: (1) unregulated, spontaneous precipitation of stored oxalic acid; (2) constitutive mineralisation involving an organic matrix that determines crystal form; and (3) regulation of crystal form by soluble agents that affect nucleation or deposition on the mineral surface. Members of Bryopsidales that deposit acicular calcium oxalate crystals do so constitutively and are not accumulators of oxalic acid; the opposite is true of many Dasycladales, which deposit calcium oxalate in the bipyramidal form as a result of spontaneous precipitation of oxalic acid. Several species of Spirogyra (Zygnematales) are the only representatives of the Charophyceae known to deposit calcium oxalate; their crystals are cruciate, have an organic matrix, and appear to be constitutive. Among red algae, some species of Antithamnion (Ceramiales) deposit acicular crystals constitutively; whereas, sequestered oxalic acid in Spyridia (Ceramiales) can precipitate to form bipyramidal crystals. Calcium oxalate-depositing members of Cladophorales are numerous; crystal morphologies and perhaps their mode of deposition vary. Vaucheria deposits morphologically diverse intracellular calcium oxalate but was also found to produce bipyramidal crystals by spontaneous precipitation. Monitoring acid dissolution of calcium oxalate by microscopic observation provides insights into crystal structure. Possible ecological and physiological functions of calcium oxalate crystals and of soluble oxalate production in algae are evaluated.