TDU MFC
4.6 Conclusions
are formed as a by-product of the release of iodinated anti-oxidants (Küpper et al., 2008). Following hypothesis 9 (Table 4.7) it was found that the measured particle counts exhibited a moderated correlation with shortwave radiation (r= 0.24,p<0) and a weak negative correlation with ozone (r= -0.15,p= 0.04).
The limited particle measurements made during the bromoform experimental period may mask stronger relationships with oxidative stresses at Cape Point. The stronger relationship found between particles and bromoform (r= 0.32, for particle counts up to 2000) suggests a concomitant driving mechanism. Above counts of 2000, the anti-oxidant processes of particles may limit the production of ROS from macroalgae and consequently retard the formation of bromoform.
Atmospheric gaseous mercury at Cape Point is known to become periodically depleted and halogens have been suggested as a possible cause of the depletion (Brunke et al., 2010; Sorensen, 2011). It has been suggested that atomic bromine may be the dominant oxidant of gaseous mercury. In the polar regions the gaseous mercury depletion events are reported to occur concurrently with an ozone depletion event (Sorensen, 2011). The mercury depletion events are, however, dominated by an alternate as yet unidentified chemical pathway. Concurrent ozone and mercury depletion events have not been recorded at Cape Point (Brunke et al., 2010). This unidentified chemical process resulting in mercury depletion may be initiated by local halide chemistry, including bromoform (E.-G. Brunke, SAWS, pers. comm. 2014). It is hypothesised that bromoform (not previously measured) in the atmosphere may initiate the episodic mercury depletion events observed at Cape Point as described in hypothesis 10 (Table 4.7). Bromoform mixing ratios at Cape Point exhibit a medium strength of association to gaseous mercury. A lag of 5 hours between measurements bromoform and gaseous mercury at Cape Point results in the strongest relationship. This suggests that a destruction process of gaseous mercury might be triggered by the presence of bromoform. While one mercury depletion event was recorded during the bromoform sampling period (7 November 2011), the full mechanisms were not explained from this study. Depletion events did not occur on every occasion bromoform was present, therefore bromoform might be an important but not independently sufficient cause of the mercury depletion.
4.6 Conclusions
The measurements of bromoform mixing ratios taken at Cape Point, presented here, represents the first extended record taken in southern Africa. The GAW station operated by the SAWS at Cape Point is an important location in these types of study, receiving air masses from numerous local and regional sources.
The dominant wind direction at the station being from the Southern Ocean provides an ideal site for the sampling of background mixing ratios of trace gases. The bulk of bromoform measurements made at Cape Point appear to be sourced from the local surrounding macroalgae beds and display a maximum of approximately 30 ppt, consistent with recently published coastal locations (Table 4.1). An episodic, simultaneous influence of the diverse sources of bromoform in the atmosphere surrounding Cape Point may result in the few extreme events of bromoform mixing ratios of the order of 60 – 80 ppt that were recorded. Some key variables affecting bromoform mixing ratios are considered below.
The north-westerly (NW) and south-easterly (SE) wind directions that dominated during this period were consistent with the wind directions expected at Cape Point (Tyson and Preston-Whyte, 2000). This combination of air masses allows the analysis of open-ocean, coastal and anthropogenically contaminated air masses from a single location. These two dominant wind directions brought bromoform from distinctly different sources; local macroalgae beds and the Benguela in the NW and Southern Ocean from the SE.
The different sources influenced the variability of the bromoform mixing ratios. The macroalgae beds surrounding Cape Point appear to have been the dominant source of bromoform to the marine atmosphere.
While the mean mixing ratios contributed by the Benguela and the Southern Ocean were comparable, the increased number of NW wind days suggests that the Benguela upwelling system and Koeberg coastal peer plant contributed more bromoform to the atmosphere during this study. The anthropogenic influence of bromoform from the NW, on air masses at Cape Point was less than that of biogenic origin, however, it could still pose a significant impact on long-term studies.
Bromoform mixing ratios at Cape Point responded to changes in wind speed through a number of processes. From the hypothesised dynamics suggested in previous studies, the two most likely dominant processes are: changes in surface state causing varying mixing between surface and bulk layers, and variations in coastal and open ocean sources via diluting effect of the local atmosphere (Liss and Merlivat, 1986; Quack and Suess, 1999). Low wind speeds were dominated by an accumulation of bromoform in the air despite low mixing ratios through a lack of dilution, resulting in elevated mixing ratios. A decrease in bromoform mixing ratios occurred as wind speed increased through dilution of the local atmosphere.
Gas flux rates were not thought to be sufficient to support the previously observed bromoform mixing ratios. Rough surface conditions resulting in strong mixing of the surface layers and the formation of bubbles produced no significant distribution of bromoform measurements with wind speed. It is contended that this resulted from variations in surface mixing and bubble formation.
The variability of the bromoform mixing ratios at Cape Point were investigated through the testing of a number of hypotheses based on previous studies. Hypotheses based on previously found relationships between bromoform and physical variables were explored. These processes included the role of shortwave radiation and ozone in affecting the variability of bromoform at Cape Point. Photosynthesis in algae results in the formation of H2O2, a ROS, which are detrimental to the algae. As discussed in Chapter 5, the bromoperoxidase enzyme may be used in algae to reduce the stress caused by ROS. A number of weak relationships were discovered (hypotheses 2, 5, 6, 9, 10 and 11, Table 4.7) suggesting mechanisms that may each explain a portion of the variability of bromoform mixing ratios observed. The hypotheses were not able to describe the full range of variability. However, the hypotheses may provide clues to further work that could shed light on the mechanisms affecting bromoform mixing ratios at Cape Point.
The calculated diurnal pattern of variability in bromoform mixing ratios was consistent with previously published reports (Ekdahl et al., 1998; Carpenter et al., 2000; Abrahamsson et al., 2004). The processes affecting variability were, therefore, likely to be similar. Bromoform mixing ratios increased during the morning with increasing shortwave radiation and then decreased in the afternoon, probably as a result
4.6. Conclusions 113 of local dilution processes or a regime shift. A second peak in bromoform mixing ratios was observed, driven by a formation through respiration processes or an accumulation through reduced destruction.
Bromoform mixing ratios were well correlated with CO, shortwave radiation and particle counts. The correlation of particles and bromoform at Cape Point may be the result of shortwave radiation inducing a comparable particle anti-oxidant and photosynthetic response in the macroalgae at Cape Point. These processes may be limited by the high levels of shortwave radiation experienced, thus peaking before local solar zenith. The relationships between the other variables was probably driven by the diurnal cycle of shortwave radiation.
The harmonic terms incorporated into the regression model were able to explain approximately 29 % of the variance of the bromoform mixing ratios observed at Cape Point. The model captured the extreme bromoform mixing ratios but failed to capture the variability at lower mixing ratios. In order to capture the extreme events accurately, the model sacrificed the variability at low mixing ratios. Harmonics of varying frequencies and an insufficient number of bromoform measurements is thought to have limited the effectiveness of the model in this case. The work described here demonstrated that this may be a viable method for modelling bromoform mixing ratios, which could be important in areas where measurements are sparse.
MBL and back trajectory analysis provides a possible explanation of the variability observed. Air parcels that transited above the MBL typically contained lower bromoform mixing ratios than those below.
While the MBL at Cape Point did not appear to act as a natural pre-concentrator, its presence as a barrier, capping surface released gases is notable as mechanism driving variability in bromoform mixing ratios at Cape Point. These gases are then prevented from mixing with air masses in the atmosphere above the top of the MBL. This mechanism could explain the reason for variability between air samples passing over the same sources, making the arrival height and direction relative to the MBL height an important consideration in bromoform variability.
The measurements of atmospheric bromoform taken at Cape Point are generally consistent with previous reports. The unique setting of Cape Point has air masses arriving from multiple, varied sources. These sources include local macroalgae beds, the Benguela upwelling system to the north and the Southern Ocean to the west and south. It is possible that interaction of multiple, simultaneous sources resulted in the episodic elevated bromoform mixing ratios above 60 ppt, observed at Cape Point. While the testing of hypotheses shed light on processes occurring at Cape Point, they were not ultimately instrumental in determining the driving mechanisms of observed bromoform variability. The quantitative detection of bromoform at Cape Point could make an important contribution to global knowledge by helping to validate chemistry models. Model output data has shown the Benguela to be an important atmospheric bromoform source (Palmer and Reason, 2009). Sparse sampling in this region has, however, prevented adequate validation to date. A longer time series is needed for the southern African region to tease out the variability of bromoform mixing ratios here. This may go toward describing the possible causes of the variability observed in the bromoform mixing ratios.
Chapter 5
Effects of oxidative stress on the
production of bromoform by two marine diatom species
This study examines the role of nutrient limitation on the production of bromoform in two axenic diatom species. The cultures were grown in laboratory conditions and nutrient limitation was achieved by the addition of sodium hydroxide or starvation. The bromoform per cell concentrations increased during the exponential growth phase, but displayed significant decreases during the limited phases of the experiments.
The work presented here was a collaborative effort with the diatom culturing being performed by Mariam Nguvava, as part of her MSc (Nguvava, 2012).