Corresponding author: Rosabruna La Ferla ( rosabruna.laferla@iamc.cnr.it ) Academic editor: Antonella Lugliè
© 2019 Maurizio Azzaro, Theodore T. Packard, Luis Salvador Monticelli, Giovanna Maimone, Alessandro Ciro Rappazzo, Filippo Azzaro, Federica Grilli, Ermanno Crisafi, Rosabruna La Ferla.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Azzaro M, Packard TT, Monticelli LS, Maimone G, Rappazzo AC, Azzaro F, Grilli F, Crisafi E, La Ferla R (2019) Microbial metabolic rates in the Ross Sea: the ABIOCLEAR Project. In: Mazzocchi MG, Capotondi L, Freppaz M, Lugliè A, Campanaro A (Eds) Italian Long-Term Ecological Research for understanding ecosystem diversity and functioning. Case studies from aquatic, terrestrial and transitional domains. Nature Conservation 34: 441-475. https://doi.org/10.3897/natureconservation.34.30631
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The Ross Sea is one of the most productive areas of the Southern Ocean and includes several functionally different marine ecosystems. With the aim of identifying signs and patterns of microbial response to current climate change, seawater microbial populations were sampled at different depths, from surface to the bottom, at two Ross Sea mooring areas southeast of Victoria Land in Antarctica. This oceanographic experiment, the XX Italian Antarctic Expedition, 2004-05, was carried out in the framework of the ABIOCLEAR project as part of LTER-Italy. Here, microbial biogeochemical rates of respiration, carbon dioxide production, total community heterotrophic energy production, prokaryotic heterotrophic activity, production (by 3H-leucine uptake) and prokaryotic biomass (by image analysis) were determined throughout the water column. As ancillary parameters, chlorophyll a, adenosine-triphosphate concentrations, temperature and salinity were measured and reported. Microbial metabolism was highly variable amongst stations and depths. In epi- and mesopelagic zones, respiratory rates varied between 52.4–437.0 and 6.3–271.5 nanol O2 l-1 h-1; prokaryotic heterotrophic production varied between 0.46–29.5 and 0.3–6.11 nanog C l-1 h-1; and prokaryotic biomass varied between 0.8–24.5 and 1.1–9.0 µg C l-1, respectively. The average heterotrophic energy production ranged between 570 and 103 mJ l-1 h-1 in upper and deeper layers, respectively. In the epipelagic layer, the Prokaryotic Carbon Demand and Prokaryotic Growth Efficiency averaged 9 times higher and 2 times lower, respectively, than in the mesopelagic one. The distribution of plankton metabolism and organic matter degradation was mainly related to the different hydrological and trophic conditions. In comparison with previous research, the Ross Sea results, here, evidenced a relatively impoverished oligotrophic microbial community, throughout the water column.
Microbial respiration, heterotrophic production, heterotrophic energy production, Ross Sea, Antarctica, LTER
The present work aims to explore the carbon fate through microbes in an area of the Ross Sea (RS) and to identify signs and patterns of microbial responses to current climate change.
The Southern Ocean (SO) plays an important role in world climate since it is considered the engine of the worldwide oceanic currents. The dramatic seasonal variability in environmental factors in SO generates a significant stress on the biota which must endure constant sunlight, oscillating temperatures and melting ice phenomena in spring-summer time. The annual pelagic primary production is largely confined to this period, when light and nutrient conditions are favourable for the phytoplankton growth. Studies on global change have revealed that the SO is becoming a larger carbon sink with over 30–40% of total carbon uptake occurring there (
One of the most productive and peculiar area of the SO continental shelf zone is the RS (
Although microbes constitute the sentinel of ecosystem evolution (
Still, none of the earlier work calculated energy production from the biochemical processes that produce this energy. They used distantly related proxies such as biomass or, the more related one, heat production (
In order to analyse the role of prokaryotic metabolism as a regulator of the organic carbon budget, seawater samples were taken in a quadrilateral area of the RS where four mooring sites were located. Fifteen stations were sampled in the epipelagic layer (from surface to 100 m) and in the mesopelagic one (from > 100 m to 800 m) to broaden the evaluation of the whole area.
The prokaryotic biomass (PB) was detected by Image Analysis cell counts and volume measurements. The ETS assay was adopted to calculate respiration rates of microplankton (< 200 µm) in terms of oxygen utilisation (OUR), carbon dioxide production (CDPR) rates and heterotrophic energy production (HEP). In some stations, the prokaryotic heterotrophic activity and the heterotrophic carbon production by 3H-leucine uptake (PHA and PHP, respectively) were measured. Ancillary parameters were chlorophyll a (CHL), adenosine triphosphate (ATP) concentrations and the hydrological parameters. We tried to simultaneously analyse the bulk C metabolism of prokaryotic assemblage, by direct measurement of different independent parameters (respiration and heterotrophic production) and their interconnections (HEP, PCD and PGE).
The aims of the paper were 1) to monitor the role of microbes as regulators of the organic carbon transfer in the biogeochemical processes, 2) to compare the obtained data with other surveys in RS and 3) to use microbial respiratory and metabolic activity patterns as proxies to describe microbial ecosystem trends.
During the XX Italian PNRA (National Programme of Antarctic Research, year 2004/05) expedition, in the framework of the ABIOCLEAR project (Antartic BIOgeochemical cycles-CLimatic and palEoclimAtic Reconstructions), an oceanographic cruise was carried out from 4 January to 14 February 2005 aboard the Italian R/V Italica. In a quadrilateral area between four mooring sites (mooring A and B, monitored in the framework of LTER-Italy activity in Antarctica and moorings H and D), a total of fifteen stations were sampled throughout the water column, from surface to 800 m depth, using a Rosette sampler with 24, 12 l, Niskin bottles. The Rosette was mounted on a CTD equipped with a Sea-Bird 9/11 plus multiparametric probe (SeaBird Electronics) that sensed temperature (T), conductivity (salinity, S) and dissolved oxygen (DO [SBE 43]). In Figure
Map of the study area within the Ross Sea in the framework of the ABIOCLEAR Project in Summer 2005. The sampling stations are included within the polygon delimited by four mooring stations.
Station names, sampling dates, geographical coordinates, maximum depths and basic parameters. PA= prokaryotic abundance; PB= prokaryotic biomass; CHLa= chorophyll a; ATP= adenosine triphosphate; ETS= electron transport system activity; PHA= prokaryotic heterotrophic activity.
Station | Date | Latitude | Longitude | Depth | Studied basic parameters |
---|---|---|---|---|---|
Abio09-D | 1/10/2005 | 75°06.77'S, 164°25.55'E | 1002 | PA, PB, CHLa, ATP, ETS | |
Abio10 | 1/11/2005 | 75°20.96'S, 166°54.77'E | 461 | PA, PB, CHLa, ATP, ETS | |
Abio19 | 1/16/2005 | 75°50.81'S, 167°14.62'E | 590 | PA, PB, CHLa, ATP, ETS | |
Abio22-A | 1/17/2005 | 76°41.49'S, 169°04.74'E | 789 | PA, PB, CHLa, ATP, ETS, PHA | |
Abio01-B | 1/30/2005 | 74°00.53'S, 175°05.67'E | 590 | PA, PB, CHLa, ATP, ETS, PHA | |
H1 | 2/1/2005 | 75°58.20'S, 177°17.64'E | 616 | PA, PB, CHLa, ATP, ETS, PHA | |
Abio05 | 2/2/2005 | 75°00.00'S, 178°19.92'E | 390 | PA, PB, CHLa, ATP, ETS | |
Abio02 | 2/3/2005 | 74°17.85'S, 171°35.14'E | 458 | PA, PB, CHLa, ATP, ETS | |
Abio06 | 2/4/2005 | 74°57.31'S, 174°35.19'E | 400 | PA, PB, CHLa, ATP, ETS, PHA | |
Abio07 | 2/5/2005 | 75°04.92'S, 171°44.70'E | 548 | PA, PB, CHLa, ATP, ETS | |
Abio20 | 2/6/2005 | 76°06.97'S, 170°09.72'E | 605 | PA, PB, CHLa, ATP, ETS, PHA | |
Abio35 | 2/8/2005 | 76°30.67'S, 172°17.60'E | 639 | PA, PB, CHLa, ATP, ETS | |
Abio21 | 2/8/2005 | 76°14.06'S, 179°06.10'E | 350 | PA, PB, CHLa, ATP, ETS | |
Abio16 | 2/9/2005 | 75°58.39'S, 176°29.49'E | 454 | PA, PB, CHLa, ATP, ETS | |
Abio17 | 2/9/2005 | 75°50.94'S, 173°35.09'E | 374 | PA, PB, CHLa, ATP, ETS |
For ATP measurements, 1 l of seawater was prefiltered through a 250 µm net and then filtered through a 0.22 µm membrane filter. The filter was immediately plunged into 3 ml boiling TRIS–EDTA phosphate buffer (pH 7.75) and the ATP was extracted at 10 °C for 3 min and kept frozen (-20 °C) until laboratory analysis in Italy (
CHLa concentrations, as an index of phytoplankton biomass, were determined in the water column from surface to a maximum of 160 m depth. The water samples (1 l) were filtered on Whatman GF/F glass-fibre filters, according to
The Trophic State Index (TSI), applied to classify the stations according to their algal biomass, was calculated from the chlorophyll measurements (
Samples for prokaryotic abundance (PA; including bacteria, archaea and cyanobacteria) were collected into sterile Falcon vials (50 ml). Each sample was immediately fixed in pre-filtered formaldehyde (0.2 µm porosity; 2% final concentration) and stored at 4 °C until analysis. Within three months, two replicates of each sample were filtered through polycarbonate black membrane filters (0.2 µm porosity; GE Water & Process Technologies) and stained for 10 min with 4’,6-diamidino-2-phenylindole (DAPI, Sigma, final concentration 10 µg ml-1) according to
ETS measurements and relative rates
Respiratory rates were quantified according to the tetrazolium reduction technique (
The specific standard deviation (i.e. the percentage of the standard deviation of the replicates on the average value of the same replicates), due to the analytical procedures and sample handling, was about 35%.
ETS (μl O2 l-1 h-1) was considered equal to the respiration rate in the epipelagic zone and converted to respiratory Carbon Dioxide Production Rates (CDPR) (μg C l-1 d-1) by using the following (Eq.1):
CDPR = (ETS*12/22.4) * (122/172) (Eq.1)
where 12 is the C atomic weight, 22.4 the O2 molar volume and 172/122 the Takahashi oxygen/carbon molar ratio (
Cell specific respiratory rates (CSRR) were calculated by dividing normalised CDPR values to the normalised cell abundance values in each station by adopting a prokaryotic contribution of 50% and 80% at epipelagic and mesopelagic layers, respectively, assuming that the activities we measured were mainly due to the prokaryotic fraction and that all the cells have similar activity levels.
Heterotrophic energy production (HEP) determination
Today, the P/O ratio is thought to be closer to 2.5 rather than 3.0 (
HEP (µJ h-1 l-1) = R (µmol O2 h-1 l-1) * 2 * 2.5 * 0.048 * 10-6 (Eq.2)
where 2 is the number of electron pairs that participate in reduction of one molecule of O2 to two molecules of H2O; 2.5 is the modified P/O of
Heterotrophic Energy production (HEP) and adenosine triphosphate (ATP) turnover time in microplankton in the Ross Sea water column.
Pelagic Zone | Depth Interval | Potential Respiration (Φ) | ATP | Respiration | HEP (ATP Production) | HEP (Energy Production) | ATP Turnover Time (τ) |
(m) | (µmol O2 l-1 h-1) | (ng l-1) | (µmol O2 l-1 h-1) | (µmol l-1 h-1) | (mJ l-1 h-1) | (min) | |
Euphotic | 2–100 | 9.13 ± 5.93 | 123 ± 72.2 | 2.38 ± 1.54 | 11.88 ± 7.71 | 570 ± 370 | 1.25 ± 0.76 |
Epipelagic | 2–160 | 8.97 ± 3.27 | 110 ± 71.0 | 2.33 ± 0.85 | 11.67 ± 4.25 | 560 ± 204 | 1.15 ± 0.77 |
Mesopelagic Upper (A) | 100–500 | 4.02 ± 3.49 | 30.35 ± 31.68 | 1.05 ± 0.91 | 5.23 ± 4.54 | 251 ± 218 | 0.74 ± 0.49 |
Mesopelagic Upper (B) | 160–500 | 3.03 ± 3.19 | 27.49 ± 32.58 | 0.86 ± 0.83 | 4.31 ± 4.15 | 207 ± 199 | 0.78 ± 0.53 |
Mesopelagic lower | 500–800 | 1.65 ± 0.88 | 13.48 ± 6.76 | 0.43 ± 0.23 | 2.15 ± 1.15 | 103 ± 55 | 0.76 ± 0.53 |
Prokaryotic heterotrophic activity (PHA) and production (PHP)
PHA was evaluated by 3H-leucine incorporation rate assay using the microtubes method described by
Prokaryotic heterotrophic production (PHP) was calculated from the 3H-leucine incorporation rate (PHA) expressed in moles incorporated per unit time and volume (
Isotopic Dilution (ID) detected from leucine incorporation rates (Vmax) in samples Abio19 (45 and 300 m depth) and Abio05 (20 and 300 m depth), respectively. ID mean value 1.25 ± 0.14.
Station | depth (m) | V max ± SD p mol l-1 h-1 | ID | confidence interval 95% |
---|---|---|---|---|
Abio19 | 45 | 18.058 ± 0.870 | 1.06 | 0.95–1.18 |
300 | 0.357 ± 0.069 | 1.24 | 1.00–1.48 | |
Abio05 | 20 | 9.785 ± 4.813 | 1.37 | 0.69–2.04 |
300 | 0.932 ± 0.225 | 1.34 | 1.01–1.66 |
PHP was expressed as production of biomass (as C) per time unit and volume.
Derived parameters as Specific Growth Rate d-1 [SGR (µ) = PHP/PB (Prokaryotic biomass)] and Biomass Turnover Time (days) [BTT (g) = ln(2)/µ] were calculated according to
In comparison, PHPD, PBD, SGRD and BTTD were calculated using 107 fg C µm-3 cell and an ID = 1 according to
The prokaryotic C requirement was computed as Prokaryotic Carbon Demand (PCD), i.e. PHP+CDPR by using normalised data, assuming the contribution of prokaryotes to total microbial community respiration as 50% in the epipelagic layer and a contribution of 80% in the mesopelagic ones (
In order to detect possible influences between environmental factors and microbial variables, Spearman Rank correlation coefficients were calculated for the microbiological data and environmental parameters using the SigmaStat software V3.0 and the Mann and Whitney test using the PAST.exe (
Data were integrated with depth according to the trapezoidal method and normalised to the depth: from 2 to 100 m for the epipelagic layer; from 100 to 800 m for the mesopelagic layer.
The depth-integrated rate (ʃR dz in mg C m2 d-1 l-1) for the water column was calculated within the depth interval between Z1 and Z2 using the following equation (Eq. 3):
ʃR dz = y (Z2(x+1) – Z1(x+1) / (x+1) (Eq.3)
Temperature ranged between -2.01 (H1, 500 m) and 1.48 °C (Abio09-D, 5 m) and salinity between 34.11 (Abio10, 25 m) and 34.79 (Abio09-D, 600 m). In Suppl. material
In Table
Range, mean and standard deviations, sampling numbers (n) of trophic parameters (CHLa, C-CHLa, ATP and C-ATP), prokaryotic abundance and biomass (PA and PB) and metabolic rates (ETS, CDPR and PHP); detected in the epipelagic (0–100 m) and mesopelagic (>100<800 m) depth layers. CHLa= cholrophyll a; C-CHLa= cholrophyll a in carbon units; ATP= adenosine triphosphate; C-ATP= adenosine triphosphate in carbon units; PA= prokaryotic abundance; PB= prokaryotic biomass; ETS= electron transport system activity; CDPR= carbon dioxide production rate; PHP= carbon prokaryotic heterotrophic production.
CHLa | C-CHLa | ATP | C-ATP | PA | PB | ETS | CDPR | PHP | |
---|---|---|---|---|---|---|---|---|---|
mg m-3 | mg C m-3 | ng l-1 | ng C l-1 | cells ml-1 | µg C l-1 | µl O2 l-1 h-1 | µg C l-1 h-1 | µg C l-1 h-1 | |
0–100 m depth | |||||||||
min | 0.019 | 1.89 | 42.76 | 10689 | 5.70E+4 | 0.8 | 0.052 | 0.0043 | 0.0005 |
max | 0.539 | 53.90 | 380.49 | 95123 | 1.44E+6 | 24.5 | 0.437 | 0.1660 | 0.0295 |
mean | 0.141 | 14.11 | 123.78 | 30944 | 2.87E+5 | 5.2 | 0.201 | 0.0740 | 0.0100 |
SD | 0.116 | 11.56 | 70.26 | 17564 | 2.55E+5 | 4.5 | 0.076 | 0.0323 | 0.0077 |
n | 67 | 67 | 39 | 39 | 67 | 67 | 66 | 66 | 25 |
100–800 m depth | |||||||||
min | 0.007 | 0.67 | 4.77 | 1193 | 5.66E+4 | 0.8 | 0.006 | 0.0002 | 0.0003 |
max | 0.244 | 24.44 | 130.69 | 32673 | 6.76E+5 | 15.1 | 0.271 | 0.0965 | 0.0061 |
mean | 0.058 | 5.77 | 29.04 | 7260 | 1.51E+5 | 3.1 | 0.086 | 0.0122 | 0.0019 |
SD | 0.051 | 5.14 | 30.66 | 7664 | 8.45E+4 | 1.9 | 0.076 | 0.0244 | 0.0015 |
n | 25 | 25 | 47 | 47 | 78 | 78 | 81 | 81 | 36 |
The average CHLa value in the epipelagic layer was 2.4 times higher than in the mesopelagic one. In Figure
Depth profiles of mean values and standard deviations of the carbon content derived from chlorophyll a (C-CHLa, a) and adenosine triphosphate (C-ATP, b), the prokaryotic biomass (PB, c) and the carbon dioxide production rates derived from ETS activity (CDPR, d) in the Ross Sea water column.
ATP sharply decreased with depth (Figure
The calculation of the average TSI, chosen to establish the station trophic state, ranged from a high of 24 at H1, the most offshore station, to a low of 16 at Abio20, a station much closer to shore (Suppl. material
Prokaryote Abundance (PA) was in the order of 104–106 cell ml-1 in the epipelagic layer and 104–105 cell ml-1 in the mesopelagic layer (Table
ETS showed a decreasing trend with depth and the values in the epipelagic layer were 3 times higher than in the mesopelagic one (Table
CDPR showed a decreasing trend with depth with a discrete variability in the deep layers (Figure
The cell specific respiratory rate (CSRR) calculated on depth-integrated and normalised data (Figure
Cell Specific Respiratory Rates (CSRR) and SD for each station in the epipelagic and mesopelagic layers (0–100 m, 100–800 m). In the upper and deeper layers, the prokaryotic contributions to total respiration were considered to be the 50% and 80%, respectively, of the total carbon dioxide production rates (CDPR).
The HEP calculations for the epipelagic and mesopelagic waters of RS are given in Table
The prokaryotic heterotrophic activity (PHA) – in term of leucine incorporation rates – varied between 0.213 and 19.035 pmol l-1 h-1 (Figure
Leucine incorporation rates (PHA expressed in nmol m-3 h-1) integrated in the depth intervals 1–100 m (epipelagic layer) and 100 m-bottom (mesopelagic layer) and normalized.
Station | depth (m) | PHA | |
1–100 m | 100 m – botton (m) | ||
Abio22-A | 789 | 12.420 | 2.093 (600) |
Abio01-B | 590 | 2.651 | 0.463 (585) |
H1 | 616 | 5.398 | 0.799 (600) |
Abio06 | 400 | 3.805 | 2.589 (390) |
Abio20 | 605 | 3.926 | 1.353 (500) |
PHP showed the same distribution of PHA throughout the water column. In the epipelagic layer, PHP varied between 0.46 and 29.51 ng C l-1 h-1 with the highest value at station Abio22-A at 25 m depth. In mesopelagic waters, PHP varied 0.33 and 6.11 ng C l-1 h- 1 and it was, on average, 5 times lower than in upper layer (Table
Prokaryotic heterotrophic production (PHP), Prokaryotic biomass (PB), Prokaryotic specific growth rate (SGR) and Biomass turnover time (BTT) calculated in the five indagated stations (ABIO22, ABIO01, H1, ABIO06 and ABIO20). PHPD, PBD, SGRD and BTTD were calculated using a different factor (107 fg C µm-3 cell-1 and ID = 1) according to
Depth | n | Mean | SD | Range | |
---|---|---|---|---|---|
PHP (ng C l-1 h-1) | 0–100 m | 22 | 12.182 | 9.492 | 0.575–36.883 |
100–800 m | 26 | 2.027 | 1.629 | 0.413–6.441 | |
PHPD (ng C l-1 h-1) | 0–100 m | 22 | 9.746 | 7.593 | 0.460–29.506 |
100–800 m | 26 | 1.622 | 1.303 | 0.330–5.153 | |
PB (µg C l-1) | 0–100 m | 20 | 5.179 | 3.594 | 1.921–15.127 |
100–800 m | 25 | 3.222 | 1.939 | 1.074–9.028 | |
PBD (µg C l-1) | 0–100 m | 20 | 1.735 | 1.242 | 0.608–5.125 |
100–800 m | 25 | 1.077 | 0.676 | 0.338–3.101 | |
SGR day-1 | 0–100 m | 20 | 0.053 | 0.047 | 0.005–0.183 |
100–800 m | 25 | 0.016 | 0.011 | 0.003–0.038 | |
SGRD day-1 | 0–100 m | 20 | 0.128 | 0.116 | 0.013–0.446 |
100–800 m | 25 | 0.040 | 0.027 | 0.006–0.095 | |
BTT (days) | 0–100 m | 20 | 28.12 | 30.43 | 3.79–128.31 |
100–800 m | 25 | 65.29 | 50.44 | 18.24–254.05 | |
BTTD (days) | 0–100 m | 20 | 11.63 | 12.50 | 1.55–52.04 |
100–800 m | 25 | 27.458 | 23.152 | 7.26–118.26 |
The normalised prokaryotic C demand (PCD) ranged between 0.035 and 0.055 mg C h-1 m-3 in the epipelagic layer with high values at Abio22-A and Abio06. In the mesopelagic layer, it ranged between 0.003 and 0.008 mg C h-1 m-3 with the highest value at Abio20 (Table
Prokaryotic carbon demand (PCD) and prokaryptic growth efficiency (PGE) depth-integrated and normalized values in the epi- and mesopelagic depth layers.
Station | 0–100 m depth | 100–800 m depth | ||
---|---|---|---|---|
PCD | PGE | PCD | PGE | |
mg C m-3 h-1 | % | mg C m-3 h-1 | % | |
Abio09-D | – | – | 0.0069 | 26.23 |
Abio22-A | 0.051 | 37.82 | 0.0052 | 60.21 |
Abio01-B | 0.0354 | 11.62 | 0.0052 | 22.28 |
H1 | 0.0417 | 19.87 | 0.0045 | 27.58 |
Abio06 | 0.0551 | 18.99 | 0.0029 | 26.21 |
Abio20 | 0.0457 | 13.31 | 0.0079 | 26.71 |
Abio16 | – | – | 0.0037 | 88.49 |
The normalised PGE ranged between 12 and 37% in epipelagic layer, with the lowest value at Abio01-B and the highest at Abio22-A. In the mesopelagic layer, PGE ranged between 22 and 88% with the minimum value at station Abio01-B and the maximum at Abio16. In contrast to the PCD, the averaged PGE was twice as high in the mesopelagic zone as in the epipelagic one (Kruskal-Wallis One-Way ANOVA: P < 0.048).
The Spearman-Rank correlation analysis of the whole dataset yielded the outputs shown in Table
Spearman–Rank correlations among microbial and environmental parameters in the whole data set. O2= dissolved oxygen; S= salinity; DEN= density; PA= prokaryotic abundance; PB= prokaryotic biomass; CCC= cell carbon content; ATP= adenosine triphosphate; CHLa= chorophyll a; PHP= prokaryotic heterotrophic production; CDPR= carbon dioxide production rates.
CHLa vs. | r | P | n | ATP vs. | r | P | n | ||||
Depth | -0.545 | 0.0000 | 80 | Depth | -0.836 | 0.0000 | 79 | ||||
T °C | 0.5 | 0.0000 | 80 | T °C | 0.776 | 0.0000 | 78 | ||||
O2 | 0.626 | 0.0000 | 73 | O2 | 0.543 | 0.0000 | 67 | ||||
S | -0.505 | 0.0000 | 73 | S | -0.762 | 0.0000 | 67 | ||||
DEN | -0.522 | 0.0000 | 73 | DEN | -0.747 | 0.0000 | 67 | ||||
PA | 0.347 | 0.0024 | 75 | PA | 0.399 | 0.0000 | 78 | ||||
PB | 0.318 | 0.0056 | 75 | CCC | -0.255 | 0.0243 | 78 | ||||
ATP | 0.899 | 0.0000 | 42 | PB | 0.229 | 0.0437 | 78 | ||||
PHP | 0.46 | 0.0139 | 28 | CHLa | 0.899 | 0.0000 | 42 | ||||
CDPR | 0.346 | 0.0028 | 73 | CDPR | 0.775 | 0.0000 | 76 | ||||
PB vs. | r | P | n | CDPR vs. | r | P | n | PHP vs. | r | P | n |
Depth | -0.285 | 0.0000 | 134 | Depth | -0.851 | 0.0000 | 136 | Depth | -0.683 | 0.0000 | 56 |
T °C | 0.197 | 0.023 | 134 | T °C | 0.714 | 0.0000 | 136 | T °C | 0.47 | 0.0000 | 56 |
O2 | 0.459 | 0.0000 | 122 | O2 | 0.607 | 0.0000 | 123 | O2 | 0.784 | 0.0000 | 47 |
S | -0.215 | 0.0174 | 122 | S | -0.731 | 0.0000 | 123 | S | -0.545 | 0.0000 | 47 |
DEN | -0.205 | 0.0237 | 122 | DEN | -0.726 | 0.0000 | 123 | DEN | -0.554 | 0.0000 | 47 |
PA | 0.898 | 0.0000 | 134 | PA | 0.304 | 0.0000 | 125 | PA | 0.662 | 0.0000 | 56 |
CCC | 0.258 | 0.0027 | 134 | CCC | -0.256 | 0.0040 | 125 | PB | 0.618 | 0.0000 | 56 |
ATP | 0.229 | 0.0437 | 78 | PB | 0.216 | 0.0157 | 125 | CHLa | 0.46 | 0.0139 | 28 |
CHLa | 0.318 | 0.0056 | 75 | CHLa | 0.346 | 0.0028 | 73 | ETS | 0.615 | 0.0000 | 54 |
PHP | 0.618 | 0.0000 | 56 | ATP | 0.775 | 0.0000 | 76 | CDPR | 0.657 | 0.0000 | 54 |
CDPR | 0.216 | 0.0157 | 125 | PHP | 0.657 | 0.0000 | 54 |
Stations and depth with minimum and maximum values of each parameters in the epi- and mesopelagic layers. CHLa= chorophyll a; ATP= adenosine triphosphate; PA= prokaryotic abundance; CCC= cell carbon content; PB= prokaryotic biomass; ETS= electron transport system activity; CDPR= carbon dioxide production rates; PHA= prokaryotic heterotrophic activity.
0–100 m | 100–800 m | |||
min | max | min | max | |
CHLa | Abio20 – 5m | Abio09(D) – 5m | Abio10 – 150m | Abio02 – 110m |
ATP | H1 – 100m | Abio09(D) – 5m | H1 – 500m | Abio02 – 110m |
PA | Abio05 – 100m | Abio09(D) – 25m | Abio22(A) – 100m | Abio07 – 500m |
CCC | Abio20 – 2m | H1 – 2m | Abio05 – 200m | Abio09(D) – 800m |
PB | Abio05 – 100m | Abio09(D) – 25m | Abio05 – 100m | Abio22(A) – 100m |
ETS | Abio22(A) – 80m | Abio10 – 25m | Abio16 – 400m | Abio10 – 150m |
CDPR | Abio16 – 2m | Abio10 – 25m | Abio16 – 400m | Abio17 – 120m |
HEP | Abio22(A) – 80m | Abio06 – 2m | Abio16 – 400m | Abio10 – 150m |
PHA | Abio01(B) – 100m | Abio22(A) – 25m | Abio06 – 270m | Abio16 – 250m |
The environmental assessment revealed a general picture of low trophism on a spatial scale as exemplified by the CHLa concentration. Analysis of the ATP concentrations also indicated modest or poor trophism. Considering ATP as a quantitative proxy for total living biomass (
PA was in a range comparable to similar measurements made in several Antarctic marine environments (see table S1 in
PB decreased with depth by a factor of 1.7. However, it was higher than other prokaryotic measurements made previously in the RS (Buitenhius et al. 2012,
The ETS assay, originally designed by
HEP calculations in the microplankton quantify the energy generation due to the decomposition of ATP by a group of enzymes (ATPases) in plasmalemma membranes of constituent bacteria and archaea as well as in constituent eukaryote mitochondrion. It represents a new metric in oceanographic analysis. The only other oceanic region, for which HEP has been calculated, is the Peru Current Upwelling at 15° S (Pisco, Peru). The HEP calculations for the epipelagic and mesopelagic waters of the Peru Upwelling at 15° S are given in
Time course experiments on PHA showed results in agreement with those detected in the VLTP-2004 project (Monticelli, personal communication). Linearity occurred within 1 and 6 hours for samples collected at 25 m depth and in a smaller time lapse for the others. Along the water column, higher PHA was always observed in the photic layers while reduced activity in the aphotic waters occurred. An increase in activity was always observed in the bottom samples, i.e. those taken a few metres from the sea floor. The increase of heterotrophic bacteria metabolisms in benthic boundary layers is a known phenomenon observed in other water columns (
At station Abio22-A, PHA was particularly high at all depths, with the highest normalised rates (12.442 nmol m-3 h-1) in the photic layer. This was one order of magnitude higher than equivalent normalised PHA calculations observed at the other four stations. The mean leucine incorporation rate observed in the 0 - 50 m layer was 8.05 pmol l-1 h-1 (sd = 4.52, n = 15), the same order of magnitude as observed by
In our experiment, an isotope dilution of 1.25 was used to calculate the CF. It was equivalent to 1.94 kg C mol leu-1. Considering the variability observed in ID determinations and the coefficient of variation (CV%) detected in the triplicate samples for leucine incorporation-rate analysis (mean = 14.3%, sd = 11.3%, n = 84), the ID should be not too far from 1 (assuming no isotope dilution). That value corresponds to a theoretical CF = 1.55 kg C mol leu-1 (
In the euphotic layer, SGR d-1 calculated with our CFs, resulted 2.4 times lower than SGRD d-1 calculated using the conversion factors used by
From mean hourly values, the CDPR/PHP ratios in the epi- and mesopelagic layers were 11.75 and 0.80 µg C l-1 h-1, respectively.
Prokaryotic (bacterial and archaeal) activity is often measured using the PGE that defines the balance between catabolic and anabolic prokaryotic processes (
However, using mean hourly normalised values, the ratio CDPR/PHP (µg C l-1 h-1) in the epipelagic layer was 3 times higher than in the mesopelagic one, presumably due to the occurrence of autotrophic respiration (
The significant relationship between CDPR and other physical and chemical parameters measured, suggests that respiration is strictly interconnected with environmental forces. Respiration varied in response to changes in hydrology according to
In all the stations, CSRR was surprisingly higher in the epipelagic layer than in the mesopelagic one. This suggests that a valuable contribution of organic matter of phytoplanktonic origin might sustain the heterotrophic metabolism in the upper layer. When we calculate CSRR by ETS Vmax, i.e. without utilisation of ETS to carbon conversion factors, almost all the mesopelagic values would be lower than surface ones with averaged CSRR value of 0.34 and 0.29 fg C cell-1 in the epi- and mesopelagic layers, respectively (data not shown). Although different from other reports from temperate seas (
The cell-specific incorporation rate (CSIR) strictly reflected the distribution of PHA throughout the water column with the highest values in the surface to 50 m depth layer. The average CSIR was similar to that detected by
Overall, in Summer 2005, the investigated area of the RS contributed in different ways to the epi- and mesopelagic layer carbon metabolism. Per sea-surface area, the autotrophic (by C-CHLa), prokaryotic (PB) and total standing stocks (C-ATP) amounted to 1545, 1681 and 4605 mg C m-2, respectively. The remaining heterotrophic component (HB) presumably accounted for a biomass of 1379 mg C m-2. The prokaryotic biomass appeared to be predominant in the mesopelagic layer with respect to the epipelagic one (depth integrated PB ratio epi/meso: 0.4). The entire heterotrophic production accounted for 1.697 mg C m-2 h-1 with a similar weight in the epi- and mesopelagic layers (depth integrated PHP ratio epi/meso was 1.03). Respiration remineralised 8.279 mg C m-2 h-1 with higher rates in the upper layers (depth integrated CDPR ratio epi/meso was 2.7).
This study was carried out within a time series of research conducted since the nineties in the Ross Sea. Through their metabolic rates, microorganisms worked as regulators of the organic carbon transfer in the Ross Sea and impacted Antarctic biogeochemical cycles. In this experiment, highly variable microbial metabolism was detected at all stations and depth layers. At the same time, coherent metabolic patterns were detected using different, independent, methodological approaches. The distribution of plankton metabolism and organic matter degradation was mainly related to the general oligotrophic conditions occurring during Summer 2005. The processes of heterotrophic production, respiration and growth efficiency revealed relatively low levels of carbon remineralisation. Compared with other cruises carried out in the Ross Sea, dramatic changes were found on an inter-annual scale. Monitoring the heterotrophic microbial patterns in long term series is proving to be an interesting approach in furthering understanding of biogeochemical trends. In contexts such as the mooring sites of LTER-Italy, it needs to be better known due to the climate-change implication of Antarctic Ocean on the global scale.
The research was funded by the XX Italian PNRA (National Programme of Antarctic Research, year 2004/05) expedition in the framework of the ABIOCLEAR project (Antartic BIOgeochemical cycles-CLimatic and palEoclimAtic Recostructions, coord. Dr. Mariangela Ravaioli of CNR-ISMAR, Institute of Marine Science) and received the financial support of P-ROSE project (Plankton biodiversity and functioning of the ROss Sea Ecosystems in a changing southern ocean, funded by PNRA, National Programme of Antarctic Research, year 2016/18, coord. Prof. Olga Mangoni of CoNISMa, National Interuniversity Consortium for Marine Sciences) and of CELEBeR project (CDW effects on glacial melting and on bulk of Fe in the Western Ross Sea, funded by PNRA, National Programme of Antarctic Research, year 2016/18, coord. Prof. Paola Rivaro of University of Genoa). The authors thank all the staff of R/V Italica for the logistics help and support. PHA saturation curves analysis and time courses were fixed in a precedent project (Victoria Land Transect Project, VLTP-2004) funded by XIX PNRA Expedition. The T.T. Packard’s contribution was supported by TIAA-CREF (USA) and Social Security (USA). The authors also thank the Editor and reviewers for their very useful comments and Mr. Alessandro Cosenza (ISP Institute of Polar Science) for figure processing.