Organic and siliceous protistan scales in north-east Atlantic abyssal sediments

Abstract

We report the occurrence of a high diversity of minute (∼1 μm diameter) organic and siliceous protistan scales in small samples (total volume ∼35 μl) of superficial sediment from the Porcupine Abyssal Plain (PAP), north-east Atlantic (4850 m water depth). Many exhibit characters by which they can be identified to species. The organic scales belong to the haptophyte genera Chrysochromulina (8–9 species), Chrysocampanula and Dolichomastix (1 species each). The siliceous scales belong to the chrysophytes Paraphysomonas vestita and Meringosphaera sp. and to the heterotrophic flagellate genus Thaumatomastix (T. dybsoeana, T. formosa, Thaumatomastix sp.). As far as we are aware, this is the first time that non-calcareous protistan scales have been observed in deep-sea sediments (although siliceous skeletal plates and cysts are reported). All scales probably originated from the upper water column and were delivered to the deep-sea floor on rapidly sinking detrital aggregates. However, naked heterotrophic flagellates are known to thrive in abyssal sediment habitats and so the possibility that some scale-bearing protists also live in benthic deep-sea environments cannot be eliminated. Many species identified at the PAP site are common in coastal marine waters around Europe; some occur as far afield as Tasmania and New Zealand. Five Chrysochromulina species are known from central oceanic areas, including parts of the North Atlantic, while another species, C. pringsheimii, is reported from a British freshwater lake. We retrieved ∼15% of the 55 named Chrysochromulina species (∼8% of the estimated total number of species in this diverse group) in the ∼35 μl of abyssal sediment. Because the scales can persist and be identified after cell death, they may provide useful time- and space-averaged information about the distribution of protist species in marine habitats. The long-term fate of the scales on the sea-floor is unknown. It is possible that at least some of the organic scales are preserved as microfossils in deep-sea sediments.