Abstract
Phytoplankton blooms exhibit a severe impact on bacterioplankton communities as they change nutrient availabilities and other environmental factors. In the current study, the response of a bacterioplankton community to a Phaeocystis globosa spring bloom was investigated in the southern North Sea. For this purpose, water samples were taken inside and reference samples outside of an algal spring bloom. Structural changes of the bacterioplankton community were assessed by amplicon-based analysis of 16S rRNA genes and transcripts generated from environmental DNA and RNA, respectively. Several marine groups responded to bloom presence. The abundance of the Roseobacter RCA cluster and the SAR92 clade significantly increased in bloom presence in the total and active fraction of the bacterial community. Functional changes were investigated by direct sequencing of environmental DNA and mRNA. The corresponding datasets comprised more than 500 million sequences across all samples. Metatranscriptomic data sets were mapped on representative genomes of abundant marine groups present in the samples and on assembled metagenomic and metatranscriptomic datasets. Differences in gene expression profiles between non-bloom and bloom samples were recorded. The genome-wide gene expression level of Planktomarina temperata, an abundant member of the Roseobacter RCA cluster, was higher inside the bloom. Genes that were differently expressed included transposases, which showed increased expression levels inside the bloom. This might contribute to the adaptation of this organism toward environmental stresses through genome reorganization. In addition, several genes affiliated to the SAR92 clade were significantly upregulated inside the bloom including genes encoding for proteins involved in isoleucine and leucine incorporation. Obtained results provide novel insights into compositional and functional variations of marine bacterioplankton communities as response to a phytoplankton bloom.
Highlights
Bacteria are major drivers in cycling of nitrogen, carbon, and other elements in marine ecosystems (Azam et al, 1983; Arrigo, 2005; DeLong and Karl, 2005)
We assume that the Active bacterial communities in the North Sea are dominated by only a few marine groups such as the Roseobacter RCA cluster
In situ experiments employing available isolates do not necessarily reflect environmental conditions and, only provide limited information on the ecological role of the studied isolates. Mapping these reads on assembled metagenomic and metatranscriptomic sequences led to an overall mapping rate of more than 85% demonstrating the power of this combined approach
Summary
Bacteria are major drivers in cycling of nitrogen, carbon, and other elements in marine ecosystems (Azam et al, 1983; Arrigo, 2005; DeLong and Karl, 2005). More than 50% of organic matter produced by phytoplankton is remineralized by marine bacteria (Cole et al, 1988; Karner and Herndl, 1992; Ducklow et al, 1993). Bacteria play an important role during and after bloom events as large amounts of organic matter are generated by primary production (Azam, 1998). Recent studies investigating bacterioplankton communities during phytoplankton blooms revealed that community structures and diversity were highly affected (Teeling et al, 2012; Liu et al, 2013; Wemheuer et al, 2014). Understanding the dynamics and interactions between bacterial communities and phytoplankton blooms is crucial to validate the ecological impact of bloom events
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