Temperature tolerance of western Baltic Sea Fucus vesiculosus - growth, photosynthesis and survival

Fucus vesiculosus L. (Phaeophyceae) is the most abundant and hence ecologically most important primary producer, carbon sink and habitat provider in the western Baltic Sea. All F. vesiculosus L. specimens were collected on 23 April 2014 from a depth of 0.2-1 m in the non-tidal Kiel Fjord, western Baltic Sea (54°27'N; 10°12'E), where this species forms dense and almost monospecific stands on stones. After sampling the algal thalli were stored in a refrigerator box with water from the sampling site, transported to Bremerhaven and stored at 10 °C for one day in filtered seawater. Experiments were conducted with vegetative apical tips (6.7±0.5 cm length), the actively growing region of F. vesiculosus, which were randomly selected and cut from 144 different individuals prior to the experiments. These tips were acclimated to laboratory conditions for three days in filtered seawater at 10 °C before the start of the experiment. Furthermore, 30 additional vegetative apices were freeze-dried to document the initial biochemical status of F. vesiculosus in its native habitat. A temperature gradient was installed in a walk-in constant cooling chamber (15 °C) in nine water baths (5, 10, 15, 20, 24, 26, 27, 28 and 29 °C ± 0.1 °C) which were tempered by thermostats (5, 10 and 15 °C: Huber Variostat CC + Pilot ONE, Peter Huber Kältemaschinen GmbH, Offenburg, Germany; 20 and 28 °C: Haake DC3, Thermo Fisher Scientific Inc., Waltham, USA; 24, 26, 27 and 29 °C: Haake DC10). Every temperature treatment consisted of four 2 L glass beakers (n = 4). In each beaker four F. vesiculosus apices were grown in 2 µm-filtered North Sea water diluted with demineralized water in a ratio of 1:1 and enriched with nutrients after Provasoli (1968; 1/10 enrichment), leading to a salinity of about 15.6 which equaled habitat conditions. The algae were exposed to an irradiance of 130 µmol photons m-2 s-1 ±10 % (Powerstar HGI-TS 150 W, OSRAM GmbH, Bad Homburg, Germany) measured at the top of the beaker under a 16:8 h L:D cycle. The media in the beakers was changed every third or fourth day and aerated with artificial air containing 380 ppm CO2 (gas mixing device; HTK Hamburg GmbH, Hamburg, Germany). Before the experiment, the algae were acclimated to the final temperatures in steps of 5 °C for 2 days each, beginning at 10 °C. After 21 days exposure time, three out of four samples per replicate were freeze-dried for further biochemical analyses, and afterwards the thermostats were turned off to reduce the temperature to 16±0.4 °C for another 10 days permitting growth under post-culture conditions.

Consumption rates of the periwinkle Littorina obtusata on the brown seaweed Ascophyllum nodosum

Anti-herbivory defenses support persistence of seaweeds. Little is known, however, about temporal dynamics in the induction of grazer-deterrent seaweed traits. In two induction experiments, consumption rates of the periwinkle Littorina obtusata (L.) on the brown seaweed Ascophyllum nodosum (L.) Le Jolis were measured in 3-d intervals. Changes in palatability of directly grazed A. nodosum were tested every 3 d with feeding assays using fresh and reconstituted seaweed pieces. Likewise, assays with fresh A. nodosum assessed changes in seaweed palatability in response to water-borne cues from nearby grazed conspecifics. Consumption rates of L. obtusata varied significantly during the 27-d induction phase of each experiment. Direct grazing by L. obtusata lowered palatability of fresh and reconstituted A. nodosum pieces to conspecific grazers after 15 d as well as after 6 and 12 d, respectively. After 12, 18, and 24 d, fresh A. nodosum located downstream of L. obtusata-grazed conspecifics was significantly less palatable than A. nodosum located downstream of ungrazed conspecifics. Changes in L. obtusata consumption rates and A. nodosum palatability during both induction experiments suggest temporal variation of grazer-deterrent responses, which may complicate experimental detection of inducible anti-herbivory defenses.

Warming and acidification experiment on early life-stage F. vesiculosus under natural fluctuation and seasonal variations (spring 2013 - 2014)

Genetic diversity of baltic F. vesiculosus is low compared to other populations which might jeopardize their potential for adaptation to climate change. Especially the early life-stage F. vesiculosus may be threaten by ocean warming and acidification. To test this, we exposed F. vesiculosus germlings to warming and acidification in the near-natural scenario in the "Kiel Outdoor Benthocosms" maintaining the natural variation of the Kiel Fjord, Germany (54°27 'N, 10°11 'W) in all seasons (spring 2013 - 2014). Warming was simulated by using a delta treatment adding 5 °C and by increasing pCO2 at 1000 µatm. Warming positively affected germlings' growth in spring and in summer but decreased non-photochemical quenching in spring and survival in summer. Acidified conditions showed much weaker effects than warming. The high genotypic variation in stress sensitivity as well as the enhanced survival at high diversity levels indicate higher potential for adaptation for genetically diverse populations. We conclude that the combination of stressors and season determines the sensitivity to environmental stress and that genetic variation is crucial for the adaptation to climate change stress.

Seawater carbonate chemistry and calcification during tropical reef studies at Moorea (French Polynesia), 1997

Community metabolism and air-sea carbon dioxide (CO2) fluxes were investigated in July 1992 on a fringing reef at Moorea (French Polynesia). The benthic community was dominated by macroalgae (85% substratum cover) and comprised of Phaeophyceae Padina tenuis (Bory), Turbinaria ornata (Turner) J. Agardh, and Hydroclathrus clathratus Bory (Howe); Chlorophyta Halimeda incrassata f. ovata J. Agardh (Howe); and Ventricaria ventricosa J. Agardh (Olsen et West), as well as several Rhodophyta (Actinotrichia fragilis Forskál (Børgesen) and several species of encrusting coralline algae). Algal biomass was 171 g dry weight/m**2. Community gross production (Pg), respiration (R), and net calcification (G) were measured in an open-top enclosure. Pg and R were respectively 248 and 240 mmol Co2/m**2/d, and there was a slight net dissolution of CaCO3 (0.8 mmol/m**2/d). This site was a sink for atmospheric CO2 (10 ± 4 mmol CO2/m**2/d), and the analysis of data from the literature suggests that this is a general feature of algal-dominated reefs. Measurement of air-sea CO2 fluxes in open water close to the enclosure demonstrated that changes in small-scale hydrodynamics can lead to misleading conclusions. Net CO2 evasion to the atmosphere was measured on the fringing reef due to changes in the current pattern that drove water from the barrier reef (a C02 source) to the study site.

Seawater carbonate chemistry and giant kelp Macrocystis pyrifera reproduction processes during experiments, 2011

The worldwide effects of ocean acidification (OA) on marine species are a growing concern. In temperate coastal seas, seaweeds are dominant primary producers that create complex habitats and supply energy to higher trophic levels. Studies on OA and macroalgae have focused on calcifying species and adult stages but, critically, they have overlooked the microscopic stages of the reproductive life cycle, which, for other anthropogenic stress e.g. UV-B radiation, are the most susceptible life-history phase. Also, environmental cues and stressors can cause changes in the sex ratio which has implications for the mating system and recruitment success. Here, we report the effects of pH (7.59-8.50) on meiospore germination and sex determination for the giant kelp, Macrocystis pyrifera (Laminariales), in the presence and absence of additional dissolved inorganic carbon (DIC). Lowered pH (7.59-7.60, using HCl-only) caused a significant reduction in germination, while added DIC had the opposite effect, indicating that increased CO2 at lower pH ameliorates physiological stress. This finding also highlights the importance of appropriate manipulation of seawater carbonate chemistry when testing the effects of ocean acidification on photosynthetic organisms. The proportion of male to female gametophytes did not vary significantly between treatments suggesting that pH was not a primary environmental modulator of sex. Relative to the baseline (pH 8.19), gametophytes were 32% larger under moderate OA (pH 7.86) compared to their size (10% increase) under extreme OA (pH 7.61). This study suggests that metabolically-active cells can compensate for the acidification of seawater. This homeostatic function minimises the negative effects of lower pH (high H+ ions) on cellular activity. The 6-9% reduction in germination success under extreme OA suggests that meiospores of M.pyrifera may be resistant to future ocean acidification.

Macroalgal responses to ocean acidification depend on nutrient and light levels

Ocean acidification may benefit algae that are able to capitalize on increased carbon availability for photosynthesis, but it is expected to have adverse effects on calcified algae through dissolution. Shifts in dominance between primary producers will have knock-on effects on marine ecosystems and will likely vary regionally, depending on factors such as irradiance (light vs. shade) and nutrient levels (oligotrophic vs. eutrophic). Thus experiments are needed to evaluate interactive effects of combined stressors in the field. In this study, we investigated the physiological responses of macroalgae near a CO2 seep in oligotrophic waters off Vulcano (Italy). The algae were incubated in situ at 0.2 m depth using a combination of three mean CO2 levels (500, 700-800 and 1200 µatm CO2), two light levels (100 and 70% of surface irradiance) and two nutrient levels of N, P, and K (enriched vs. non-enriched treatments) in the non-calcified macroalga Cystoseira compressa (Phaeophyceae, Fucales) and calcified Padina pavonica (Phaeophyceae, Dictyotales). A suite of biochemical assays and in vivo chlorophyll a fluorescence parameters showed that elevated CO2 levels benefitted both of these algae, although their responses varied depending on light and nutrient availability. In C. compressa, elevated CO2 treatments resulted in higher carbon content and antioxidant activity in shaded conditions both with and without nutrient enrichment-they had more Chla, phenols and fucoxanthin with nutrient enrichment and higher quantum yield (Fv/Fm) and photosynthetic efficiency (alpha ETR) without nutrient enrichment. In P. pavonica, elevated CO2 treatments had higher carbon content, Fv/Fm, alpha ETR, and Chla regardless of nutrient levels-they had higher concentrations of phenolic compounds in nutrient enriched, fully-lit conditions and more antioxidants in shaded, nutrient enriched conditions. Nitrogen content increased significantly in fertilized treatments, confirming that these algae were nutrient limited in this oligotrophic part of the Mediterranean. Our findings strengthen evidence that brown algae can be expected to proliferate as the oceans acidify where physicochemical conditions, such as nutrient levels and light, permit.