Growth and mortality of coccolithophores during spring in a temperate Shelf Sea (Celtic Sea, April 2015)

Coccolithophores are key components of phytoplankton communities, exerting a critical impact on the global carbon cycle and the Earth’s climate through the production of coccoliths made of calcium carbonate (calcite) and bioactive gases. Microzooplankton grazing is an important mortality factor in coccolithophore blooms, however little is currently known regarding the mortality (or growth) rates within non-bloom populations. Measurements of coccolithophore calcite production (CP) and dilution experiments to determine microzooplankton (≤63 µm) grazing rates were made during a spring cruise (April 2015) at the Central Celtic Sea (CCS), shelf edge (CS2), and within an adjacent April bloom of the coccolithophore Emiliania huxleyi at station J2. CP at CCS ranged from 10.4 to 40.4 µmol C m−3 d−1 and peaked at the height of the spring phytoplankton bloom (peak chlorophyll-a concentrations ∼6 mg m−3). Cell normalised calcification rates declined from ∼1.7 to ∼0.2 pmol C cell−1 d−1, accompanied by a shift from a mixed coccolithophore species community to one dominated by the more lightly calcified species E. huxleyi and Calciopappus caudatus. At the CCS, coccolithophore abundance increased from 6 to 94 cells mL−1, with net growth rates ranging from 0.06 to 0.21 d−1 from the 4th to the 28th April. Estimates of intrinsic growth and grazing rates from dilution experiments, at the CCS ranged from 0.01 to 0.86 d−1 and from 0.01 to 1.32 d−1, respectively, which resulted in variable net growth rates during April. Microzooplankton grazers consumed 59 to >100% of daily calcite production at the CCS. Within the E. huxleyi bloom a maximum density of 1986 cells mL−1 was recorded, along with CP rates of 6000 µmol C m−3 d−1 and an intrinsic growth rate of 0.29 d−1, with ∼80% of daily calcite production being consumed. Our results show that microzooplankton can exert strong top-down control on both bloom and non-bloom coccolithophore populations, grazing over 60% of daily growth (and calcite production). The fate of consumed calcite is unclear, but may be lost either through dissolution in acidic food vacuoles, and subsequent release as CO2, or export to the seabed after incorporation into small faecal pellets. With such high microzooplankton-mediated mortality losses, the fate of grazed calcite is clearly a high priority research direction

Microzooplankton grazing in Phaeocystis and diatom-dominated waters in the southern North Sea in spring

The impact of microzooplankton grazing upon phytoplankton production was quantified in surface waters of the Southern Bight of the North Sea, during April 1998. Two sites were studied in order to examine the impact of microzooplankton grazing on phytoplankton communities dominated by either Phaeocystis globosa and large phytoplankton or small phytoplankton taxa. The nearshore site was characterised by a phytoplankton community comprised mainly of P. globosa and chains of diatoms with high productivity (av. 346 ± 185 gC l?1 d?1) and biomass (280 ± 171 gC l?1 d?1). In contrast, in the offshore waters relatively small diatoms dominated the phytoplankton where productivity and biomass were more than ca. five times lower than in nearshore waters. Contrary to expectations, the nearshore site supported a high biomass of microzooplankton (av. 22.4 ± 10.6 gC l?1 d?1) which was dominated by large heterotrophic dinoflagellates, mostly Gyrodinium cf. spirale. Offshore the microzooplankton community contained one-third the biomass of the nearshore community and was dominated by smaller individuals, in particular oligotrich ciliates, Strombidium spp. Dilution experiments were conducted in order to quantify phytoplankton growth and losses due to microzooplankton grazing in the <200 m size fraction. Phytoplankton specific growth rates (<200 m) ranged between 0.13 and 0.67 d?1 with highest values associated with offshore waters. In contrast, phytoplankton mortality due to microzooplankton grazing (0.27 to 1.14 d?1) was highest at the nearshore site and exceeded the growth rates of the <200 m phytoplankton. Biomass specific grazing rates were three-fold higher in nearshore (av. 0.33 ± 0.23 d?1) waters compared to those offshore (av. 0.11 ± 0.09 d?1). These results show that microzooplankton were grazing more vigorously nearshore than offshore and were consistently cropping the production of the <200 m phytoplankton. This high grazing pressure is likely to drive a shift in community composition from smaller to larger cells, in particular colonies of P. globosa. The high grazing rates on smaller phytoplankton demonstrated in this study illustrate that microzooplankton grazing may be one of the driving forces behind the evolution of the Phaeocystis life-history strategy that involves a transition between solitary and colonial cells

Is dimethyl sulphide production related to microzooplankton herbivory in the southern North Sea?

Microzooplankton herbivory is considered to be a key process by which dimethylsulphoniopropionate (DMSP) in phytoplankton is transformed to climatically active dimethyl sulphide (DMS). However, there is little firm evidence to show that this occurs in natural waters. We used direct measurements of microzooplankton grazing rates and net DMS production in the southern North Sea to examine the impact of herbivory on DMS production. Estimates of the particulate DMSP ingested by microzooplankton in the form of Phaeocystis sp. were found to account for the DMS production rates observed

Microzooplankton grazing in <i>Phaeocystis</i> and diatom-dominated waters in the southern North Sea in spring

The impact of microzooplankton grazing upon phytoplankton production was quantified in surface waters of the Southern Bight of the North Sea, during April 1998. Two sites were studied in order to examine the impact of microzooplankton grazing on phytoplankton communities dominated by either Phaeocystis globosa and large phytoplankton or small phytoplankton taxa. The nearshore site was characterised by a phytoplankton community comprised mainly of P. globosa and chains of diatoms with high productivity (av. 346 F 185 µgC l-1d-1) and biomass (280 F 171 µgC l-1d-1). In contrast, in the offshore waters relatively small diatoms dominated the phytoplankton where productivity and biomass were more than ca. five times lower than in nearshore waters. Contrary to expectations, the nearshore site supported a high biomass of microzooplankton (av. 22.4 F 10.6 µgC l-1d-1) which was dominated by large heterotrophic dinoflagellates, mostly Gyrodinium cf. spirale. Offshore the microzooplankton community contained one-third the biomass of the nearshore community and was dominated by smaller individuals, in particular oligotrich ciliates, Strombidium spp. Dilution experiments were conducted in order to quantify phytoplankton growth and losses due to microzooplankton grazing in the -1 with highest values associated with offshore waters. In contrast, phytoplankton mortality due to microzooplankton grazing (0.27 to 1.14 d-1) was highest at the nearshore site and exceeded the growth rates of the -1) waters compared to those offshore (av. 0.11 ± 0.09 d-1). These results show that microzooplankton were grazing more vigorously nearshore than offshore and were consistently cropping the production of the < 200 Am phytoplankton. This high grazing pressure is likely to drive a shift in community composition from smaller to larger cells, in particular colonies of P. globosa. The high grazing rates on smaller phytoplankton demonstrated in this study illustrate that microzooplankton grazing may be one of the driving forces behind the evolution of the Phaeocystis life-history strategy that involves a transition between solitary and colonial cells

Cadmium uptake by marine micro-organisms in the English Channel and Celtic Sea

A series of shipboard experiments using the radiotracer 109Cd investigated the role of phytoplankton and bacteria in the uptake of dissolved Cd in the English Channel and Celtic Sea. The results demonstrate that Cd uptake is related to rates of primary production and bacterial numbers. Statistical analysis of plankton species abundance infer that Rhizosolenia, Chaetoceros and Pseudonitzschia diatom species are largely responsible for the higher Cd uptake observed in the &gt;5 ?m size fraction during a diatom-dominated spring bloom. Total Cd uptake rates during winter non-bloom conditions were between 0.04 and 0.29 pmol l–1 h–1, and increased to between 0.43 and 1.23 pmol l–1 h–1 during diatom bloom conditions. These uptake rates are consistent with the seasonal surface depletion of Cd reported in the Celtic Sea and attributed to uptake by phytoplankton bloom material. A calculated Cd:C ratio of 3.1 ?mol mol–1 for natural plankton samples of the Celtic Sea agrees well with results of previous culture studies, which have reported ratios between 0.1 and 5.0 ?mol mol–1 for the coastal diatoms Thalassiosira weissflogii and T. pseudonana. Cd uptake was also linearly related to bacterial numbers, which was attributed to surface adsorption of Cd ions onto bacterial particles which have relatively high specific surface areas. These results demonstrate surface adsorption of Cd onto bacterial surfaces, and other biogenic non-living particles, i.e. ‘passive Cd uptake’, which is significantly augmented during a spring diatom bloom

Growth and mortality of coccolithophores during spring in a temperate Shelf Sea (Celtic Sea, April 2015)

Coccolithophores are key components of phytoplankton communities, exerting a critical impact on the global carbon cycle and the Earth's climate through the production of coccoliths made of calcium carbonate (calcite) and bioactive gases. Microzooplankton grazing is an important mortality factor in coccolithophore blooms, however little is currently known regarding the mortality (or growth) rates within non-bloom populations. Measurements of coccolithophore calcite production (CP) and dilution experiments to determine microzooplankton (≤63 µm) grazing rates were made during a spring cruise (April 2015) at the Central Celtic Sea (CCS), shelf edge (CS2), and within an adjacent April bloom of the coccolithophore Emiliania huxleyi at station J2. CP at CCS ranged from 10.4 to 40.4 µmol C m−3 d−1 and peaked at the height of the spring phytoplankton bloom (peak chlorophyll-a concentrations ∼6 mg m−3). Cell normalised calcification rates declined from ∼1.7 to ∼0.2 pmol C cell−1 d−1, accompanied by a shift from a mixed coccolithophore species community to one dominated by the more lightly calcified species E. huxleyi and Calciopappus caudatus. At the CCS, coccolithophore abundance increased from 6 to 94 cells mL−1, with net growth rates ranging from 0.06 to 0.21 d−1 from the 4th to the 28th April. Estimates of intrinsic growth and grazing rates from dilution experiments, at the CCS ranged from 0.01 to 0.86 d−1 and from 0.01 to 1.32 d−1, respectively, which resulted in variable net growth rates during April. Microzooplankton grazers consumed 59 to &gt;100% of daily calcite production at the CCS. Within the E. huxleyi bloom a maximum density of 1986 cells mL−1 was recorded, along with CP rates of 6000 µmol C m−3 d−1 and an intrinsic growth rate of 0.29 d−1, with ∼80% of daily calcite production being consumed. Our results show that microzooplankton can exert strong top-down control on both bloom and non-bloom coccolithophore populations, grazing over 60% of daily growth (and calcite production). The fate of consumed calcite is unclear, but may be lost either through dissolution in acidic food vacuoles, and subsequent release as CO2, or export to the seabed after incorporation into small faecal pellets. With such high microzooplankton-mediated mortality losses, the fate of grazed calcite is clearly a high priority research direction.</p

Effect of bumetanide on toluene diisocyanate induced contractions in guinea pig airways.

BACKGROUND: The loop diuretic frusemide has been shown to inhibit the bronchoconstrictor response to exercise, inhaled allergen, distilled water, adenosine, and sodium metabisulphite. Toluene diisocyanate contracts smooth muscle by activating capsaicin sensitive nerves and causes asthma that shares many features with allergen induced asthma. METHODS: The study was designed to assess the effect of two loop diuretics, bumetanide (10 and 100 microM) and frusemide (100 microM), on smooth muscle contraction induced by toluene diisocyanate (0.03-1000 microM) in guinea pig airways with and, in the case of bumetanide, without epithelium. The effect of bumetanide on the response to acetylcholine, neurokinin A, and electrical field stimulation in guinea pig bronchial smooth muscle rings was also examined. RESULTS: Bumetanide (10 and 100 microM) had no effect on toluene diisocyanate induced contraction whether airway epithelium was present or not. Frusemide (100 microM) caused no significant inhibition of toluene diisocyanate induced contraction (mean reduction on the entire curve 25%). Bumetanide inhibited non-adrenergic, non-cholinergic contraction induced by electrical field stimulation of bronchi pretreated with atropine (1 microM) and indomethacin (5 microM) and this inhibition was inversely related to the frequency of stimulation, suggesting that bumetanide may be inhibiting transmitter release at the prejunctional level. Bumetanide and frusemide did not inhibit the responses to exogenous acetylcholine (0.1 microM) or neurokinin A (1 nM). CONCLUSIONS: Bumetanide and frusemide in doses that are known to inhibit non-adrenergic, non-cholinergic contraction due to electrical field stimulation failed to inhibit the response to toluene diisocyanate in guinea pig airways