Dynamics of nutrients, total organic carbon, prokaryotes and viruses in onboard incubations of cold-water corals

The potential influence of the cold-water corals (CWCs) Lophelia pertusa and Madrepora oculata on the dynamics of inorganic nutrient and total organic carbon (TOC) concentrations and the abundances of prokaryotes and viruses in bottom water was assessed in onboard incubation experiments. Ammonium, nitrite, dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP) and TOC concentrations and N:P ratios were typically higher in incubation water with corals than in controls, whereas nitrate concentrations did not reveal a clear trend. Mucus release (normalized to coral surface) was estimated by the net increase rate of TOC concentrations and averaged 23 +/- 6 mg C m(-2) h(-1) for L. pertusa and 21 +/- 8 mg C m(-2) h(-1) for M. oculata. Prokaryotic and viral abundance and turnover rates were typically stimulated in incubation water with corals. This estimated prokaryotic stimulation averaged 6.0 +/- 3.0x10(9) cells m(-2) h(-1) for L. pertusa and 8.4 +/- 2.9x10(9) cells m(-2) h(-1) for M. oculata, whereas the estimated viral stimulation averaged 15.6 +/- 12.7x10(9) particles m(-2) h(-1) for L. pertusa and 4.3 +/- 0.4x10(9) particles m(-2) h(-1) M. oculata. Our data suggest that prokaryotes and viruses are released from corals and that nutrient and mucus release enhanced prokaryotic and viral production. The result of this stimulation could be a fuelling of bottom water in CWC reefs with nutrients and organic matter and consequently an enhancement of microbe-mediated processes

A 13C labelling study on carbon fluxes in Arctic plankton communities under elevated CO2 levels

The effect of CO2 on carbon fluxes in Arctic plankton communities was investigated during the 2010 EPOCA mesocosm study in Ny Ålesund, Svalbard. Nine mesocosms were set up with initial pCO2 levels ranging from 185 to 1420 μatm for 5 weeks. 13C labelled bicarbonate was added at the start of the experiment to follow the transfer of carbon from dissolved inorganic carbon (DIC) into phytoplankton, bacteria, total particulate organic carbon (POC), zooplankton, and settling particles. Polar lipid derived fatty acids (PLFA) were used to trace carbon dynamics of phytoplankton and bacteria and allowed distinction of two groups of phytoplankton: phyto I (autotrophs) and phyto II (mixotrophs). Nutrients were added on day 13. A nutrient-phytoplankton-zooplankton-detritus model amended with 13C dynamics was constructed and fitted to the data to quantify uptake rates and carbon fluxes in the plankton community during the phase prior to nutrient addition (phase 1, days 0–12). During the first 12 days, a phytoplankton bloom developed that was characterized by high growth rates (0.87 days−1) for phyto I and lower growth rates (0.18 days−1) for phyto II. A large part of the carbon fixed by phytoplankton (~31%) was transferred to bacteria, while mesozooplankton grazed only ~6% of the production. After 6 days, the bloom collapsed and part of the organic matter subsequently settled into the sediment traps. The sedimentation losses of detritus in phase 1 were low (0.008 days−1) and overall export was only ~7% of production. Zooplankton grazing and detritus sinking losses prior to nutrient addition were sensitive to CO2: grazing decreased with increasing CO2, while sinking increased. Phytoplankton production increased again after nutrient addition on day 13. Although phyto II showed initially higher growth rates with increasing CO2 (days 14–22), the overall production of POC after nutrient addition (phase 2, days 14–29) decreased with increasing CO2. Significant sedimentation occurred towards the end of the experiment (after day 24) and much more material settled down in the sediment traps at low CO2

Phytoplankton-bacteria coupling under elevated CO₂ levels: a stable isotope labelling study

The potential impact of rising carbon dioxide (CO2) on carbon transfer from phytoplankton to bacteria was investigated during the 2005 PeECE III mesocosm study in Bergen, Norway. Sets of mesocosms, in which a phytoplankton bloom was induced by nutrient addition, were incubated under 1× (~350 μatm), 2× (~700 μatm), and 3× present day CO2 (~1050 μatm) initial seawater and sustained atmospheric CO2 levels for 3 weeks. 13C labelled bicarbonate was added to all mesocosms to follow the transfer of carbon from dissolved inorganic carbon (DIC) into phytoplankton and subsequently heterotrophic bacteria, and settling particles. Isotope ratios of polar-lipid-derived fatty acids (PLFA) were used to infer the biomass and production of phytoplankton and bacteria. Phytoplankton PLFA were enriched within one day after label addition, whilst it took another 3 days before bacteria showed substantial enrichment. Group-specific primary production measurements revealed that coccolithophores showed higher primary production than green algae and diatoms. Elevated CO2 had a significant positive effect on post-bloom biomass of green algae, diatoms, and bacteria. A simple model based on measured isotope ratios of phytoplankton and bacteria revealed that CO2 had no significant effect on the carbon transfer efficiency from phytoplankton to bacteria during the bloom. There was no indication of CO2 effects on enhanced settling based on isotope mixing models during the phytoplankton bloom, but this could not be determined in the post-bloom phase. Our results suggest that CO2 effects are most pronounced in the post-bloom phase, under nutrient limitation

Effect of increased pCO2 on the planktonic metabolic balance during a mesocosm experiment in an Arctic fjord

The effect of ocean acidification on the balance between gross community production (GCP) and community respiration (CR) (i.e., net community production, NCP) of plankton communities was investigated in summer 2010 in Kongsfjorden, west of Svalbard. Surface water, which was characterized by low concentrations of dissolved inorganic nutrients and chlorophyll a (a proxy of phytoplankton biomass), was enclosed in nine mesocosms and subjected to eight pCO2 levels (two replicated controls and seven enhanced pCO2 treatments) for one month. Nutrients were added to all mesocosms on day 13 of the experiment, and thereafter increase of chlorophyll a was provoked in all mesocosms. No clear trend in response to increasing pCO2 was found in the daily values of NCP, CR, and GCP. For further analysis, these parameters were cumulated for the following three periods: phase 1 – end of CO2 manipulation until nutrient addition (t4 to t13); phase 2 – nutrient addition until the second chlorophyll a minimum (t14 to t21); phase 3 – the second chlorophyll a minimum until the end of this study (t22 to t28). A significant response was detected as a decrease of NCP with increasing pCO2 during phase 3. CR was relatively stable throughout the experiment in all mesocosms. As a result, the cumulative GCP significantly decreased with increasing pCO2 during phase 3. After the nutrient addition, the ratios of cumulative NCP to cumulative consumption of NO3 and PO4 showed a significant decrease during phase 3 with increasing pCO2. The results suggest that elevated pCO2 influenced cumulative NCP and stoichiometric C and nutrient coupling of the plankton community in a high-latitude fjord only for a limited period. However provided that there were some differences or weak correlations between NCP data based on different methods in the same experiment, this conclusion should be taken with caution

The important role of sponges in carbon and nitrogen cycling in a deep-sea biological hotspot

Deep-sea sponge grounds are hotspots of biodiversity, harbouring thriving ecosystems in the otherwise barren deep sea. It remains unknown how these sponge grounds survive in this food-limited environment. Here, we unravel how sponges and their associated fauna sustain themselves by identifying their food sources and food-web interactions using bulk and compound-specific stable isotope analysis of amino and fatty acids. We found that sponges with a high microbial abundance had an isotopic composition resembling organisms at the base of the food web, suggesting that they are able to use dissolved resources that are generally inaccessible to animals. In contrast, low microbial abundance sponges had a bulk isotopic composition that resembles a predator at the top of a food web, which appears to be the result of very efficient recycling pathways that are so far unknown. The compound-specific-isotope analysis, however, positioned low-microbial abundance sponges with other filter-feeding fauna. Furthermore, fatty-acid analysis confirmed transfer of sponge-derived organic material to the otherwise food-limited associated fauna. Through this subsidy, sponges are key to the sustenance of thriving deep-sea ecosystems and might have, due to their ubiquitous abundance, a global impact on biogeochemical cycles.publishedVersio

Giant sponge grounds of Central Arctic seamounts are associated with extinct seep life

The Central Arctic Ocean is one of the most oligotrophic oceans on Earth because of its sea-ice cover and short productive season. Nonetheless, across the peaks of extinct volcanic seamounts of the Langseth Ridge (87°N, 61°E), we observe a surprisingly dense benthic biomass. Bacteriosponges are the most abundant fauna within this community, with a mass of 460 g C m-2 and an estimated carbon demand of around 110 g C m-2 yr-1, despite export fluxes from regional primary productivity only sufficient to provide <1% of this required carbon. Observed sponge distribution, bulk and compound-specific isotope data of fatty acids suggest that the sponge microbiome taps into refractory dissolved and particulate organic matter, including remnants of an extinct seep community. The metabolic profile of bacteriosponge fatty acids and expressed genes indicate that autotrophic symbionts contribute significantly to carbon assimilation. We suggest that this hotspot ecosystem is unique to the Central Arctic and associated with extinct seep biota, once fueled by degassing of the volcanic mounts

The important role of sponges in carbon and nitrogen cycling in a deep-sea biological hotspot

Deep-sea sponge grounds are hotspots of biodiversity, harbouring thriving ecosystems in the otherwise barren deep sea. It remains unknown how these sponge grounds survive in this food-limited environment. Here, we unravel how sponges and their associated fauna sustain themselves by identifying their food sources and food-web interactions using bulk and compound-specific stable isotope analysis of amino and fatty acids. We found that sponges with a high microbial abundance had an isotopic composition resembling organisms at the base of the food web, suggesting that they are able to use dissolved resources that are generally inaccessible to animals. In contrast, low microbial abundance sponges had a bulk isotopic composition that resembles a predator at the top of a food web, which appears to be the result of very efficient recycling pathways that are so far unknown. The compound-specific-isotope analysis, however, positioned low-microbial abundance sponges with other filter-feeding fauna. Furthermore, fatty-acid analysis confirmed transfer of sponge-derived organic material to the otherwise food-limited associated fauna. Through this subsidy, sponges are key to the sustenance of thriving deep-sea ecosystems and might have, due to their ubiquitous abundance, a global impact on biogeochemical cycles. Read the free Plain Language Summary for this article on the Journal blog