Diversity in small molecular weight solutes and osmoregulation

Photosynthesis is closely linked to the generation of low molecular weight organic solutes (Figure 4.6) that are involved in osmoregulation. The formation of simple sugars, in particular, results in the formation of a range of osmotically-active compounds which are diagnostic for particular algal groups (Table 4.5). These compounds are important in the osmotic balance within freshwater environments, where algal cells are surrounded by a hypotonic medium. In such conditions the tendency for water to enter by endosmosis is either counterbalanced by cell wall pressure, or (in naked algae) requires continuous expulsion of water by contractile vacuole activity. In conditions of increasing salinity, where the external medium becomes hypertonic, higher intracellular concentrations of osmotically-active solutes are required to balance the elevated external molarity.

The importance of light in the osmoregulatory process is illustrated by the diatom Cyclindrotheca fusiformis, where photosynthesis is responsible for maintaining the free internal mannose concentration at an adequate level to balance outside osmolarity (Paul, 1979). In the dark, free mannose can be

Table 4.5 Some examples of osmotically-active low molecular weight organic solutes in different algal groups

Algal group Environmental groups Low MW solute

Table 4.5 Some examples of osmotically-active low molecular weight organic solutes in different algal groups

Algal group Environmental groups Low MW solute

Blue-green algaea

Freshwater environments

Glucosyl-glycerol

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