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: Ocean acidification (OA) and organic matter enrichment (due to coastal eutrophication) could act in concert to shift coral reef carbonate sediments from a present state of net calcification to a future state of net dissolution, but no studies have examined the combined effect of these stressors on sediment metabolism and dissolution. This study used 22-hour incubations in flume aquaria with captive sediment communities to measure the combined effect of OA and organic matter (OM) enrichment, on coral reef sediment gross primary productivity (GPP), respiration (R), and net calcification (Gnet). Relative to control sediment communities, both OA ( 1000 µatm) and OM enrichment (+ 40 µmol C/L) significantly decreased rates of sediment Gnet by 98% and 15% mmol CaCO3/m**2/h, respectively , but the mechanism behind this decrease differed. The OA-mediated transition to net dissolution was geochemical, as rates of GPP and R remained unaffected and dissolution was solely enhanced by a decline in the aragonite saturation state (Omega arg) of the overlying water column. In contrast, the OM-mediated decline in Gnet was due to a decline in GPP/R, thereby biologically reducing overlying seawater Ωarg due to the increased respiratory addition of CO2. The decrease in Gnet in response to a combination of both stressors was additive (- 10% relative to OA alone) but this decrease did not significantly differ from the effect of OA alone. In this study OA was the primary driver of future carbonate sediment dissolution, but longer-term experiments with chronic organic matter enrichment are required.

Description: The interaction between current flow and topography (e.g., surface ripples) in shallow, permeable coral reef carbonate sediments establishes pressure gradients that increase the rate of sediment-water solute exchange relative to fluid shear along a flat bottom. It is currently unknown how this effect from surface ripples will modify the rate at which the sediment porewater is exposed to future chemical changes in the overlying water column, such as elevated pCO2 that is causing ocean acidification (OA). To address this question, this study used a series of 22-hour incubations in flume aquaria with permeable calcium carbonate sediment communities and examined the interactive effect of pCO2 (400 and 1000 µatm) and surface topography (flat and rippled sediments) on carbonate sediment metabolism and dissolution. According to dissolved oxygen optode image analysis, the presence of surface ripples increased the oxygenated area below the sediment surface by 295% relative to flat sediments. This was reflected in the sediment-to-water column fluxes of dissolved oxygen, where rippled sediments exhibited rates of respiration (R) and gross primary production (GPP) that were ~ 45% and ~ 50% higher, respectively, than flat sediments. An increase in pCO2 shifted the sediments in the flat flumes from net calcifying (Gnet 〉 0) to net dissolving (Gnet 〈 0), an effect that was amplified an additional ~ 60% in rippled sediments. These results suggest that current estimates of coral reef carbonate sediment Gnet may be underestimating the dissolution response to OA where the carbonate sediment environment exhibits ripples in the topography.

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.