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Abstract. The cold-water coral Lophelia pertusa is one of the few species able to build reef-like structures and a 3-dimensional coral framework in the deep oceans. Furthermore, deep cold-water coral bioherms may be among the first marine ecosystems to be affected by ocean acidification. Colonies of L. pertusa were collected during a cruise in 2006 to cold-water coral bioherms of the Mingulay reef complex (Hebrides, North Atlantic). Shortly after sample collection onboard these corals were labelled with calcium-45. The same experimental approach was used to assess calcification rates and how those changed due to reduced pH during a cruise to the Skagerrak (North Sea) in 2007. The highest calcification rates were found in youngest polyps with up to 1% d −1 new skeletal growth and average rates of 0.11±0.02% d −1 (±S.E.). Lowering pH by 0.15 and 0.3 units relative to the ambient level resulted in calcification being reduced by 30 and 56%. Lower pH reduced calcification more in fast growing, young polyps (59% reduction) than in older polyps (40% reduction). Thus skeletal growth of young and fast calcifying corallites suffered more from ocean acidification. Nevertheless, L. pertusa exhibited positive net calcification (as measured by 45 Ca incorporation) even at an aragonite saturation state ( a ) below 1.

ABSTRACT:The ocean absorbs about 25% of anthropogenic CO 2 emissions, which alters its chemistry. Among the changes of the carbonate system are an increase in the partial pressure of CO 2 (pCO 2 ) and a decline of pH; hence, the whole process is often referred to as 'ocean acidification'. Many microbial processes can be affected either directly or indirectly via a cascade of effects through the response of non-microbial groups and/or through changes in seawater chemistry. We briefly review the current understanding of the impact of ocean acidification on microbial diversity and processes, and highlight the gaps that need to be addressed in future research. The focus is on Bacteria, Archaea, viruses and protistan grazers but also includes total primary production of phytoplankton as well as species composition of eukaryotic phytoplankton. Some species and communities exhibit increased primary production at elevated pCO 2 . In contrast to their heterocystous counterparts, nitrogen fixation by non-heterocystous cyanobacteria is stimulated by elevated pCO 2 . The experimental data on the response of prokaryotic production to ocean acidification are not consistent. Very few other microbial processes have been investigated at environmentally relevant pH levels. The potential for microbes to adapt to ocean acidification, at either the species level by genetic change or at the community level through the replacement of sensitive species or groups by non-or less sensitive ones, is completely unknown. Consequently, the impact of ocean acidification on keystone species and microbial diversity needs to be elucidated. Most experiments used a short-term perturbation approach by using cultured organisms; few were conducted in mesocosms and none in situ. There is likely a lot to be learned from observations in areas naturally enriched with CO 2 , such as vents, upwelling and near-shore areas.

Viral abundance and processes in the water column and sediments are well studied for some systems; however, we know relatively little about virus-host interactions on particles and how particles influence these interactions. Here we review virus-prokaryote interactions on inorganic and organic particles in the water column. Profiting from recent methodological progress, we show that confocal laser scanning microscopy in combination with lectin and nucleic acid staining is one of the most powerful methods to visualize the distribution of viruses and their hosts on particles such as organic aggregates. Viral abundance on suspended matter ranges from 10 5 to 10 11 ml -1. The main factors controlling viral abundance are the quality, size and age of aggregates and the exposure time of viruses to aggregates. Other factors such as water residence time likely act indirectly. Overall, aggregates appear to play a role of viral scavengers or reservoirs rather than viral factories. Adsorption of viruses to organic aggregates or inorganic particles can stimulate growth of the free-living prokaryotic community, e.g. by reducing viral lysis. Such mechanisms can affect microbial diversity, food web structure and biogeochemical cycles. Viral lysis of bacterio-and phytoplankton influences the formation and fate of aggregates and can, for example, result in a higher stability of algal flocs. Thus, viruses also influence carbon export; however, it is still not clear whether they short-circuit or prime the biological pump. Throughout this review, emphasis has been placed on defining general problems and knowledge gaps in virus-particle interactions and on providing avenues for further research, particularly those linked to global change.

The Mingulay reef complex in the Sea of the Hebrides west of Scotland was first mapped in 2003 with a further survey in 2006 revealing previously unknown live coral reef areas at 120 to 190 m depth. Habitat mapping confirmed that distinctive mounded bathymetry was formed by reefs of Lophelia pertusa with surficial coral debris dating to almost 4000 yr. Benthic lander and mooring deployments revealed 2 dominant food supply mechanisms to the reefs: a regular rapid downwelling of surface water delivering pulses of warm fluorescent water, and periodic advection of high turbidity bottom waters. Closed chamber respirometry studies suggest that L. pertusa responds to seawater warming, such as that seen during the rapid downwelling events, with increases in metabolic rate. Lipid biomarker analysis implies that corals at Mingulay feed predominantly on herbivorous calanoid copepods. Integrating geophysical and hydrographical survey data allowed us to quantify the roles of these environmental factors in controlling biodiversity of attached epifaunal species across the reefs. Longitudinal structuring of these communities is striking: species richness (α) and turnover (β) change significantly west to east, with variation in community composition largely explained by bathymetric variables that are spatially structured on the reef complex. Vibro-cores through the reef mounds show abundant coral debris with significant hiatuses. High resolution side-scan sonar revealed trawl marks in areas south of the coral reefs where vessel monitoring system data showed the highest density of local fishing activity. The interdisciplinary approach in this study allowed us to record the food supply and hydrographic environment experienced by L. pertusa and determine how it may be ecophysiologically adapted to these conditions. Improved basic understanding of cold-water coral biology and biodiversity alongside efforts to map and date these long-lived habitats are vital to development of future conservation policies. KEY WORDS: Ecological engineer · Lophelia pertusa · Seamounts · Internal waves Resale or republication not permitted without written consent of the publisher Contribution to the Theme Section 'Conservation and management of deep-sea corals and coral reefs'

Global environmental changes, including ocean acidification, have been identified as a major threat to scleractinian corals. General predictions are that ocean acidification will be detrimental to reef growth and that 40 to more than 80 per cent of present-day reefs will decline during the next 50 years. Coldwater corals (CWCs) are thought to be strongly affected by changes in ocean acidification owing to their distribution in deep and/or cold waters, which naturally exhibit a CaCO 3 saturation state lower than in shallow/warm waters. Calcification was measured in three species of Mediterranean cold-water scleractinian corals (Lophelia pertusa, Madrepora oculata and Desmophyllum dianthus) on-board research vessels and soon after collection. Incubations were performed in ambient sea water. The species M. oculata was additionally incubated in sea water reduced or enriched in CO 2. At ambient conditions, calcification rates ranged between 20.01 and 0.23% d 21 . Calcification rates of M. oculata under variable partial pressure of CO 2 (pCO 2 ) were the same for ambient and elevated pCO 2 (404 and 867 matm) with 0.06 + 0.06% d 21 , while calcification was 0.12 + 0.06% d 21 when pCO 2 was reduced to its pre-industrial level (285 matm). This suggests that present-day CWC calcification in the Mediterranean Sea has already drastically declined (by 50%) as a consequence of anthropogenic-induced ocean acidification.

Understanding external deciding factors in growth and morphology of reef corals is essential to elucidate the role of corals in marine ecosystems, and to explain their susceptibility to pollution and global climate change. Here, we extend on a previously presented model for simulating the growth and form of a branching coral and we compare the simulated morphologies to three-dimensional (3D) images of the coral species Madracis mirabilis. Simulation experiments and isotope analyses of M. mirabilis skeletons indicate that external gradients of dissolved inorganic carbon (DIC) determine the morphogenesis of branching, phototrophic corals. In the simulations we use a first principle model of accretive growth based on local interactions between the polyps. The only species-specific information in the model is the average size of a polyp. From flow tank and simulation studies it is known that a relatively large stagnant and diffusion dominated region develops within a branching colony. We have used this information by assuming in our model that growth is entirely driven by a diffusion-limited process, where DIC supply represents the limiting factor. With such model constraints it is possible to generate morphologies that are virtually indistinguishable from the 3D images of the actual colonies.

Ocean acidification caused by anthropogenic uptake of CO2 is perceived to be a major threat to calcifying organisms. Cold-water corals were thought to be strongly affected by a decrease in ocean pH due to their abundance in deep and cold waters which, in contrast to tropical coral reef waters, will soon become corrosive to calcium carbonate. Calcification rates of two Mediterranean cold-water coral species, Lophelia pertusa and Madrepora oculata, were measured under variable partial pressure of CO2 (pCO2) that ranged between 380 µatm for present-day conditions and 930 µatm for the end of the century. The present study addressed both short- and long-term responses by repeatedly determining calcification rates on the same specimens over a period of 9 months. Besides studying the direct, short-term response to elevated pCO2 levels, the study aimed to elucidate the potential for acclimation of calcification of cold-water corals to ocean acidification. Net calcification of both species was unaffected by the levels of pCO2 investigated and revealed no short-term shock and, therefore, no long-term acclimation in calcification to changes in the carbonate chemistry. There was an effect of time during repeated experiments with increasing net calcification rates for both species, however, as this pattern was found in all treatments, there is no indication that acclimation of calcification to ocean acidification occurred. The use of controls (initial and ambient net calcification rates) indicated that this increase was not caused by acclimation in calcification response to higher pCO2. An extrapolation of these data suggests that calcification of these two cold-water corals will not be affected by the pCO2 level projected at the end of the century.