Mixed approach on Chroodactylon ornatum (Stylonematophyceae, Rhodophyta) tolerance to hyposalinity: growth, photosynthetic performance and carbohydrate analysis

Chroodactylon ornatum is a cosmopolitan non-Florideophycean red alga with a simple morphology, generally described as a euryhaline species. Herein, we compared the response of a marine strain under exposure to different salinities (3, 7.5, 15 and 30 psu) through the analyses of growth, morphology, chlorophyll a fluorescence, floridean starch properties, and mucilage composition. In 3 psu, the cultures consisted of solitary cells whereas the typical pseudofilamentous morphology of increasing length was found at higher salinities. The species grew in the whole range of salinities, although growth response together with photosynthetic efficiency were maximum in full seawater. The maximum relative electron transport rate (rETR) was significantly lower under hyposaline conditions, concomitant with the increase in photophysical decay but not in non-photochemical quenching of chlorophyll fluorescence (NPQ). Higher phycocyanin content at the highest salinity tested (30 psu) could be related to a more efficient energy transfer to the reaction centre. The amylopectin:amylose ratios were higher in 7.5 and 30 psu. Agarans extracted at different salinities showed similar sulphation degrees, but there was an increase in xylose content in the mucilage from lower salinities. This could be related to a change in physical properties of the sheath polysaccharide, yielding a polymer with lower gelling capacity which hampered the formation and/or elongation of pseudofilaments.