Ecological role of surface phosphatase activities of Rivulariaceae

Abstract

The literature suggests that the cyanobacterium Rivularia is found at sites where organic phosphorus (P), at times forms a high proportion of total P in the environment. Its ability to utilise organic P through "surface" phosphatase activity may be important in its success. The aim of this thesis was to investigate this in detail using both field material (from a freshwater stream and from a marine intertidal zone) and axenic isolates of Rivulariaceae. At both sites inorganic P concentrations peaked: in March/May (1992-4) at the freshwater site, and in June (1992-3) at the marine site (although in 5 of the pools containing Rivularia, inorganic P was maximal in February/March 1993). Pools associated with rotting seaweed had higher concentrations of inorganic P (which made up most of the total P) during peaks. It is likely that the high tide resulted in the mixing of weed pool water with Rivularia pool water, slightly lower down the eulittoral zone, and also influenced the retreating seawater. Organic P was a greater proportion of total P in the pools containing Rivularia and was found to increase in these pools during the tidal cycle, suggesting internal generation. At the freshwater site organic P concentrations were higher in pools associated with peat than in stream water. At the freshwater site phosphorus fractions were often below detection limits, but combined nitrogen was rarely this low; the reverse was the case at the marine site. At the freshwater site phosphomonoesterase activity of Rivularia was generally high, except when hormogonia were present in the colonies. At the marine site, phosphatase activity was usually low, with a peak using p-nitrophenyl phosphate (pNPP) as a substrate in July/August and, using 4-methyIumbelliferyl phosphate (4-MUP) as a substrate in September/October, especially in 1992. Phosphatase activity of Rivularia at both sites was negatively influenced by inorganic P and positively correlated with the presence of hairs. K(_m) (Michaelis-Menten constant) was lower using 4-MUP than pNPP in all organisms assayed. Apparent negative cooperativity was found in 7-day cultures of Calothrix parietina D550 using 4-MUP and in whole colonies of freshwater Rivularia using pNPP. These results were discussed with reference to the relationship between nitrogen (N) and P at the sites and the enzyme kinetics of field organisms and axenic isolates. Rivularia is successful in these apparently different environments probably because organic P can be utilised when phosphorus is limiting and colonies are able to fix N2 when N is limiting. Seasonal peaks in inorganic P probably allow regeneration of the Rivularia population.