Temporal distribution of the archaeal community in the Changjiang Estuary hypoxia area and the adjacent East China Sea as determined by denaturing gradient gel electrophoresis and multivariate analysis

Evaluating Production of Cyclopentyl Tetraethers by Marine Group II Euryarchaeota in the Pearl River Estuary and Coastal South China Sea: Potential Impact on the TEX86 Paleothermometer

TEX₈₆ [TetraEther indeX of glycerol dialkyl glycerol tetraethers (GDGTs) with 86 carbon atoms] has been widely applied to reconstruct (paleo-) sea surface temperature. Marine Group I (MG-I) Thaumarchaeota were thought to be the primary source of GDGTs constituting the TEX₈₆ formula; however, recent research has suggested that Marine Group II (MG-II) Euryarchaeota may also contribute significantly to the GDGT pool in the ocean. Little is known regarding the potential impact of MG-II Euryarchaeota-derived GDGTs on TEX₈₆ values recorded in marine sediments. In this study, we assessed the relationship between distributions of GDGTs and MG-II Euryarchaeota and evaluated its potential effect on the TEX₈₆ proxy. Lipid and DNA analyses were performed on suspended particulate matter and surface sediments collected along a salinity gradient from the lower Pearl River (river water) and its estuary (mixing water) to the coastal South China Sea (SCS, seawater). TEX₈₆-derived temperatures from the water column and surface sediments were significantly correlated and both were lower than satellite-based temperatures. The ring index (RI) values in these environments were higher than predicted from the calculated TEX₈₆-RI correlation, indicating that the GDGT pool in the water column of the PR estuary and coastal SCS comprises relatively more cyclopentane rings, which thereby altered TEX₈₆ values. Furthermore, the abundance of MG-II Euryarchaeota 16S rRNA gene in the mixing water was two to three orders of magnitude higher than those observed in the river or seawater. Significant linear correlations were observed between the gene abundance ratio of MG-II Euryarchaeota to total archaea and the fractional abundance of GDGTs with cyclopentane rings. Collectively, these results suggest that MG-II Euryarchaeota likely produce a large proportion of GDGTs with 1–4 cyclopentane moieties, which may bias TEX₈₆ values in the water column and sediments. As such, valid interpretation of TEX₈₆ values in the sediment record, particularly in coastal oceans, should consider the contribution from MG-II Euryarchaeota.

Planktonic Euryarchaeota are a significant source of archaeal tetraether lipids in the ocean

Archaea are ubiquitous in marine plankton, and fossil forms of archaeal tetraether membrane lipids in sedimentary rocks document their participation in marine biogeochemical cycles for >100 million years. Ribosomal RNA surveys have identified four major clades of planktonic archaea but, to date, tetraether lipids have been characterized in only one, the Marine Group I Thaumarchaeota. The membrane lipid composition of the other planktonic archaeal groups--all uncultured Euryarchaeota--is currently unknown. Using integrated nucleic acid and lipid analyses, we found that Marine Group II Euryarchaeota (MG-II) contributed significantly to the tetraether lipid pool in the North Pacific Subtropical Gyre at shallow to intermediate depths. Our data strongly suggested that MG-II also synthesize crenarchaeol, a tetraether lipid previously considered to be a unique biomarker for Thaumarchaeota. Metagenomic datasets spanning 5 y indicated that depth stratification of planktonic archaeal groups was a stable feature in the North Pacific Subtropical Gyre. The consistent prevalence of MG-II at depths where the bulk of exported organic matter originates, together with their ubiquitous distribution over diverse oceanic provinces, suggests that this clade is a significant source of tetraether lipids to marine sediments. Our results are relevant to archaeal lipid biomarker applications in the modern oceans and the interpretation of these compounds in the geologic record.

Molecular ecology techniques reveal both spatial and temporal variations in the diversity of archaeal communities within the athalassohaline environment of Rambla Salada, Spain

We have studied the distribution of the archaeal communities in Rambla Salada (Murcia, Spain) over three different seasons and observed the influence upon them of the environmental variables, salinity, pH, oxygen and temperature. Samples were collected from three representative sites in order to gain an insight into the archaeal population of the rambla as a whole. Denaturing gradient gel electrophoresis patterns and diversity indexes indicate that the diversity of the archaeal community in Rambla Salada changed mainly according to the season. We found no significant differences between the types of sample studied: watery sediments and soils. The upwelling zone showed most diversity in its archaeal community. The overall archaeal community was composed mainly of Halobacteriales and Thermoplasmatales, accounting for 72.6 and 12.1 % of the total, respectively. Haloarcula was the most abundant genus, being present at all three sites during all three seasons. Some few Crenarchaeota were always found, mainly at low-salinity levels. Ordination canonical correspondence analysis demonstrated that salinity affected the structure of the community significantly, whilst pH, oxygen and temperature did so to a lesser extent. Most Halobacteriales correlated positively with salinity and pH, whilst Thermoplasmatales correlated negatively with both salinity and pH and positively with temperature and oxygen. The archaeal community with the highest diversity was sampled during June 2006, the season with the highest salt concentration. Catalyzed reporter deposition-fluorescence in situ hybridization showed that the percentage of archaea in Rambla Salada compared to the total number of microorganisms (as measured by DAPI) ranged from 11.1 to 16.7 %. Our research group had isolated the most abundant taxon, Haloarcula, previously in Rambla Salada using classical culture techniques, but on this occasion, using culture-independent methods, we were also able to identify some phylotypes, Halorubrum, Methanolobus, Natronomonas, Halomicrobium, Halobacterium, Halosimplex, uncultured Thermoplasmatales and uncultured Crenarchaeota, that had remained undetected during our earlier studies in this habitat.

Dynamics of autotrophic marine planktonic thaumarchaeota in the East China Sea

The ubiquitous and abundant distribution of ammonia-oxidizing Thaumarchaeota in marine environments is now well documented, and their crucial role in the global nitrogen cycle has been highlighted. However, the potential contribution of Thaumarchaeota in the carbon cycle remains poorly understood. Here we present for the first time a seasonal investigation on the shelf region (bathymetry≤200 m) of the East China Sea (ECS) involving analysis of both thaumarchaeal 16S rRNA and autotrophy-related genes (acetyl-CoA carboxylase gene, accA). Quantitative PCR results clearly showed a higher abundance of thaumarchaeal 16S and accA genes in late-autumn (November) than summer (August), whereas the diversity and community structure of autotrophic Thaumarchaeota showed no statistically significant difference between different seasons as revealed by thaumarchaeal accA gene clone libraries. Phylogenetic analysis indicated that shallow ecotypes dominated the autotrophic Thaumarchaeota in the ECS shelf (86.3% of total sequences), while a novel non-marine thaumarchaeal accA lineage was identified in the Changjiang estuary in summer (when freshwater plumes become larger) but not in autumn, implying that Changjiang freshwater discharge played a certain role in transporting terrestrial microorganisms to the ECS. Multivariate statistical analysis indicated that the biogeography of the autotrophic Thaumarchaeota in the shelf water of the ECS was influenced by complex hydrographic conditions. However, an in silico comparative analysis suggested that the diversity and abundance of the autotrophic Thaumarchaeota might be biased by the ‘universal’ thaumarchaeal accA gene primers Cren529F/Cren981R since this primer set is likely to miss some members within particular phylogenetic groups. Collectively, this study improved our understanding of the biogeographic patterns of the autotrophic Thaumarchaeota in temperate coastal waters, and suggested that new accA primers with improved coverage and sensitivity across phylogenetic groups are needed to gain a more thorough understanding of the role of the autotrophic Thaumarchaeota in the global carbon cycle.