Description: Ocean acidity has increased by 30% since preindustrial times due to the uptake of anthropogenic CO2 and is projected to rise by another 120% before 2100 if CO2 emissions continue at current rates. Ocean acidification is expected to have wide-ranging impacts on marine life, including reduced growth and net erosion of coral reefs. Our present understanding of the impacts of ocean acidification on marine life, however, relies heavily on results from short-term CO2 perturbation studies. Here, we present results from the first long-term CO2 perturbation study on the dominant reef-building cold-water coral Lophelia pertusa and relate them to results from a short-term study to compare the effect of exposure time on the coral's responses. Short-term (1 week) high CO2 exposure resulted in a decline of calcification by 26–29% for a pH decrease of 0.1 units and net dissolution of calcium carbonate. In contrast, L. pertusa was capable to acclimate to acidified conditions in long-term (6 months) incubations, leading to even slightly enhanced rates of calcification. Net growth is sustained even in waters sub-saturated with respect to aragonite. Acclimation to seawater acidification did not cause a measurable increase in metabolic rates. This is the first evidence of successful acclimation in a coral species to ocean acidification, emphasizing the general need for long-term incubations in ocean acidification research. To conclude on the sensitivity of cold-water coral reefs to future ocean acidification further ecophysiological studies are necessary which should also encompass the role of food availability and rising temperatures.

Description: Biomineralization in the marine phytoplankton Emiliania huxleyi is a stringently controlled intracellular process. The molecular basis of coccolith production is still relatively unknown although its importance in global biogeochemical cycles and varying sensitivity to increased pCO2 levels has been well documented. This study looks into the role of several candidate Ca2+, H+ and inorganic carbon transport genes in E. huxleyi, using quantitative reverse transcriptase PCR. Differential gene expression analysis was investigated in two isogenic pairs of calcifying and non-calcifying strains of E. huxleyi and cultures grown at various Ca2+ concentrations to alter calcite production. We show that calcification correlated to the consistent upregulation of a putative HCO3- transporter belonging to the solute carrier 4 (SLC4) family, a Ca2+/H+ exchanger belonging to the CAX family of exchangers and a vacuolar H+-ATPase. We also show that the coccolith-associated protein, GPA is downregulated in calcifying cells. The data provide strong evidence that these genes play key roles in E. huxleyi biomineralization. Based on the gene expression data and the current literature a working model for biomineralization-related ion transport in coccolithophores is presented.

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: Carbon acquisition was investigated in three marine bloom-forming dinoflagellates – Prorocentrum minimum, Heterocapsa triquetra and Ceratium lineatum. In vivo activities of extracellular and intracellular carbonic anhydrase (CA), photosynthetic O2 evolution, CO2 and HCO3– uptake rates were measured by membrane inlet mass spectrometry (MIMS) in cells acclimated to low pH (8.0) and high pH (8.5 or 9.1). A second approach used short-term 14C-disequilibrium incubations to estimate the carbon source utilized by the cells. All three species showed negligible extracellular CA (eCA) activity in cells acclimated to low pH and only slightly higher activity when acclimated to high pH. Intracellular CA (iCA) activity was present in all three species, but it increased only in P. minimum with increasing pH. Half-saturation concentrations (K1/2) for photosynthetic O2 evolution were low compared to ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) kinetics. Moreover, apparent affinities for inorganic carbon (Ci) increased with increasing pH in the acclimation, indicating the operation of an efficient CO2 concentration mechanism (CCM) in these dinoflagellates. Rates of CO2 uptake were comparably low and could not support the observed rates of photosynthesis. Consequently, rates of HCO3– uptake were high in the investigated species, contributing more than 80% of the photosynthetic carbon fixation. The affinity for HCO3– and maximum uptake rates increased under higher pH. The strong preference for HCO3– was also confirmed by the 14C-disequilibrium technique. Modes of carbon acquisition were consistent with the 13C-fractionation pattern observed and indicated a strong species-specific difference in leakage. These results suggest that photosynthesis in marine dinoflagellates is not limited by Ci even at high pH, which may occur during red tides in coastal waters.

Description: Climate warming will affect the phytoplankton spring bloom. Experiments conducted at IFM-GEOMAR indicate, that bloom biomass of phytoplankton will become less, average cell size will become smaller, the biological carbon pump will be weakened and the timing of the bloom will be advanced a bit in a warming climate.

Description: The cold waters of the polar seas are naturally low in carbonate saturation. As uptake of anthropogenic CO2 continues to acidify the oceans worldwide, the Arctic Ocean will be the first to pass the chemical threshold where surface seawater becomes undersaturated, i.e. corrosive for unprotected shells and skeletons of calcifying organisms. While the overall impacts of ocean acidification on the Arctic ecosystems are still unknown, the predicted changes in seawater chemistry are expected to make it increasingly difficult for calcareous organisms to inhabit these regions.

Description: Human-induced climate change is causing a warming of the surface ocean. Due to widely differing temperature sensitivities of key biological processes, this may have profound implications for marine food web interactions and the biogeochemical cycling of key elements such as carbon. Using a novel indoor-mesocosm approach, we show that rising sea surface temperature shifts the balance between photosynthetic production and respiratory consumption of organic carbon in a plankton community. This may weaken the ocean’s capacity to sequester atmospheric CO2, hence providing a positive feedback to anthropogenic climate change.

Description: Gravitational sinking of photosynthetically fixed particulate organic carbon (POC) constitutes a key component of the biological carbon pump. The fraction of POC leaving the surface ocean depends on POC sinking velocity (SV) and remineralization rate (Cremin), both of which depend on plankton community structure. However, the key drivers in plankton communities controlling SV and Cremin are poorly constrained. In fall 2014, we conducted a 6 weeks mesocosm experiment in the subtropical NE Atlantic Ocean to study the influence of plankton community structure on SV and Cremin. Oligotrophic conditions prevailed for the first 3 weeks, until nutrient‐rich deep water injected into all mesocosms stimulated diatom blooms. SV declined steadily over the course of the experiment due to decreasing CaCO3 ballast and – according to an optical proxy proposed herein – due to increasing aggregate porosity mostly during an aggregation event after the diatom bloom. Furthermore, SV was positively correlated with the contribution of picophytoplankton to the total phytoplankton biomass. Cremin was highest during a Synechococcus bloom under oligotrophic conditions and in some mesocosms during the diatom bloom after the deep‐water addition while it was particularly low during harmful algal blooms. The temporal changes were considerably larger in Cremin (max. 15‐fold) than in SV (max. 3‐fold). Accordingly, estimated POC transfer efficiency to 1000 m was mainly dependent on how the plankton community structure affected Cremin. Our approach revealed key players and interactions in the plankton food web influencing POC export efficiency thereby improving our mechanistic understanding of the biological carbon pump. Key points Sinking velocity was higher during oligotrophy than during blooms which is linked to ballast, porosity, and phytoplankton size structure Remineralization was highly variable but tended to be higher during Synechococcus or diatom blooms and lower during harmful algal blooms Plankton community structure had a considerably larger influence on particle remineralization rate than on sinking velocity

Description: Marine nitrogen (N2) fixation supports significant primary productivity in the global ocean. However, in one of the most productive regions of the world ocean, the northern Humboldt Upwelling System (HUS), the magnitude and spatial distribution of this process remains poorly characterized. This study presents a spatially resolved dataset of N2 fixation rates across six coastal transects of the northern HUS off Peru (8°S – 16°S) during austral summer. N2 fixation rates were detected throughout the waters column including within the OMZ between 12°S and 16°S. N2 fixation rates were highest where the subsurface Oxygen Minimum Zone (OMZ, O2 〈20 µmol L-1) was most intense and estimated nitrogen (N) loss was highest. There, rates were measured throughout the water column. Hence the vertical and spatial distribution of rates indicates colocation of N2 fixation with N loss in the coastal productive waters of the northern HUS. Despite high phosphate and total dissolvable iron (TdFe) concentrations throughout the study area, N2 fixation was still generally low (1.19 ± 3.81 nmol L-1 d-1) and its distribution could not be directly explained by these two factors. Our results suggest that the distribution was likely influenced by a complex interplay of environmental factors including phytoplankton biomass and organic matter availability, and potentially iron, or other trace metal (co)-limitation of both N2 fixation and primary production. In general, our results support previous conclusions that N2 fixation in the northern HUS plays a minor role as a source of new N and to replenish the regional N loss. Key Points: A north-to-south pattern in N2 fixation rates was observed implying increased N turnover between 12°S and 16°S where N loss was pronounced Highest N2 fixation rates were measured in coastal productive waters above and within the OMZ, showing no clear relationship with Fe or P The magnitude of N2 fixation was low compared to predictions, estimated to account for ∼0.3% of primary production and 〈2% of local N loss