Diatom quantification and their distribution with salinity brines in coastal sediments of Terra Nova Bay (Antarctica)

Abstract

Benthic diatoms represent an important element of global nutritional productivity; to raise attention on their role, which is often neglected due to analytical difficulties, surface (1 cm top layer) coastal sediments from Gerlache inlet to Penguin Bay at Terra Nova Bay were collected and stored at −20 °C. DNA amplification by real-time PCR, based on diatom-specific oligonucleotide primers designed on small-subunit rRNA (SSU rRNA), was performed in addition to diatom conventional cell counting and spectrophotometric determination of photo-pigments. Moreover, cations and anions were determined in sediments with the aim to identify factors involved in the control of diatom abundance. Diatom distribution was found quite heterogeneous displaying significant differences from site to site. The salinity in sediments ranged from 45.1 at Gerlache inlet to 76.2 at Penguin Bay and it was correlated with cell abundance, biodiversity, amount of pigments and amplified DNA. The dominant species, Fragilariopsis curta, was associated to sediment salinity brines.

Introduction

Diatoms are responsible for a substantial part of the carbon fixation (Arrigo et al., 1999), especially at Ross Sea (Antarctica), which is characterized by relatively high primary productivity due to the Polynya phenomenon. Pennate diatoms can be found in water column and in sediments and, at high latitudes they are the most conspicuous organisms in the pack ice, where they could reach such high concentrations that their photosynthetic pigments discolor the ice brown (Thomas and Dieckman, 2002). Several diatom species have been found in this harsh habitat, where low ice temperatures are linked to high levels of salinity brine (Zhang et al., 1999). These unicellular photosynthetic micro algae are mainly found on the underside of the ice where the light is more intense than in the rest of the water column. However, their abundance can be significant also in the surface sediments of the coastal zone where the light reaches the bottom and in some areas they were found to be more productive than phytoplankton (Brotas and Catarino, 1995, Cabeçadas et al., 1983). Several pennate diatoms manage to migrate through sediment layers but 80% of the living community is concentrated in the top few millimeters (MacIntyre et al., 1996).

Diatom colonization of sediments is a poorly studied topic due to sampling and analytical difficulties. Microscopy observations are complicated by the presence of sediment grains and organic matter, hence, frustule cleaning with acids or hydrogen peroxide is essential. In such conditions the distinction between live cells, dead cells still carrying intact biomolecules and empty frustules is very difficult to achieve. However, as it is still unclear which may be the role of benthic diatoms in the sediment organic carbon enrichment, it is of primary interest to improve the investigation methods and to obtain coherent correlation among data. Moreover, recent in situ productivity studies based on detection of biopolymeric carbon (BPC; the sum of carbon equivalents of proteins, carbohydrates and lipids) in coastal sediments at Terra Nova Bay during the Austral summer 2004–2005 suggest a key role for diatoms (Baldi et al., 2010).

The few information available indicate that the dominant species in the settled material (hence coming from the water column) of the southwestern Ross Sea, as well as in the surface sediments North of Drygalski Ice Tongue (Western Ross Sea, South of Terra Nova Bay), is Fragilariopsis curta, which is commonly associated with both sea ice and the retreating ice edge, suggesting that it is probably seeded by melting ice (Leventer and Dunbar, 1996, Cunningham and Leventer, 1998). F. curta was found to characterize the surface sediments also in other regions of the Antarctic shelf, such as Prydz Bay, representing up to the 60% of total diatoms (Rathburn et al., 1997).

The aim of this study was to provide information on diatom abundance and distribution in poorly studied coastal sediments of Terra Nova Bay (Ross Sea, Antarctica). Such organisms may represent an essential food source and it is important to understand their role in the trophic chain mainly in a region characterized by extreme environmental conditions but also able to guarantee the survival of high trophic level animals (e.g. penguins; Sweeney et al., 2000). In order to reduce the analytical difficulties linked to the determination of benthic community different techniques were used. Conventional cell counting was related to quantitative determination of photo-pigments, nutrients and PCR amplified DNA in the attempt to distinguish between DNA-containing diatoms and empty frustules settled in the sediments. Moreover, the correlation between biological and chemical data may furnish indication on the factor controlling diatom distribution in such an extreme habitat.