Food selectivity and processing by the cold-water coral Lophelia pertusa

Cold-water corals form prominent reef ecosystems along ocean margins that depend on suspended resources produced in surface waters. In this study, we investigated food processing of 13C and 15N labelled bacteria and algae by the cold-water coral Lophelia pertusa. Coral respiration, tissue incorporation of C and N and metabolic-derived C incorporation into the skeleton were traced following the additions of different food concentrations (100, 300, 1300 µg C/l) and two ratios of suspended bacterial and algal biomass (1:1, 3:1). Respiration and tissue incorporation by L. pertusa increased markedly following exposure to higher food concentrations. The net growth efficiency of L. pertusa was low (0.08±0.03), which is consistent with their slow growth rates. The contribution of algae and bacteria to total coral assimilation was proportional to the food mixture in the two lowest food concentrations, but algae were preferred over bacteria as food source at the highest food concentration. Similarly, the stoichiometric uptake of C and N was coupled in the low and medium food treatment, but was uncoupled in the high food treatment and indicated a comparatively higher uptake or retention of bacterial carbon as compared to algal nitrogen. We argue that behavioural responses for these small-sized food particles, such as tentacle behaviour, mucus trapping and physiological processing, are more likely to explain the observed food selectivity as compared to physical-mechanical considerations. A comparison of the experimental food conditions to natural organic carbon concentrations above CWC reefs suggests that L. pertusa is well adapted to exploit temporal pulses of high organic matter concentrations in the bottom water caused by internal waves and down-welling events.

Diversity of zooplankton assemblages at Station L4, Western English Channel, measured using next generation DNA sequencing

Background: Zooplankton play an important role in our oceans, in biogeochemical cycling and providing a food source for commercially important fish larvae. However, difficulties in correctly identifying zooplankton hinder our understanding of their roles in marine ecosystem functioning, and can prevent detection of long term changes in their community structure. The advent of massively parallel Next Generation Sequencing technology allows DNA sequence data to be recovered directly from whole community samples. Here we assess the ability of such sequencing to quantify the richness and diversity of a mixed zooplankton assemblage from a productive monitoring site in the Western English Channel. Methodology/Principle Findings: Plankton WP2 replicate net hauls (200 µm) were taken at the Western Channel Observatory long-term monitoring station L4 in September 2010 and January 2011. These samples were analysed by microscopy and metagenetic analysis of the 18S nuclear small subunit ribosomal RNA gene using the 454 pyrosequencing platform. Following quality control a total of 419,042 sequences were obtained for all samples. The sequences clustered in to 205 operational taxonomic units using a 97% similarity cut-off. Allocation of taxonomy by comparison with the National Centre for Biotechnology Information database identified 138 OTUs to species level, 11 to genus level and 1 to order, <2.5% of sequences were classified as unknowns. By comparison a skilled microscopic analyst was able to routinely enumerate only 75 taxonomic groups. Conclusions: The percentage of OTUs assigned to major eukaryotic taxonomic groups broadly aligns between the metagenetic and morphological analysis and are dominated by Copepoda. However, the metagenetics reveals a previously hidden taxonomic richness, especially for Copepoda and meroplankton such as Bivalvia, Gastropoda and Polychaeta. It also reveals rare species and parasites. We conclude that Next Generation Sequencing of 18S amplicons is a powerful tool for estimating diversity and species richness of zooplankton communities.

Lipids and nitrogen isotopes of deep-water corals

The lipid and organic nitrogen isotopic (delta15N) compositions of two common deep-water corals (Lophelia pertusa and Madrepora oculata) collected from selected locations of the NE Atlantic are compared to the composition of suspended particulate organic matter, in order to determine their principle food source. Initial results suggest that they may feed primarily on zooplankton. This is based on the increased abundances of mono-unsaturated fatty acids and alcohols and the different ratios of the polyunsaturated fatty acids, 22:6/20:5 of the corals when compared to those of the suspended particulate organic matter. There is enrichment in L. pertusa of mono-unsaturated fatty acids and of delta15N relative to M. oculata. It is unclear whether this reflects different feeding strategies or assimilation/storage efficiencies of zooplankton tissue or different metabolism in the two coral species.

Fatty acid composition, element concentration and isotope ratios of sea ice algae sampled in Rijpfjorden, Svalbard

The accelerating decrease of Arctic sea ice substantially changes the growth conditions for primary producers, particularly with respect to light. This affects the biochemical composition of sea ice algae, which are an essential high-quality food source for herbivores early in the season. Their high nutritional value is related to their content of polyunsaturated fatty acids (PUFAs), which play an important role for successful maturation, egg production, hatching and nauplii development in grazers. We followed the fatty acid composition of an assemblage of sea ice algae in a high Arctic fjord during spring from the early bloom stage to post bloom. Light conditions proved to be decisive in determining the nutritional quality of sea ice algae, and irradiance was negatively correlated with the relative amount of PUFAs. Algal PUFA content decreased on average by 40 % from April to June, while algal biomass (measured as particulate carbon, C) did not differ. This decrease was even more pronounced when algae were exposed to higher irradiances due to reduced snow cover. The ratio of chlorophyll a (chl a) to C, as well as the level of photoprotective pigments, confirmed a physiological adaptation to higher light levels in algae of poorer nutritional quality. We conclude that high irradiances are detrimental to sea ice algal food quality, and that the biochemical composition of sea ice algae is strongly dependent on growth conditions.

Limacina helicina size weight relationship in Kongsfjorden Svalbard, May 2009

Thecosome pteropods (pelagic mollusks) can play a key role in the food web of various marine ecosystems. They are a food source for zooplankton or higher predators such as fishes, whales and birds that is particularly important in high latitude areas. Since they harbor a highly soluble aragonitic shell, they could be very sensitive to ocean acidification driven by the increase of anthropogenic CO2 emissions. The effect of changes in the seawater chemistry was investigated on Limacina helicina, a key species of Arctic pelagic ecosystems. Individuals were kept in the laboratory under controlled pCO2 levels of 280, 380, 550, 760 and 1020 µatm and at control (0°C) and elevated (4°C) temperatures. The respiration rate was unaffected by pCO2 at control temperature, but significantly increased as a function of the pCO2 level at elevated temperature. pCO2 had no effect on the gut clearance rate at either temperature. Precipitation of CaCO3, measured as the incorporation of 45Ca, significantly declined as a function of pCO2 at both temperatures. The decrease in calcium carbonate precipitation was highly correlated to the aragonite saturation state. Even though this study demonstrates that pteropods are able to precipitate calcium carbonate at low aragonite saturation state, the results support the current concern for the future of Arctic pteropods, as the production of their shell appears to be very sensitive to decreased pH. A decline of pteropod populations would likely cause dramatic changes to various pelagic ecosystems.

Benthic foraminifera in surface sediments of the Gulf of Lions

Surface sediment was sampled at two bathyal sites in the southwestern Gulf of Lions in the western Mediterranean Sea in February and August 1997 to study the distribution and microhabitat of living (Rose Bengal stained) deep sea benthic foraminifera. Both standing stock and diversity of the faunas, and the microhabitat of distinct species mirror the trophic situation and the depth of the oxidised layer at the different sites. Our results suggest that the faunas do not comprise highly opportunistic species and are adapted to rather stable environments. In the axial channel of the Lacaze-Duthiers Canyon, organic matter fluxes are enhanced due to advective transport of organic matter resulting in elevated oxygen consumption rates in the surface sediment and a rather thin oxidised layer. The corresponding benthic foraminiferal fauna is characterised by rather high standing stock and diversity, and a well-developed deep infauna. In addition to freshly deposited phytodetritus, more degraded organic matter seems to be an important food source. In contrast, at the open slope, organic matter fluxes and oxygen consumption rates in the surface sediment are lower and the oxidised layer is much thicker than inside the canyon. The corresponding benthic foraminiferal fauna comprises mainly epifaunal and shallow-infaunal species with much lower standing stocks and clear differences between February and August. In August standing stocks are higher and the average living depths of most species shift towards the sediment surface. These differences can be attributed to patchiness or represent a seasonal trophic signal.

Stable isotopic ratios, shell lengths and organochlorine compounds in four bivalve species collected in July 2009 around Svalbard

Organochlorine compounds (OC) were determined in Arctic bivalves (Mya truncata, Serripes groenlan-dicus, Hiatella arctica and Chlamys islandica) from Svalbard with regard to differences in geographic location, species and variations related to their size and age. Higher chlorinated polychlorinated biphenyls (PCB 101-PCB 194), chlordanes and alpha-hexachlorocyclohexane (alpha-HCH) were consistently detected in the bivalves and PCBs dominated the OC load in the organisms. OC concentrations were highest in Mya truncata and the lowest in Serripes groenlandicus. Species-specific OC levels were likely related to differences in the species' food source, as indicated by the d13C results, rather than size and age. Higher OC concentrations were observed in bivalves from Kongsfjorden compared to the northern sampling locations Liefdefjorden and Sjuoyane. The spatial differences might be related to different water masses influencing Kongsfjorden (Atlantic) and the northern locations (Arctic), with differing phytoplankton bloom situations.

Fatty acid content of POM, sediment, foraminifer and peracarid samples obtained during POLARSTERN cruise ANT-XXIV/2 (PS71)

In order to investigate the diversity of diet composition in macrobenthic peracarid crustaceans from the Antarctic shelf and deep sea, the fatty acid (FA) composition of different species belonging to the orders Isopoda, Amphipoda, Cumacea and Tanaidacea was analysed. Multivariate analyses of the FA composition confirmed general differences between the orders, but also distinct differences within these orders. To gain information on the origin of the FAs found, the potential food sources sediment, POM and foraminiferans were included in the study. Most of the analysed amphipod species displayed high 18:1(n-9)-18:1(n-7) ratios, widely used as an indicator for a carnivorous component in the diet. Cumaceans were characterised by increased phytoplankton FA markers such as 20:5(n-3) (up to 29% of total FAs), suggesting a diet based on phytodetritus. High values of the FA 20:4(n-6) were found in some munnopsid isopods (up to 21% of total FAs) and some tanaidacean species (up to 19% of total FAs). 20:4(n-6) also occurred in high proportions in some foraminiferan samples (up to 21% of total fatty acids), but not in sediment and POM, possibly indicating the ingestion of foraminiferans by some peracarid crustaceans.

Benthic foraminifera extinctions in the Cenozoic record

In the late Pliocene-middle Pleistocene a group of 95 species of elongate, cylindrical, deep-sea (lower bathyal-abyssal) benthic foraminifera became extinct. This Extinction Group (Ext. Gp), belonging to three families (all the Stilostomellidae and Pleurostomellidae, some of the Nodosariidae), was a major component (20-70%) of deep-sea foraminiferal assemblages in the middle Cenozoic and subsequently declined in abundance and species richness before finally disappearing almost completely during the mid-Pleistocene Climatic Transition (MPT). So what caused these declines and extinction? In this study 127 Ext. Gp species are identified from eight Cenozoic bathyal and abyssal sequences in the North Atlantic and equatorial Pacific Oceans. Most species are long-ranging with 80% originating in the Eocene or earlier. The greatest abundance and diversity of the Ext. Gp was in the warm oceanic conditions of the middle Eocene-early Oligocene. The group was subjected to significant changes in the composition of the faunal dominants and slightly enhanced species turnover during and soon after the rapid Eocene-Oligocene cooling event. Declines in the relative abundance and flux of the Ext. Gp, together with enhanced species loss, occurred during middle-late Miocene cooling, particularly at abyssal sites. The overall number of Ext. Gp species present began declining earlier at mid abyssal depths (in middle Miocene) than at upper abyssal (in late Pliocene-early Pleistocene) and then lower bathyal depths (in MPT). By far the most significant Ext. Gp declines in abundance and species loss occurred during the more severe glacial stages of the late Pliocene-middle Pleistocene. Clearly, the decline and extinction of this group of deep-sea foraminifera was related to the function of their specialized apertures and the stepwise cooling of global climate and deep water. We infer that the apertural modifications may be related to the method of food collection or processing, and that the extinctions may have resulted from the decline or loss of their specific phytoplankton or prokaryote food source, that was more directly impacted than the foraminifera by the cooling temperatures.

Production, oxygen uptake, and sinking velocity of copepod fecal pellets originated from the central North Sea

Production, oxygen uptake, and sinking velocity of copepod fecal pellets egested by Temora longicornis were measured using a nanoflagellate (Rhodomonas sp.), a diatom (Thalassiosira weissflogii), or a coccolithophorid (Emiliania huxleyi) as food sources. Fecal pellet production varied between 0.8 pellets ind**-1 h**-1 and 3.8 pellets ind**-1 h**-1 and was significantly higher with T. weissflogii than with the other food sources. Average pellet size varied between 2.2 x 10**5 µm**3 and 10.0 x 10**5 µm**3. Using an oxygen microsensor, small-scale oxygen fluxes and microbial respiration rates were measured directly with a spatial resolution of 2 µm at the interface of copepod fecal pellets and the surrounding water. Averaged volume-specific respiration rates were 4.12 fmol O2 µm**-3 d**-1, 2.86 fmol O2 µm**-3 d**-1, and 0.73 fmol O2 µm**-3 d**-1 in pellets produced on Rhodomonas sp., T. weissflogii, and E. huxleyi, respectively. The average carbon-specific respiration rate was 0.15 d**-1 independent on diet (range: 0.08-0.21 d**-1). Because of ballasting of opal and calcite, sinking velocities were significantly higher for pellets produced on T. weissflogii (322 +- 169 m d**-1) and E. huxleyi (200 +- 93 m d**-1) than on Rhodomonas sp. (35 +- 29 m d**-1). Preservation of carbon was estimated to be approximately 10-fold higher in fecal pellets produced when T. longicornis was fed E. huxleyi or T. weissflogii rather than Rhodomonas sp. Our study directly demonstrates that ballast increases the sinking rate of freshly produced copepod fecal pellets but does not protect them from decomposition.

Pliocene/Pleistocene benthic foraminiferal abundances from six drilling cores

Twenty percent (19 genera, 95 species) of cosmopolitan, deep-sea (500-4000 m), benthic foraminiferal species became extinct during the late Pliocene-Middle Pleistocene (3-0.12 Ma), with the peak of extinctions (76 species) occurring during the mid-Pleistocene Climate Transition (MPT, 1.2-0.55 Ma). One whole family (Stilostomellidae, 30 species) was wiped out, and a second (Pleurostomellidae, 29 species) was decimated with just one species possibly surviving through to the present. Our studies at 21 deep-sea core sites show widespread pulsed declines in abundance and diversity of the extinction group species during more extreme glacials, with partial interglacial recoveries. These declines started in the late Pliocene in southern sourced deep water masses (Antarctic Bottom Water, Circumpolar Deep Water) and extending into intermediate waters (Antarctic Intermediate Water, North Atlantic Deep Water) in the MPT, with the youngest declines in sites farthest downstream from high-latitude source areas for intermediate waters. We infer that the unusual apertural types that were targeted by this extinction period were adaptations for a specific kind of food source and that it was probably the demise of this microbial food that resulted in the foraminiferal extinctions. We hypothesize that it may have been increased cold and oxygenation of the southern sourced deep water masses that impacted on this deep water microbial food source during major late Pliocene and Early Pleistocene glacials when Antarctic ice was substantially expanded. The food source in intermediate water was not impacted until major glacials in the MPT when there were significant expansion of polar sea ice in both hemispheres and major changes in the source areas, temperature, and oxygenation of global intermediate waters.

(Table 1) Composition of stomach and gut content of the amphipod Onisimus litoralis, and algal availability near Barrow, Alaska

Sea ice algae have been widely discussed as a potential food source for pelagic and benthic animals in ice-covered waters, specifically in the light of current substantial changes in the Arctic ice regime. Stomach and gut contents of the Arctic nearshore lysianassid amphipod Onisimus litoralis sampled from February to May 2003 indicate that Arctic ice algae were dominant food no earlier than the onset of ice melt. Crustaceans, common prey in a previous study, were absent in stomachs and guts during the survey period. Our data support the concept that sea ice-derived organic carbon is of specific relevance for Arctic plankton and benthos during the period of ice melt.

Fecal pellet ingestion rate and feeding behavior of Oithona similis, Calanus helgolandicus and Pseudocalanus elongatus from the North Sea

Studies of fecal pellet flux show that a large percentage of pellets produced in the upper ocean is degraded within the surface waters. It is therefore important to investigate these degradation mechanisms to understand the role of fecal pellets in the oceanic carbon cycle. Degradation of pellets is mainly thought to be caused by coprophagy (ingestion of fecal pellets) by copepods, and especially by the ubiquitous copepods Oithona spp. We examined fecal pellet ingestion rate and feeding behavior of O. similis and 2 other dominant copepod species from the North Sea (Calanus helgolandicus and Pseudocalanus elongatus). All investigations were done with fecal pellets as the sole food source and with fecal pellets offered together with an alternative suitable food source. The ingestion of fecal pellets by all 3 copepod species was highest when offered together with an alternative food source. No feeding behavior was determined for O. similis due to the lack of pellet capture in those experiments. Fecal pellets offered together with an alternative food source increased the filtration activity by C. helgolandicus and P. elongatus and thereby the number of pellets caught in their feeding current. However, most pellets were rejected immediately after capture and were often fragmented during rejection. Actual ingestion of captured pellets was rare (<37% for C. helgolandicus and <24% for P. elongatus), and only small pellet fragments were ingested unintentionally along with alternative food. We therefore suggest coprorhexy (fragmentation of pellets) to be the main effect of copepods on the vertical flux of fecal pellets. Coprorhexy turns the pellets into smaller, slower-sinking particles that can then be degraded by other organisms such as bacteria and protozooplankton.