The abundance of heterotrophic protists associated with intertidal seaweeds

Seaweeds have long been known to support abundant populations of bacteria so it is surprising that few studies have considered the abundance of heterotrophic protists on seaweed surfaces. A year-long study was conducted to investigate the numbers of heterotrophic protists on undamaged and damaged tissue of a number of intertidal species of seaweed. Generally, amoebae and flagellates numbered less than 20 cells cm^-2 on undamaged seaweed tissue but were found in greater numbers (more than 20 cm^-2) on damaged tissue. Ciliates were around 1 or 2 cells cm^-2 on undamaged seaweeds and between 1 and 5 cm^-2 on damaged tissue. Numbers of heterotrophic dinoflagellates and heterotrophic diatoms were similar to those of ciliates in late summer and autumn when seaweeds were producing increased amounts of dissolved organic carbon. By assuming that 'true' surface protists inhabited the thin surface film of water on seaweeds, comparisons were drawn with the numbers of protists in nearby open waters. After using appropriate conversion factors, both amoebae and flagellates were more abundant in the surface film (per ml) than in the open water. In particular, surface amoebae averaged around 1800 cells ml^-1 but only reached around 19 cells ml^-1 in open water. The results are discussed in relation to the various sources of nutrition available to heterotrophic protists on macroalgae. The high numbers of amoebae suggest that they are the major micropredators of bacteria on seaweed surfaces. (C) 2000 Academic Press.