Distribution and abundance of thraustochytrids in different Mediterranean coastal habitats

Abstract

Habitat, seasonality and species distribution of the marine fungoid protists, thraustochytrids, were investigated in a coastal area of the Ligurian Sea (NW Mediterranean). Three habitats (the sandy surf area, the middle of a 5 m seawater column, and its underlying sandy sea bottom) were monitored monthly for 1 yr. Thraustochytrids in the surf and sea-bottom sandy samples yielded average densities of 4.4 ± 4 and 6.1 ± 5.3 × 10 cells 11, respectively. In both sandy habitats, a similar seasonal trend was observed: the highest densities were recorded in late spring, while a significant drop in densities occurred during the autumn-winter period. In the water column, the density of thraustochytrids was much lower, 1.3 ± 1 × 102 cells 11 and did not show significant seasonal variations. A positive relationship between organic matter and thraustochytrid densities occurred in the sandy sea bottom. Among the nutrients assessed in the water column, only total phosphorus was related to variations of thraustochytrid densities. The 3 habitats analyzed showed marked differences in species number and assemblage. A higher number of species were found in the sandy sea bottom than in the sandy surf area and in the seawater column. The genus Thraustochytrium was the most abundant in the sandy bottom habitat. Among Thraustochytrium species, those characterized by sporangia with proliferation bodies were dominant. Species of the genus Schizochytrium, typically forming a cluster of sporangia, were dominant in the sandy surf area. Here, the possible ecological significances of these species' features are discussed.