Description: A large-scale multidisciplinary mesocosm experiment in an Arctic fjord (Kongsfjorden, Svalbard; 78°56.2′N) was used to study Arctic marine food webs and biogeochemical elements cycling at natural and elevated future carbon dioxide (CO2) levels. At the start of the experiment, marine-derived chromophoric dissolved organic matter (CDOM) dominated the CDOM pool. Thus, this experiment constituted a convenient case to study production of autochthonous CDOM, which is typically masked by high levels of CDOM of terrestrial origin in the Arctic Ocean proper. CDOM accumulated during the experiment in line with an increase in bacterial abundance; however, no response was observed to increased pCO2 levels. Changes in CDOM absorption spectral slopes indicate that bacteria were most likely responsible for the observed CDOM dynamics. Distinct absorption peaks (at ~ 330 and ~ 360 nm) were likely associated with mycosporine-like amino acids (MAAs). Due to the experimental setup, MAAs were produced in absence of ultraviolet exposure providing evidence for MAAs to be considered as multipurpose metabolites rather than simple photoprotective compounds. We showed that a small increase in CDOM during the experiment made it a major contributor to total absorption in a range of photosynthetically active radiation (PAR, 400–700 nm) and, therefore, is important for spectral light availability and may be important for photosynthesis and phytoplankton groups composition in a rapidly changing Arctic marine ecosystem.

Description: The sea-surface microlayer (SML) is the ocean's uppermost boundary to the atmosphere and in control of climate relevant processes like gas exchange and emission of marine primary organic aerosols (POA). The SML represents a complex surface film including organic components like polysaccharides, proteins, and marine gel particles, and harbors diverse microbial communities. Despite the potential relevance of the SML in ocean-atmosphere interactions, still little is known about its structural characteristics and sensitivity to a changing environment such as increased oceanic uptake of anthropogenic CO2. Here we report results of a large-scale mesocosm study, indicating that ocean acidification can affect the abundance and activity of microorganisms during phytoplankton blooms, resulting in changes in composition and dynamics of organic matter in the SML. Our results reveal a potential coupling between anthropogenic CO2 emissions and the biogenic properties of the SML, pointing to a hitherto disregarded feedback process between ocean and atmosphere under climate change.

Description: Coccolithophores are important calcifying phytoplankton predicted to be impacted by changes in ocean carbonate chemistry caused by the absorption of anthropogenic CO2. However, it is difficult to disentangle the effects of the simultaneously changing carbonate system parameters (CO2, bicarbonate, carbonate and protons) on the physiological responses to elevated CO2. Here, we adopted a multifactorial approach at constant pH or CO2 whilst varying dissolved inorganic carbon (DIC) to determine physiological and transcriptional responses to individual carbonate system parameters. We show that Emiliania huxleyi is sensitive to low CO2 (growth and photosynthesis) and low bicarbonate (calcification) as well as low pH beyond a limited tolerance range, but is much less sensitive to elevated CO2 and bicarbonate. Multiple up-regulated genes at low DIC bear the hallmarks of a carbon-concentrating mechanism (CCM) that is responsive to CO2 and bicarbonate but not to pH. Emiliania huxleyi appears to have evolved mechanisms to respond to limiting rather than elevated CO2. Calcification does not function as a CCM, but is inhibited at low DIC to allow the redistribution of DIC from calcification to photosynthesis. The presented data provides a significant step in understanding how E. huxleyi will respond to changing carbonate chemistry at a cellular level.

Description: About 50 Gt of carbon is fixed photosynthetically by surface ocean phytoplankton communities every year. Part of this organic matter is reprocessed within the plankton community to form aggregates which eventually sink and export carbon into the deep ocean. The fraction of organic matter leaving the surface ocean is partly dependent on aggregate sinking velocity which accelerates with increasing aggregate size and density, where the latter is controlled by ballast load and aggregate porosity. In May 2011, we moored nine 25 m deep mesocosms in a Norwegian fjord to assess on a daily basis how plankton community structure affects material properties and sinking velocities of aggregates (Ø 80–400 µm) collected in the mesocosms' sediment traps. We noted that sinking velocity was not necessarily accelerated by opal ballast during diatom blooms, which could be due to relatively high porosity of these rather fresh aggregates. Furthermore, estimated aggregate porosity (Pestimated) decreased as the picoautotroph (0.2–2 µm) fraction of the phytoplankton biomass increased. Thus, picoautotroph-dominated communities may be indicative for food webs promoting a high degree of aggregate repackaging with potential for accelerated sinking. Blooms of the coccolithophore Emiliania huxleyi revealed that cell concentrations of ~1500 cells/mL accelerate sinking by about 35–40%, which we estimate (by one-dimensional modeling) to elevate organic matter transfer efficiency through the mesopelagic from 14 to 24%. Our results indicate that sinking velocities are influenced by the complex interplay between the availability of ballast minerals and aggregate packaging; both of which are controlled by plankton community structure.

Description: To understand the influence of changing surface ocean pH and carbonate chemistry on the coccolithophore Emiliania huxleyi, it is necessary to characterize mechanisms involved in pH homeostasis and ion transport. Here, we measured effects of changes in seawater carbonate chemistry on the fluorescence emission ratio of BCECF (2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein) as a measure of intracellular pH (pH(i)). Out of equilibrium solutions were used to differentiate between membrane permeation pathways for H+, CO(2) and HCO(3)-. Changes in fluorescence ratio were calibrated in single cells, resulting in a ratio change of 0.78 per pH(i) unit. pH(i) acutely followed the pH of seawater (pH(e)) in a linear fashion between pH(e) values of 6.5 and 9 with a slope of 0.44 per pH(e) unit. pH(i) was nearly insensitive to changes in seawater CO(2) at constant pH(e) and HCO(3)-. An increase in extracellular HCO(3)- resulted in a slight intracellular acidification. In the presence of DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid), a broad-spectrum inhibitor of anion exchangers, E. huxleyi acidified irreversibly. DIDS slightly reduced the effect of pH(e) on pH(i). The data for the first time show the occurrence of a proton permeation pathway in E. huxleyi plasma membrane. pH(i) homeostasis involves a DIDS-sensitive mechanism.

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.