Abundance of thraustochytrids and ciliated protozoans in a Mediterranean sandy shore determined by an improved, direct method

Thraustochytrids (fungoid protoctists) and ciliated protozoans are major co mponents of marine sandy shore microcommunities. However they are rarely en umerated together. This report describes the results obtained through appli cation of an improved method for directly enumerating both of these microor ganisms from sandy sediments. Thraustochytrids and ciliates were extracted by repeated washing with glutaraldehyde-seawater solution. To separate both microorganisms from non-living material the supernatant was centrifuged in a non-linear silica gel gradient, and then stained with 3 different fluoro chromes. Sampling was carried out on a sheltered, medium-grain-sized sandy shore in the Ligurian Sea (NW Mediterranean). Ninety-six 1 mi replicated sa nd samples were collected. They were arranged following a nested ANOVA desi gn in which factors were 3 successively smaller spatial scales (sampling un its of different size, ranging between meters and centimeters). Sampling wa s repeated 3 times during the autumn-winter period. Average ciliate abundan ce was 44 +/- 26 ml(-1), showing a significant decrease with time (from aut umn to winter). The average abundance of thraustochytrids was 42.4 +/- 35.2 ml(-1), with no significant decrease over time. Neither group of protists showed a spatial structure at the scales explored. Variability among replic ated 1 mi samples was the main variance source for both microorganisms. The findings presented here are the first to address the distribution of thrau stochytrids in marine sediments as determined by direct enumeration.