Course:EOSC 475/ResearchProject/pCO2HeterotrophicBacteria
Critical Analysis: Testing the effect of CO2 concentration on the dynamics of marine heterotrophic bacterioplankton.
Authours: Hans-Peter Grossart, Martin Allgaier, Uta Passow, and Ulf Riebesell
By: Louisa Hadley 20096111
Introduction
Since the industrial revolution, humans have been emitting substantial amounts of carbon dioxide into the atmosphere. Of the anthropogenic carbon emissions being released into the atmosphere, roughly one third are being taken up by oceans (Grossart et al. 2006). This is causing decreased pH levels in the oceans, with an estimated drop of 0.35 units by 2100 (Wolf-Gladrow et al. 1999).
Prior Literature
Many studies have been conducted analyzing how the predicted decrease in pH will affect phytoplankton. These studies have shown evidence for changes in the physiology and composition of phytoplankton (Burkhardt et al. 2001, Tortell et al. 2002), as well as a reduction of biogenic calcification from foraminifera and coccolithophores (Bijma et al. 1999; Riebesell et al. 2000). This suggests a higher loss of particulate organic carbon (Bijma et al. 1999; Riebesell et al. 2000; Engel 2002).
Reduced calcification will likely result in less vertical transport of calcium carbonate to the deep sea, as well as an increase in the extracellular organic matter and concentration of transparent exopolymer particles (TEP) (TEP has a high C:N, and C:P ratio relative to the Redfield ratio) (Klaas and Archer 2002). Therefore, studies on phytoplankton suggest that the C:N:P ratio of sinking particles in the ocean may change as a result of increased carbon emissions.
These changes in the quantity and quality of dissolved and particulate organic matter could impact the activities of free and particle-associated bacteria, however, very few studies have been done on the topic. The studies that have been done have mixed results. One study conducted in the lab showed a direct effect of decreased seawater pH from CO2 dissolution on bacterial production in the deep sea (Coffin et al. 2004). It provided evidence that a change in pH will alter the microbial community structure (favouring certain strains) (Coffin et al. 2004). On the other hand, a mesocosm study observed no relationship between total bacteria abundance and pCO2 in the surface water (Rochelle-Newall et al. 2004). This was the first study to suggest that bacterial abundances and pCO2 are independent. It is important to note that while these two experiments are addressing the same topic, they did so using very different methods, and this could account for why different conclusions were drawn. The first study looked at deep sea water, in the lab setting, and its’ measure of the effect of pCO2 on heterotrophic bacteria was bacterial production. The second study analyzed the effects on surface water, was done using mesocosms, and it’s measure was bacterial abundance.
Main Finding from the Paper
In response to the limited studies and mixed results of the effect of increased pCO2 levels on bacterial communities, Grossart et al. (2006) conducted an experiment to explore the effect of changes in pCO2 on bacterial abundance and activities in surface waters. Looking at the effect of pCO2 on the dynamics of surface water marine heterotrophic bacteria, as well as bacterial abundance, is further building on the work of Rochelle-Newall et al. (2004) who only investigated bacterial abundance. This was done in a controlled environmental study, using mesocosms with different pCO2 levels: 190 ppmV as a proxy for the past, 370 ppmV as a proxy for the present, and 700 ppmV as a proxy for the future. It was conducted in the Raunefjorden (60.38N, 5.28E) at the Large Scale Facility in Bergen, Norway, in May 2003.
The first main finding was that the abundance of total bacteria and free bacteria did not change with increasing pCO2, supporting the findings from the study conducted by Rochelle-Newall et al. (2004). Although there was no change in total abundance, there were measurable but indirect effects of the changes in pCO2 on bacterial activities (mainly linked to phytoplankton and particle dynamics). For example, bacterial protein production (BPP) was highest in the future mesocosm (700 ppmV CO2), and the averages of total protease were also highest at the elevated pCO2 levels. Additionally, numbers of attached compared to free bacteria were significantly higher in the future mesocosm (700 ppmV CO2).
Influence on the Field of Research Downstream
A Look at Combined Effects
Grossart et al. (2006) pointed out that to reliably predict bacterioplankton dynamics in the presence of increased oceanic pCO2, there needs to be repeated studies under controlled environmental conditions. In the discussion, the paper also emphasized that rising surface ocean temperatures, increased stratification, and decreasing nutrient supply in the surface layer will accompany future changes in pCO2 levels. It drew attention to the importance of broadening the focus of research on this topic to include combined effects of all projected changes in the environment (not just pCO2).
In response to this critique, some studies have since been undertaken to look at combined effects. One recent study found that interactive effects behave differently than if each factor was looked at separately (Piontek et al. 2015). It concluding that complex, combined effects must be considered when predicting how marine systems will react to the increasing CO2 levels associated with climate change (Piontek et al. 2015). We can expect that more research will be conducted to asses what the results of these combined effects might be.
The Debate Continues
In response to the suggestion of Grossart et al. (2006) that increasing pCO2 levels will affect bacterial activities, Joint et al. (2011) presented an argument against it. They noted that there is a lot of natural variability in pH with season, depth, and along productivity gradients that was ignored in the assessment. Many microbes accommodate large, rapid changes in pH, and are accustomed to doing so. Joint et al. (2011) suggests that major biogeochemical processes won’t actually be that different in future. Oliver et al. (2014) conducted a mesocom study that supported this proposition. The study showed no significant difference in community abundance, structure, or composition with elevated pCO2, suggesting that marine bacterial communities are highly resistant to elevated pCO2 and lower pH conditions (Oliver et al. 2014).
In response to this conclusion however, it has been noted that while there is a lot of natural variability in pH, this isn’t representative of long-term exposure. To better assess the effects of long-term exposure, a study was conducted along a natural CO2 gradient in sediments. The gradient was caused by venting systems, and gave a representation of the microbial communities present at high pCO2 levels (resembling those of the future). The results from this study suggested that the microbial community structure in sediments will likely be affected by long term exposure to pCO2 levels predicted in the future (Raulf et al. 2015). It is important to note that this study was done along sediments; the effects of increased pCO2 may be different in surface waters.
Critical Reflection
Strengths
One of the main strengths of this paper is that it looked at the dynamics of heterotrophic bacteria. While changes in abundance of bacteria in response to increasing pCO2 had been looked at before in a mesocosm study, this was the first mesocosm study to directly relate changes in pCO2 to changes in the dynamics of heterotrophic bacteria in the surface waters. Looking at this was crucial in understanding that bacterial communities and pCO2 levels likely are not independent, as would be concluded if only bacterial abundance were assessed. While total bacterial abundance may have been found to be constant at different pCO2 levels, changes were observed in the BPP, which can be used as a proxy for bacterial growth rate.
Abundance is function of growth rate minus death rate (either natural or through predation). Therefore, the fact that abundance remains constant across different pCO2 levels while BPP differs, indicates that changes in pCO2 also effect the death rate. This can be through increased predation (possibly from increased abundance of predators); or increased death rates due to an alteration in the life cycle of heterotrophic bacteria, or maybe from alterations in virus abundance/infection. This suggests that the changes in pCO2 could possibly be affecting another part of the ecosystem directly, for example the abundance of organisms that consume bacteria, and therefore having an indirect effect on bacteria. These findings act as a reminder that heterotrophic bacteria are part of a food web, and if changing pCO2 effects any part of this web, all parts will likely be affected. This leads to another one of the strengths of the study.
Mesocosm studies are environmental experiments, and are much better at taking into account ecosystem dynamics than lab experiments. Therefore, they give a better representation of how the ecosystem will react as a whole. The benefit of it being an experiment as opposed to a correlation study, is that the researchers were able to control the forcing factor, CO2 concentration, to mimic the conditions that are predicted in the future.
Additionally, one more strength is that bacterial production and growth was determined with a dual label approach using both 14[C]-leucine, and 3[H]-thymidine. Using both methods added credibility to the results.
Weaknesses
The study looked at bacterial abundance and activity over 20 days during an algal bloom. I don’t think these results can then be generalized to make claims of how bacteria will be affected year round. For example, in temperate regions in winter and summer months when there are no algal blooms, bacterial activity may be affected differently by increased pCO2 compared to during an algal bloom. Also, exposure to increased pCO2 on the timescale of days (20 in this case) may not produce the same effects that long-term exposure to increased pCO2 would produce.
The major weakness of this paper is the small sample size. While nine mesocosms where installed, with the intention of having three replicates at each of the three different pCO2 levels, due to limitations in time and resources, only one mesocosm of each pCO2 level was sampled for dissolved amino acids and bacterial parameters. One replicate for each treatment level is definitely not sufficient for drawing any reliable conclusions. For example, the paper notes that differences in DCAA concentrations between treatments were significantly lower in the future mesocosm, compared to the past mesocosm. It then goes on to explain that this statistical difference was mainly due to one data point (the peak concentration of DCAA during the first maxima was very high in the past mesocosm). One data point should not be capable of creating a significant difference. This is indicative that there is definitely not enough data/replicates to produce reliable results.
Additionally, statistical analyses for the exponential phase (days 0 to 12) and the decline phase (days 14 to 20) were conducted separately. The paper notes that they only took four time points during the decline phase, and therefore they state that the significance levels for all measurements in this phase are rather uncertain. Again this comes back to the point that not enough data was collected.
Summary of Impact
Despite the weaknesses listed above, this paper has had an impact on the understanding of how heterotrophic bacteria may react to the anticipated pCO2 conditions of the future. It drew attention to areas that need further research, and identified short comings in the research that was already completed on the topic. While I don’t necessarily think the results are reliable, I think they were an important driving force for further research. For example, this paper emphasized the role of combined environmental effects associated with global warming on heterotrophic bacteria, and new research is still being produced addressing this concern. Additionally, since there is not yet a consensus as to what the effect will be of increased pCO2 on heterotrophic bacteria, or if there even will be an effect, new research is still coming out on this topic.
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