Abstract
Abstract. While picocyanobacteria (PC) are important actors in carbon and nutrient cycles in aquatic systems, factors controlling their interannual dynamics and diversity are poorly known due to the general lack of long-term monitoring surveys. This study intended to fill this gap by applying a DNA-based paleolimnological approach to sediment records from a deep subalpine lake that has experienced dramatic changes in environmental conditions during the last century (eutrophication, re-oligotrophication and large-scale climate changes). In particular, we investigated the long-term (100 yr) diversity and dynamics of Synechococcus,, PC that have presumably been affected by both the lake trophic status changes and global warming. The lake's morphological and environmental conditions provided the ideal conditions for DNA preservation in the sediment archives. Generalised additive models applied to quantitative PCR (qPCR; quantitative Polymerase Chain Reaction) results highlighted that an increase in summer temperature could have a significant positive impact on the relative abundance of Synechococcus, (fraction of Synechococcus, in total cyanobacteria). The diversity of Synechococcus, in Lake Bourget was studied by phylogenetic analyses of the 16S rRNA gene and the following internally transcribed spacer (ITS). Up to 23 different OTUs (based on 16S rRNA), which fell into various cosmopolitan or endemic clusters, were identified in samples from the past 100 yr. Moreover, the study of ITS revealed a higher diversity within the major 16S rRNA-defined OTUs. Changes in PC diversity were related to the lake's trophic status. Overall, qPCR and sequencing results showed that environmental changes (in temperature and phosphorus concentration) affected Synechococcus, community dynamics and structure, translating into changes in genotype composition. These results also helped to re-evaluate the geographical distribution of some Synechococcus, clusters. Providing such novel insights into the long-term history of an important group of primary producers, this study illustrates the promising approach that consists in coupling molecular tools and paleolimnology to reconstruct a lake's biodiversity history.
Highlights
CnaurablolynficxyecdlecaorfbmonanisTyahltatrekiebsuC.teIrndydteooestdh,peuhppiectoorpe7h0yt%oploafntkhteonaninultra-oligotrophic waters (Caron et al, 1985; Callieri, 2008; Wilhelm et al, 2006)
total organic carbon content (TOC) remained high from the late 1950s to the most recent time period, in spite of the lake’s trophic status decreasing from eutrophic to oligo-mesotrophic during the latest 20 yr, as illustrated by the diatom-inferred www.biogeosciences.net/10/3817/2013/
The highest levels of total cyanobacteria and Synechococcus were found in the most superficial sediment layer (BF1) with up to 5.13 × 106 (±0.110 × 106) and 0.15 × 106 (±0.004 × 106) cells g−1 of dry sediment, respectively
Summary
CnaurablolynficxyecdlecaorfbmonanisTyahltatrekiebsuC.teIrndydteooestdh,peuhppiectoorpe7h0yt%oploafntkhteonaninultra-oligotrophic waters (Caron et al, 1985; Callieri, 2008; Wilhelm et al, 2006). Previous studies highlighted that temperate European lakes support mainly phycoerythrin-rich Synechococcus cells, while phycocyanin-rich cells are rare (Katano et al, 2005; Callieri, 2008; Personnic et al, 2009) These Synechococcus-type cells contribute significantly to the total primary production of the lakes (Stockner et al, 2000), our current understanding of their ecology, diversity, distribution and taxonomy in fresh waters is still limited. The recent development and application of molecular tools on DNA archived in lake sediments (Willerslev et al, 2007; Boere et al, 2011; Coolen et al, 2008; Epp et al, 2011) provide the opportunity to both complement the classical biological proxies (Coolen and Gibson, 2009) and to more exhaustively reconstruct past changes in the biodiversity and compositions of phytoplanktonic assemblages with high taxonomic resolution Such long-term reconstructions may thereafter be used to identify the major environmental factors structuring the assemblages. Local and regional environmental forces (here, phosphorus concentration vs. temperature) responsible for shifts in Synechococcus diversity and structure were identified using multivariate analyses
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