Physiological responses of marine and brackish Fucus vesiculosus L. with respect to salinity

Abstract: The intertidal brown alga Fucus vesiculosus L. is mainly a marine species (34 practical salinity units, psu), but the alga also grows in the sublittoral of the brackish Bothnian Sea (part of the Baltic Sea; 5 psu). The conditions at the growth sites are clearly different between the Bothnian Sea and the Norwegian Sea (part of Atlantic) with constant low salinity and a lack of tides in the Bothnian Sea. The objectives of the thesis were to compare the physiology in marine and brackish ecotypes of F. vesiculosus with respect to salinity and the ability of F. vesiculosus to acclimate to different salinities. A study of photosynthetic maximum capacity and relative amount of Rubisco in relation to salinity in brackish F. vesiculosus were also performed. The results showed that both ecotypes of F. vesiculosus have the same potential to use the available excitation energy for photochemistry. The results also suggest that this is relatively independent of salinity changes. There were a higher number of water soluble organic compounds, higher mannitol content (mmol kg‐1 DW), lower chlorophyll (Chl) content (mg g‐1 DW) and higher tolerance to desiccation in the marine ecotype. The number of water soluble carbon compounds did not change when the algae were treated to either high or low salinities and it was suggested that the differences were due to an intertidal or sublittoral acclimation, and not salinity. Both ecotypes showed changed mannitol content as a response to changed salinity but the changes were different between the ecotypes and seasons. The content of mannitol and the osmotic adjustment by mannitol in a longer timescale than 24 h appears to be closely connected to irradiance and photosynthesis in addition to the salinity. The main reason for higher rate of photosynthesis in higher salinity for the brackish ecotype is not clarified because no correlation could be detected between photosynthesis and the relative amount of Rubisco. The Chl content increased in darkness and the differences between the ecotypes are probably due to a compensation for low irradiance in the sublittoral growth site. Higher tolerance for desiccation in marine ecotype was concluded to be due to a lower rate of water loss because of more mannitol and thicker thallus.