Effects of increased atmospheric CO<sub>2</sub> on small and intermediate sized osmotrophs during a nutrient induced phytoplankton bloom

Abstract

Abstract. We report the transient population dynamic response of the osmotrophic community initiated by a nutrient pulse in mesocosms exposed to different pCO 2 levels. Differences in phytoplankton and heterotrophic bacteria abundances associated with the CO 2 treatment are also described. Coastal seawater was enclosed in floating mesocosms (27 m 3 ) and nutrients were supplied initially in order to stimulate growth of microbial organisms, including the coccolitophorid Emiliania huxleyi. The mesocosms were modified to achieve 350 µatm (1×CO 2 ), 700 µatm (2×CO 2 ) and 1050 µatm (3×CO 2 ) CO 2 pressure. The temporal dynamics was related to nutrient conditions in the enclosures. Numerically small osmotrophs (picoeukaryotes and Synechoccocus sp.) dominated initially and towards the end of the experiment, whereas intermediate sized osmotrophs bloomed as the initial bloom of small sized osmotrophs ceased. Maximum concentrations of E. huxleyi were approximately 4.6×10 3 cells ml −1 whereas other intermediate sized osmotrophs reached approximately twice as high concentrations. The osmotrophic succession pattern did not change, and neither were we able to detect differences with regard to presence or absence of specific osmotrophic taxa as a consequence of altered pCO 2 . Towards the end of the experiment we did, however, record significantly higher picoeukaryoticand lower Synechococcus-abundances in the higher CO 2 treatments. Slightly increased cell concentrations of E. huxleyi and other nanoeukaryotes were also recorded at elevated pCO 2 on certain days.

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

“…The subsequent decline in abundance was likely due to topdown control from the heterotrophic nanoflagellate community, which displayed an increase in abundance at this time (Hancock et al, 2017). Bacterial tolerance to high CO 2 has been reported previously in this region Westwood et al, 2018) and has also been reported in numerous studies in the Arctic (Grossart et al, 2006;Allgaier et al, 2008;Paulino et al, 2008;Baragi et al, 2015;Wang et al, 2016), suggesting that the marine bacterial community will be resilient to increasing CO 2 .…”

“…Kim et al, 2006;Hopkinson et al, 2010;Riebesell et al, 2013;Paul et al, 2015;Bach et al, 2016;Bunse et al, 2016). Studies in the Arctic reported increases in phytoplankton primary productivity, growth, and organic matter concentration at CO 2 levels ≥ 800 µatm under nutrient-replete conditions (Bellerby et al, 2008;Egge et al, 2009;Engel et al, 2013;Schulz et al, 2013), whilst the bacterial community was unaffected (Grossart et al, 2006;Allgaier et al, 2008;Paulino et al, 2008;Baragi et al, 2015). These studies also highlight the importance of nutrient availability in the community response to elevated CO 2 , with substantial differences in primary and bacterial productivity, chlorophyll a (Chl a), and elemental stoichiometry observed between nutrient-replete and nutrient-limited conditions Schulz et al, 2013;Sperling et al, 2013;Bach et al, 2016).…”

“…Bacterial populations respond to increases in phytoplankton primary productivity by increasing their productivity and abundance, with maximum abundance often occurring after the peak of the phytoplankton bloom (Pearce et al, 2007). 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).…”

Arctic microbial community dynamics influenced by elevated CO<sub>2</sub> levels

“…The higher abundances of picophytoplankton I at high pCO 2 resulted thus from somewhat enhanced growth rates. Paulino et al (2008) reported an increased abundance of picophytoplankton in a high CO 2 mesocosm; however, this was towards the end of the experiment when all nutrients were depleted. During another Norwegian fjord mesocosm experiment, still nutrient-replete, the total number of small-sized picophytoplankton did not change under increased pCO 2 (750 µatm; Hopkins et al, 2010;Newbold et al, 2012).…”

Effect of elevated CO<sub>2</sub> on the dynamics of particle-attached and free-living bacterioplankton communities in an Arctic fjord

“…Effects on other bacterioplankton activities and overall performance have been found to be more variable between experiments. While for one in situ mesocosm experiment at the Bergen large-scale facility higher bacterial protein production (BPP), higher growth rates for particle-associated and free-living bacteria, as well as higher bacterial biomass but not abundance have been reported under high CO 2 (Grossart et al, 2006;Paulino et al, 2008;de Kluijver et al, 2010), in two similar experiments at the same site, no significant changes in bacterial abundance or activity could be observed Newbold et al, 2012). Apart from some minor differences in experimental design, these differences in the response of bacterioplankton activity were possibly due to differences in initial limitation of the bacterial community, as noted by Riebesell et al (2008).…”