Description: Marine bacteria are the main consumers of freshly produced organic matter. Many enzymatic processes involved in the bacterial digestion of organic compounds were shown to be pH sensitive in previous studies. Due to the continuous rise in atmospheric CO2 concentration, seawater pH is presently decreasing at a rate unprecedented during the last 300 million years but the consequences for microbial physiology, organic matter cycling and marine biogeochemistry are still unresolved. We studied the effects of elevated seawater pCO2 on a natural plankton community during a large-scale mesocosm study in a Norwegian fjord. Nine Kiel Off-Shore Mesocosms for Future Ocean Simulations (KOSMOS) were adjusted to different pCO2 levels ranging initially from ca. 280 to 3000 µatm and sampled every second day for 34 days. The first phytoplankton bloom developed around day 5. On day 14, inorganic nutrients were added to the enclosed, nutrient-poor waters to stimulate a second phytoplankton bloom, which occurred around day 20. Our results indicate that marine bacteria benefit directly and indirectly from decreasing seawater pH. During the first phytoplankton bloom, 5-10% more transparent exopolymer particles were formed in the high pCO2 mesocosms. Simultaneously, the efficiency of the protein-degrading enzyme leucine aminopeptidase increased with decreasing pH resulting in up to three times higher values in the highest pCO2/lowest pH mesocosm compared to the controls. In general, total and cell-specific aminopeptidase activities were elevated under low pH conditions. The combination of enhanced enzymatic hydrolysis of organic matter and increased availability of gel particles as substrate supported up to 28% higher bacterial abundance in the high pCO2 treatments. We conclude that ocean acidification has the potential to stimulate the bacterial community and facilitate the microbial recycling of freshly produced organic matter, thus strengthening the role of the microbial loop in the surface ocean.

Description: Rates of gross primary production (GPP), respiration (R), and net calcification (Gnet) in coral reef sediments are expected to change in response to global warming (and the consequent increase in sea surface temperature) and coastal eutrophication (and the subsequent increase in the concentration of organic matter (OM) being filtered by permeable coral reef carbonate sediments). To date, no studies have examined the combined effect of seawater warming and OM enrichment on coral reef carbonate sediment metabolism and dissolution. This study used 22-hour in situ benthic chamber incubations to examine the combined effect of temperature (T) and OM, in the form of coral mucus and phytodetritus, on GPP, R, and Gnet in the permeable coral reef carbonate sediments of Heron Island lagoon, Australia. Compared to control incubations, both warming (+2.4 ºC) and OM increased R and GPP. Under warmed conditions, R was enhanced to a greater extent than GPP, resulting in a shift to net heterotrophy and net dissolution. Under both phytodetritus and coral mucus treatments, GPP was enhanced to a greater extent than R, resulting in a net increase in GPP/R and Gnet. The combined effect of warming and OM enhanced R and GPP, but the net effect on GPP/R and Gnet was not significantly different from control incubations. These findings show that a shift to net heterotrophy and dissolution due to short-term increases in seawater warming may be countered by a net increase GPP/R and Gnet due to short-term increases in nutrient release from OM.

Description: Calculated fCO2 levels at which growth, calcification and photosynthetic rates are inhibited to half of maximum for E. huxleyi, G. oceanica, S. apsteinii, C. leptoporus and C. pelagicus. PIC:POC ratios were then calculated from modelled metabolic rates (MR) of calcification and photosynthesis at CO2 concentrations corresponding to the above fCO2 values for each rate for each species.

Description: This dataset contains measured and calculated seawater properties of two (2 months each) deployments of a SeapHOx (Seabird Electronics) in Cape Byron Marine Park, Australia between 08/2017 and 01/2018.

Description: Global warming (and the consequent increase in sea surface temperature) is expected to modify rates of gross primary production (GPP), respiration (R), and net calcium carbonate (CaCO3) dissolution in permeable coral reef carbonate sediments. Previous simulations of seawater warming on coral reef sediments found a decline in the GPP/R ratio and an associated increase in CaCO3 dissolution but were only conducted over a short timescale (〈 24 h). To date, no studies have examined the prolonged (〉 24 h) effect of seawater warming on coral reef CaCO3 sediment metabolism and dissolution, which may allow the benthic community to acclimatise. This study used 600-L flume aquaria to examine the effect of seawater warming on GPP, R, and CaCO3 dissolution in the permeable coral reef CaCO3 sediments of Mo'orea, French Polynesia over a period of 15 d. On average, when exposed to warmed seawater (+ 2.8 ºC), R in the CaCO3 sediments was enhanced (+ 58 %) to a greater extent than GPP (+19 %), resulting in a decline in GPP/R (- 23 %) and an associated increase in net CaCO3 dissolution (+ 126 %). The magnitude of these warming-mediated metabolic changes increased each day until reaching a plateau after about 8 days, indicating that 24-h experiments may be underestimating the effect of warming over longer timescales. Interestingly, the increase in dissolution relative to control treatments was more striking during the day (+ 163 %) than at night (+ 89 %), suggesting that warming acted to both enhance geochemical dissolution and reduce biogenic calcification or inorganic precipitation. Together, these data indicate that, over the timescale observed here, photosynthesis and associated inorganic and biogenic CaCO3 precipitation do not exhibit the ability to counterbalance the warming-mediated increase in sediment heterotrophy and CaCO3 dissolution.