Effect of salinity, temperature, organic and inorganic nutrients on growth of cultured Fibrocapsa japonica (Raphidophyceae) from the northern Adriatic Sea

The effects of temperature, salinity, inorganic and organic nutrients on the growth and physiological performance of Fibrocapsa japonica have been investigated in strains isolated from the northern Adriatic Sea, where intense and regular blooms have occurred since 1997 in coastal waters during summer. Strains isolated at different times and from different locations appeared homogeneous in terms of both physiological responses and molecular (ITS-5.8S rDNA) characteristics. Growth rates were higher at temperatures between 20 and 26 °C and in a salinity range of 30–35 (0.7 div day⁻¹). The temperature of 16 °C inhibited growth, more markedly at the lowest and highest salinity values, a result also confirmed by a lower photosynthetic efficiency and by an increase in cell volume due to impaired division. Higher cell concentrations were obtained with macronutrients at f/2 levels than in a fivefold diluted medium. Comparing the utilization of 200 μM nitrate to that of different N sources (inorganic and organic), F. japonica showed an efficient growth with equivalent amounts of ammonia, urea and amino acids, such as glycine and tryptophan; glutamate was less effective, while methionine had toxic effects. Organic phosphate, administered as glycerophosphate, could also sustain F. japonica growth, probably on account of an alkaline phosphatase whose activity was enhanced in the presence of the organic form. Vitamins were necessary for growth, though no further stimulation was observed when a surplus of vitamin B12 (3 nM) was added. The addition of 11.7 μM iron instead of 2.3 μM, as well as that of humic acid, with or without macronutrients, did not enhance algal growth either. These results led us to hypothesise that F. japonica blooms became more frequent due to a general seawater temperature increase and to the availability of organic forms which, in coastal anthropized areas, are especially abundant in summer periods.     

The diversity of relationships between Antarctic sponges and diatoms: the case of Mycale acerata Kirkpatrick, 1907 (Porifera,Demospongiae)

The diatom assemblage associated with the Antarctic sponge Mycale acerata was studied through an analysis of the diatom frustule and pigment concentrations in both the sponge ectosome and choanosome. Sponges were sampled weekly from November 2001 to February 2002 at Terra Nova Bay, Antarctica, at a depth of 25–35 m. The most abundant diatoms were Porannulus contentus, Fragilariopsis curta, Thalassiosira cf. gracilis, T. perpusilla and Plagiotropis sp. High abundances of P. contentus were found on the sponge ectosome up to the beginning of November, before the ice melted, while later frustules were incorporated inside, indicating that P. contentus lives epibiontically on M. acerata and represents a potential food source for the sponge. The presence of other diatom species was mainly related to the summer phytoplankton bloom. The sponge incorporates diatoms from the water column and utilises them as a food source, accumulating frustules inside the choanosome. The lack of planktonic diatom frustules at the beginning of the summer indicates that they are expelled or dissolved during the cold season.