In this mesocosm study with water from the northern Baltic Sea, we showed effects of increased tDOM and temperature in shaping bacterial community composition. These findings add important understanding of the bacterioplankton population dynamics in the Baltic Sea mesocosm experiment of Lefébure et al. (
2013), who established that tDOM and temperature significantly affected bulk microbial activities. This is in agreement with studies in as diverse environments as the western Arctic, equatorial Pacific Ocean, and the Baltic Sea, all showing substantial combined effects of increased DOM and temperatures on bacterioplankton bulk activities (Kirchman and Rich
1997; Degerman et al.
2013) and overall general community structure (Degerman et al.
2013). Our findings further highlight that increased tDOM and temperatures promoted or suppressed a spectrum of individual populations (Fig.
2; Tables
1,
2). This study thus provides a comprehensive analysis of which bacterial populations may respond or not to future anthropogenic-induced shifts in environmental conditions.
Differential response
Among the top 200 OTUs, around one-third showed relatively similar abundances in the tDOMH + T and control mesocosms, suggesting that they were not affected by the induced changes in growth conditions—at least not within the time frame of the experiment. Another one-third of the OTUs increased in the tDOMH + T mesocosms. Moreover, one-third of the OTUs were more abundant in control mesocosms, suggesting that they were negatively affected by increased temperature and tDOM. Overall, the analysis thus showed that 62 % of the top 200 OTUs in the current experiment were affected either positively or negatively by changes in environmental conditions. This suggests that persistent changes over periods from several months to years in temperatures and tDOM loading have the potential to cause profound changes in bacterioplankton community composition.
Several major bacterial groups that are abundant in the Baltic Sea were also abundant in our experiment. We find for example that Alphaproteobacteria were generally responsive (Fig.
2). Also Betaproteobacteria OTUs were mostly responsive, whereas Actinobacteria and phytoplankton were generally sensitive (Fig.
2). Still, within all major groups, there were both responsive and sensitive OTUs and even resistant ones. A possible reason for detecting equal increase or resistance among bacterial populations between control and tDOM
H + T mesocosms could potentially be the “bottle-effect”. Such effects are frequently seen in incubation experiments, especially among Gammaproteobacteria (e.g., Dinasquet et al.
2013). Nevertheless, despite possible “bottle-effects”, there were not only striking differences in terms of responsiveness, sensitivity, and resistance at low taxonomic resolution between major phylogenetic groups, but also differences within each phyla/class. Thus, we conclude that a majority of the responses observed were distinctive of the tDOM
H + T treatment as compared to the controls.
The following is an account of distinct distribution patterns for important individual populations. Betaproteobacterial OTUs like
Burkholderia and
Comamonadaceae were positively influenced by increased tDOM and temperature (Fig.
2; Tables
1,
2). In accordance, a study investigating the effects of continental runoff from the Iberian Peninsula on bacterioplankton showed a strong positive correlation between humic DOM and Betaproteobacteria (Teira et al.
2009). Although Betaproteobacteria are frequently found in small numbers in the Baltic Sea in general, specific members can reach up to several percent of the total community in the Baltic Sea (Herlemann et al.
2011, Lindh et al.
2015). In addition, the northern Baltic Sea contains on average more Betaproteobacteria than elsewhere in the Baltic Sea (Herlemann et al.
2011), possibly related to the lower salinity and/or higher levels of tDOM in this region. This suggests that the
Burkholderia and BAL58 OTUs, in particular, and Betaproteobacteria, in general, may have an increased biogeochemical role in the cycling of carbon in the Baltic Sea and estuarine environments under future predicted climate change scenarios.
Alphaproteobacteria were generally stimulated by increased tDOM and temperatures, i.e., responsive, albeit some were found in both tDOM
H + T and control mesocosms, i.e., resistant. One particularly abundant Alphaproteobacteria was the resistant
Roseobacter clade OTU UMU_000004 (Table
1). Members of the
Roseobacter clade are often dominant (up to 25 % of total abundance) in marine surface waters around the globe (Newton et al.
2010) and relatives of this particular OTU have previously been found in the Baltic Sea (Sjöstedt et al.
2012). Many
Roseobacter members contain metabolic features that allow them to be successful in various marine environments and are therefore of major importance for the cycling of carbon (Wagner-Dobler and Biebl
2006; Newton et al.
2010). Regarding members of the SAR11 clade bacteria, which are characterized as oligotrophs (Morris et al.
2002; Tripp
2013), it was surprising that two SAR11 OTUs were responsive to increased tDOM and temperatures, while a third was found primarily in the controls (Fig.
2; Table
1). In a previous Baltic Sea climate change experiment, a close relative of these SAR11 OTUs was predominant in higher temperatures (6 °C) but absent at lower temperatures (3 °C) (Lindh et al.
2013). Thus, although SAR11 clade bacteria are generally oligotrophs, it appears that different OTUs in this clade have a noticeable capacity to respond to changes in temperature and tDOM availability, considering that their abundance in seawater can have major implications for defining bacterioplankton community structure.
Actinobacteria are generally found in high abundance across the Baltic Sea, particularly in the northern basins (Bothnian Bay, Bothnian Sea) (Herlemann et al.
2011; Dupont et al.
2014, Lindh et al.
2015). Two important examples of sensitive actinobacterial OTUs, i.e., more abundant in control than in tDOM
H + T mesocosms, were members of the hgcI clade and the CL500-29 clade. The hgcI clade has previously not been described extensively among OTUs of the Baltic Sea, but relatives have been detected in high abundance in experiments with Baltic seawater (Sjöstedt et al.
2012). In lakes, members of the hgcI clade are often dominant components of the bacterioplankton, where they have a competitive advantage in waters with low DOC concentrations at low temperature (Glöckner et al.
2000). Still, bacteria in the hgcI clade remain poorly characterized and their functional traits in marine/brackish environments are unknown. The CL500-29 clade OTU found in high abundance in our control mesocosms has previously been found to be a generalist in terms of utilization of different carbon compounds in Baltic Sea microcosm experiments (Gomez-Consarnau et al.
2012). Thus, our results suggest a major decrease in the abundance of presently abundant actinobacterial populations in the northern Baltic Sea under predicted climate change scenarios.
Within Verrucomicrobia, we found 4 distinct OTUs related to the abundant but relatively unknown
Candidatus Spartobacterium baltica that showed different responses in our mesocosms. This taxon is spatially widespread and abundant in the Baltic Sea (Herlemann et al.
2011), particularly during summer at times of cyanobacterial blooms and high temperatures (Andersson et al.
2009; Herlemann et al.
2011). This is likely due to the ability to utilize phytoplankton-derived high-molecular weight polysaccharides (Herlemann et al.
2013). In contrast, in our tDOM
H + T mesocosms, one of the Verrucomicrobial OTUs was outcompeted by other populations, which may suggest that it is less adapted to degrade and utilize terrigenous carbon-like humic substances. Still, it is important to note that other close relatives were either responsive or resistant to the control and tDOM
H + T conditions investigated here.
A similar distribution of differential responses was seen in the SAR86 clade, where one SAR86 OTU was responsive in tDOM
H + T mesocosms, and another was sensitive. Different members of this clade seem to have the capacity to degrade and utilize specific carbon compounds (Dupont et al.
2012), suggesting a possible differentiation into ecotypes. Thus, for several taxa, ecotype-level differentiation among closely related populations is important to consider when interpreting responses to changes in environmental conditions.
Bacteroidetes are often abundant in the Baltic Sea (Andersson et al.
2009; Herlemann et al.
2011, Lindh et al.
2015), and they are generally recognized for having an arsenal of enzymes to degrade phytoplankton-derived polysaccharides and peptides (Kirchman
2002; Fernandez-Gomez et al.
2013). Within the Bacteroidetes, there was substantial variation in response to the mesocosm conditions. For example,
Owenweeksia OTUs were responsive, while members of the genus
Fluviicola and the NS3a clade were sensitive to increased tDOM and temperature. Bacteroidetes often respond strongly to changes in growth conditions, either positively or negatively depending on which specific taxon/genus they belong to (Pinhassi et al.
2004; Andersson et al.
2009; Diez-Vives et al.
2014; von Scheibner et al.
2014). Substantial differences within Bacteroidetes in the number of glycoside hydrolases and peptidases are proposed to indicate a differentiation among taxa for distinct DOM utilization patterns (Fernandez-Gomez et al.
2013). This could account for parts of the variability among Bacteroidetes populations in degrading humic substances found in tDOM in our study.
In addition to major differences in the increase/decrease of OTUs, it was also curious to note that a few OTUs increased in relative abundance from being undetected at the onset of the experiment (Table
2). In particular, opportunistic populations, such as
Comamonadaceae (Betaproteobacteria) and
Desulfuromonadales (Deltaprotebacteria) OTUs, increased substantially in tDOM
H + T mesocosms. Rare, or initially undetected OTUs that becomes abundant also occurs in situ in the marine environment and has been observed in experimental incubations following environmental perturbations, emphasizing the role of the rare biosphere in responding to change in environmental conditions (Campbell et al.
2011; Lennon and Jones
2011; Sjöstedt et al.
2012; Alonso-Saez et al.
2014). For example, change in salinity promoted previously rare or undetected OTUs in chemostat transplants between Skagerrak seawater and Baltic Sea water (Sjöstedt et al.
2012).
It is also important to note that the observed changes among bacterial populations in our experiment are the result of adaptation in a closed system. The distribution of bacterial populations in the natural marine environment is limited by few physical barriers and, in the perspective of climate change, dispersal is likely an important factor for bacterioplankton responses to environmental change. Nevertheless, our observations highlight substantial effects of climate change-induced shifts in the local environmental conditions for regulating bacterioplankton community composition