The distribution of epiphytic algae and sesslle invertebrates on the leaves of the seagrasses Posidonla australis Hooker f. and P, sinuosa Cambridge and Kuo is not random. Epiphyte load on the leaves of both species increases with increasmg distance away from the basal meristem There are approximately 3 times as many epiphytic algal species as invertebrate species, and many of these epiphytes grow at distinct locations on the leaves. Epiphytic invertebrates were found primarily on the basal sections of the leaves, whereas algae were most abundant near the leaf apex. Distribution of epiphyte load across the leaf surface was also non-random, with initial settlement of epiphyte propagules occurring at the marqns of the leaves. The structure of the epiphytic community is strongly correlated with leaf age, with a greater abundance of epiphytic species occurring on the older leaves It is clear that leaf morphology also plays a significant role in the distribution of the epiphytes. There is no apparent difference m the epiphytic community between the sides of the flat P. australis leaf whereas, in P. sinuosa, the concave side of the curved leaf supports a more diverse epiphytic community than the convex side. Similar patterns in the distribution of epiphytic organisms were observed on artificial seagrass leaves, indicating that, although a temporal component is involved, epiphyte distribution is influenced mainly by the relative position upon the leaf surface as well as leaf morphology, which affects the water flow pattern over the leaf.

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Summary• Phosphate uptake rates were measured in Synechococcus R-2 incubated in artificial secondary-and tertiary-treated sewage.• Phosphate uptake was measured using chemical assay and 32 P incorporation. Intracellular pH was measured using accumulation of 14 C-labelled weak acids and bases and membrane potentials using 86 Rb + /valinomycin.• Synechococcus cells are capable of very rapid, opportunistic uptake of phosphate (10 -30 nmol m − 2 s − 1 ) even though net uptake by growing cultures was < 0.5 nmol m − 2 s − 1 . Km and Vmax in the light were not significantly different at pH o 7.5 and 10. The mean Km values were 1.91 ± 0.41 mmol m − 3 and 0.304 ± 0.055 mmol m − 3 for P-sufficient (secondary-treated sewage) and P-deficient (tertiarytreated sewage) cells, respectively. The transport systems probably recognize both H 2 PO 4 − and HPO 4 2 − . Intracellular inorganic phosphate is +28 to +56 kJ mol − 1 from electrochemical equilibrium. In P-sufficient cells uptake is very slow in the dark ( c . 0.1 nmol m − 2 s − 1 ) but phosphate-starved cells can opportunistically take up P about 100 times faster.• Two separate ATP-driven phosphate uptake mechanisms (1 PO 4 in per ATP) appear to be responsible for phosphate uptake by the cells. They have different Km values, different light/dark responses and electrical behaviour.

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