Response of bacterioplankton activity in an Arctic fjord system to elevated <i>p</i>CO<sub>2</sub>: results from a mesocosm perturbation study

Abstract

Abstract. The effect of elevated seawater carbon dioxide (CO 2 ) on the activity of a natural bacterioplankton community in an Arctic fjord system was investigated by a mesocosm perturbation study in the frame of the European Project on Ocean Acidification (EPOCA). A pCO 2 range of 175-1085 µatm was set up in nine mesocosms deployed in the Kongsfjorden (Svalbard). The activity of natural extracellular enzyme assemblages increased in response to acidification. Rates of β-glucosidase and leucine-aminopeptidase increased along the gradient of mesocosm pCO 2 . A decrease in seawater pH of 0.5 units almost doubled rates of both enzymes.Heterotrophic bacterial activity was closely coupled to phytoplankton productivity in this experiment. The bacterioplankton community responded to rising chlorophyll a concentrations after a lag phase of only a few days with increasing protein production and extracellular enzyme activity. Time-integrated primary production and bacterial protein production were positively correlated, strongly suggesting that higher amounts of phytoplankton-derived organic matter were assimilated by heterotrophic bacteria at increased primary production. Primary production increased under high pCO 2 in this study, and it can be suggested that the efficient heterotrophic carbon utilisation had the potential to counteract the enhanced autotrophic CO 2 fixation. However, our results also show that beneficial pCO 2 -related effects on bacterial activity can be mitigated by the top-down control of bacterial abundances in natural microbial communities.

Ocean acidification of a coastal Antarctic marine microbial community reveals a critical threshold for CO<sub>2</sub> tolerance in phytoplankton productivity

“…This promotion of growth may be linked to an increase in diatom abundance observed in this treatment (Hancock et al, 2017). The coupling of bacterial growth with phytoplankton productivity has been reported by numerous studies on natural marine microbial communities (Allgaier et al, 2008;Grossart et al, 2006;Engel et al, 2013;Piontek et al, 2013;Sperling et al, 2013;Bergen et al, 2016). Thus, it is likely that the bacterial community was controlled more by grazing and nutrient availability than by CO 2 level.…”

“…High CO 2 levels have been observed to have either no effect on abundance and productivity (Grossart et al, 2006;Allgaier et al, 2008;Paulino et al, 2008;Baragi et al, 2015;Wang et al, 2016) or increase growth rate and production only during the postbloom phase of an experiment (Grossart et al, 2006;Sperling et al, 2013;Westwood et al, 2018). Thus, bacterial communities appear to be relatively tolerant to ocean acidification, with bacterial growth indirectly affected by the ocean acidification responses of the phytoplankton community (Grossart et al, 2006;Allgaier et al, 2008;Engel et al, 2013;Piontek et al, 2013;Sperling et al, 2013;Bergen et al, 2016).…”

The influence of ocean acidification on nitrogen regeneration and nitrous oxide production in the northwest European shelf sea

“…However, the response to P CO 2 treatments was ambiguous. While direct observations of NH + 4 regeneration rate responses to simulated OA conditions are lacking to our knowledge, Piontek et al (2013) measured enhanced rates of bacterial extracellular hydrolytic enzyme activity during both acidification bioassay and mesocosm studies, indicating the potential for enhanced nutrient regeneration under OA conditions. By contrast, observations suggest that OA conditions inhibit NH + 4 oxidation (Beman et al, 2011;Huesemann et al, 2002) and implications for N 2 O concentration would be anticipated.…”

“…The subsequent release of fixed nitrogen as NH + 4 and dissolved organic nitrogen (DON) can support microbial growth (Mulholland et al, 2006). Heterotrophic degradation of DON, through the activity of extracellular hydrolytic enzymes for example (Piontek et al, 2013) Herfort et al, 2007) provides oxidised inorganic nitrogen for autotrophic assimilation. Consequently, pelagic nitrogen regenerating processes directly influence the concentration and composition of the dissolved inorganic nitrogen (DIN) pool, which in turn directly influences the rate, extent and composition of phytoplankton community growth Maguer et al, 1999;Rees et al, 2002;Maguer et al, 2011).…”

“…Somewhat surprisingly, there are few studies which have investigated the potential influence of OA upon nitrogen cycle processes, although nitrogen regeneration responses have been inferred during mesocosm studies Piontek et al, 2013). The process of NH + 4 oxidation (i.e.…”