Description: Ocean acidification reduces the concentration of carbonate ions and increases those of bicarbonate ions in seawater compared with the present oceanic conditions. This altered composition of inorganic carbon species may, by interacting with ultraviolet radiation (UVR), affect the physiology of macroalgal species. However, very little is known about how calcareous algae respond to UVR and ocean acidification. Therefore, we conducted an experiment to determine the effects of UVR and ocean acidification on the calcified rhodophyte Corallina officinalis using CO2-enriched cultures with and without UVR exposure. Low pH increased the relative electron transport rates (rETR) but decreased the CaCO3 content and had a miniscule effect on growth. However, UVA (4.25 W m-2) and a moderate level of UVB (0.5 W m-2) increased the rETR and growth rates in C. officinalis, and there was a significant interactive effect of pH and UVR on UVR-absorbing compound concentrations. Thus, at low irradiance, pH and UVR interact in a way that affects the multiple physiological responses of C. officinalis differently. In particular, changes in the skeletal content induced by low pH may affect how C. officinalis absorbs and uses light. Therefore, the light quality used in ocean acidification experiments will affect the predictions of how calcified macroalgae will respond to elevated CO2.

Description: Seagrass meadows are a crucial component of tropical marine reef ecosystems. The seagrass plants are colonized by a multitude of epiphytic organisms that contribute to determining the ecological role of seagrasses. To better understand how environmental changes like ocean acidification might affect epiphytic assemblages, the microbial community composition of the epiphytic biofilm of Enhalus acroides was investigated at a natural CO2 vent in Papua New Guinea using molecular fingerprinting and next generation sequencing of 16S and 18S rRNA genes. Both bacterial and eukaryotic epiphytes formed distinct communities at the CO2-impacted site compared to the control site. This site-related CO2 effect was also visible in the succession pattern of microbial epiphytes. We further found an increased abundance of bacterial types associated with coral diseases at the CO2-impacted site (Fusobacteria, Thalassomonas) whereas eukaryotes such as certain crustose coralline algae commonly related to healthy reefs were less diverse. These trends in the epiphytic community of E. acroides suggest a potential role of seagrasses as vectors of coral pathogens and may support previous predictions of a decrease in reef health and prevalence of diseases under future ocean acidification scenarios.

Description: Kelps (Phaeophyceae, Laminariales) function as ecosystem engineers along many Arctic rocky shores. With ongoing climate change, the frequency and intensity of marine heatwaves are increasing. Further, extensive meltwater plumes darken Arctic fjords. It was the aim of this study to analyse the future development of Arctic kelp forest ecosystems. We conducted a laboratory experiment, in which we determined physiological and biochemical responses of Agarum clathratum and Saccharina latissima to a marine heatwave scenario (4, 7, and 10°C) being acclimated to either low-light (3 µmol photons m²/s) or in-situ-light (120 µmol photons m²/s) conditions. Grown sporophytes were sampled in Nuup Kangerlua, Greenland from a sampling depth between 7-10 m. Meristematic discs were cut (diameter 2 cm) and distributed equally between treatments and replicates. The discs acclimated to the light conditions for two days, before the 12 days heatwave scenario started, followed by a five-day recovery period. Maximum quantum yield of photosystem II (Fv/Fm) measurements were measured every second day, using a pulse amplitude modulated fluorometer (Portable Chlorophyll Fluorometer PAM-2100, Heinz Walz GmbH, Effeltrich, Germany). The following kelp response parameters were measured before (day 6), at the peak (day 18) and after (day 23) the experimental heatwave. Growth was analysed by measuring the discs dry weight (g). Rapid light curves were measured with a pulse amplitude modulated fluorometer (Portable Chlorophyll Fluorometer PAM-2100, Heinz Walz GmbH, Effeltrich, Germany), using irradiance steps between 0-611 µmol photons m²/s to derive photosynthesis vs. irradiance curve parameters (Pmax, alpha, EK). Dark respiration and net photosynthetic rates were measured with a 4-channel optode set-up (FireStingO2 Fibre-Optic Oxygen Meter FSO2-C4, PyroScience Sensor technology, Aachen, Germany) by analysing the oxygen evolution in response to different light intensities within a 25 ml Schott bottle, each containing meristematic discs. Pigment content was analysed with a High-Performance Liquid Chromatograph (HPLC, LaChromElite® system, L-2200 autosampler (chilled), DA-detetctor L-2450; VWR-Hitachi International GmbH, Darmstadt, Germany). Total phlorotannin concentration was analysed with the Folin-Ciocalteau assay, using a phloroglucinol dilution series (C6H6O3, Sigma-Aldrich: 0–1000 µg/ml).