The qualitative and quantitative occurrence of low molecular weight carbohydrates (LMWCs) in the Bangiophyceae is surveyed. Members of the orders Erythropeltidales (Sahlingia and Erythrotrichia), Compsopogonales (Compsopogon), and Bangiales (Bangia and Porphyra) all contain floridoside and, in some cases, D‐ and L‐isofloridoside, making the group chemotaxonomically the most homogeneous; L‐isofloridoside occurred only in the Bangiales. The Porphyridiales showed great variation in carbohydrate types: Chroodactylon contained only sorbitol, Porphyridium contained only floridoside, Dixoniella and Rhodella species exhibited only mannitol, and Rhodosorus showed digeneaside and sorbitol, whereas Stylonema contained floridoside, D‐isofloridoside, digeneaside, and sorbitol. The extensive variation in LMWC composition within and between the different orders suggests that the enzyme systems for each might have developed repeatedly in different Bangiophyceae and Florideophyceae. Nonetheless, a phylogenetic tree based on small‐subunit ribosomal RNA supports the LMWC patterns in the Bangiophyceae.

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The effects of organic osmolytes synthesised and accumulated by red algae from mangrove habitats were investigated on the in vitro activities of two major enzymes, one of the citric acid cycle (malate dehydrogenase, MDH) and one of the oxidative pentose phosphate pathway (glucose-6- phosphate dehydrogenase, G6PDH). These enzymes were extracted from the mangrove algae Bostrychia tenella, Caloglossa leprieurii, Catenella nipae and Stictosiphonia hookeri. In each case, activity of the enzymes was inhibited with increasing NaCl concentrations up to 600 mM . In contrast, equimolar concentrations of mannitol (the major osmolyte in C. leprieurii), sorbitol (the major osmolyte in B. Tenella and S. hookeri) and a heteroside mixture (of which floridoside is the major osmolyte in C. nipae) did not inhibit enzyme function. Dulcitol, the second most important organic osmolyte in B. tenella, exerted no negative effect at its maximum solubility of 180 rnM on the salt-sensitive MDH. These data are all consistent with the proposed function of these organic compounds as compatible solutes.