Abstract
Temperature, nutrients, and salinity are among the important factors constraining the distribution and abundance of microorganisms in the ocean. Marine Group II (MGII) belonging to Euryarchaeota commonly dominates the planktonic archaeal community in shallow water and Marine Group I (MGI, now is called Thaumarchaeota) in deeper water in global oceans. Results of quantitative PCR (qPCR) and 454 sequencing in our study, however, showed the dominance of MGII in planktonic archaea throughout the water column of the northeastern South China Sea (SCS) that is characterized by strong water mixing. The abundance of ammonia-oxidizing archaea (AOA) representing the main group of Thaumarchaeota in deeper water in the northeastern SCS was significantly lower than in other oceanic regions. Phylogenetic analysis showed that the top operational taxonomic units (OTUs) of the MGII occurring predominantly below 200 m depth may be unique in the northeastern SCS based on the observation that they are distantly related to known sequences (identity ranging from 90–94%). The abundance of MGII was also significantly correlated with total bacteria in the whole column, which may indicate that MGII and bacteria may have similar physiological or biochemical properties or responses to environmental variation. This study provides valuable information about the dominance of MGII over AOA in both shallow and deep water in the northeastern SCS and highlights the need for comprehensive studies integrating physical, chemical, and microbial oceanography.
Highlights
Microorganisms are the majority of life in the ocean and play fundamental roles in ecological functions and biogeochemical cycles (Fuhrman, 2009)
Planktonic archaea have been recognized to play important roles in global carbon and nitrogen cycles (Karner et al, 2001; Francis et al, 2005; Ingalls et al, 2006). They were initially divided into Marine Group I (MGI; called Thaumarchaeota) and Marine Group II (MGII) that belong to Euryarchaeota (DeLong, 1992); the latter has generally been observed to dominate the surface ocean in archaeal composition, whereas the former becomes increasingly abundant at greater depths (Massana et al, 2000; Karner et al, 2001; Herndl et al, 2005; Lincoln et al, 2014)
It is worth mentioning that operational taxonomic units (OTUs) of MGII in the top 100 m (B7) depth intervals showed greater distribution similarity than those from most depths below; OTUs of MGII from the top 5 m at D5, did not show distribution similarity with those from any depth below (Figure 5A), which suggests that surface water or water in the upper photic zone had less mixing with water from deeper depths
Summary
Microorganisms are the majority of life in the ocean and play fundamental roles in ecological functions and biogeochemical cycles (Fuhrman, 2009). Planktonic archaea have been recognized to play important roles in global carbon and nitrogen cycles (Karner et al, 2001; Francis et al, 2005; Ingalls et al, 2006) They were initially divided into Marine Group I (MGI; called Thaumarchaeota) and Marine Group II (MGII) that belong to Euryarchaeota (DeLong, 1992); the latter has generally been observed to dominate the surface ocean in archaeal composition, whereas the former becomes increasingly abundant at greater depths (Massana et al, 2000; Karner et al, 2001; Herndl et al, 2005; Lincoln et al, 2014). The mechanisms controlling the distribution of MGII in different water columns of the ocean are poorly known
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