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Dark CO 2 fixation by bacteria is believed to be particularly important in oligotrophic ecosystems. However, only a few studies have characterized the role of bacterial dissolved inorganic carbon (DIC) fixation in global carbon dynamics. Therefore, this study quantified the primary production (PP), total bacteria dark CO 2 fixation (TB DIC fixation), and heterotrophic bacterial production (HBP) in the warm and oligotrophic Red Sea using stable-isotope labeling and cavity ring-down spectroscopy (13 C–CRDS). Additionally, we assessed the contribution of bacterial DIC fixation (TB DIC %) relative to the total DIC fixation (total DIC fixation). Our study demonstrated that TB DIC fixation increased the total DIC fixation from 2.03 to 60.45 µ g C L -1 d -1 within the photic zone, contributing 13.18 % to 71.68 % with an average value of 33.95 ± 0.02 % of the photic layer total DIC fixation. The highest TB DIC fixation values were measured at the surface and deep (400 m) water with an average value of 5.23 ± 0.45 and 4.95 ± 1.33 µ g C L -1 d -1 , respectively. These findings suggest that the non-photosynthetic processes such as anaplerotic DIC reactions and chemoautotrophic CO 2 fixation extended to the entire oxygenated water column. On the other hand, the percent of TB DIC contribution to total DIC fixation increased as primary production decreased (R2=0.45 , p<0.0001), suggesting the relevance of increased dark DIC fixation when photosynthetic production was low or absent, as observed in other systems. Therefore, when estimating the total carbon dioxide production in the ocean, dark DIC fixation must also be accounted for as a crucial component of the carbon dioxide flux in addition to photosynthesis. [ABSTRACT FROM AUTHOR]

Dark CO2 fixation by bacteria is believed to be particularly important in oligotrophic ecosystems. However, only a few studies have characterized the role of bacterial dissolved inorganic carbon (DIC) fixation in global carbon dynamics. Therefore, this study quantified the primary production (PP), total bacteria dark CO2 fixation (TBDIC fixation), and heterotrophic bacterial production (HBP) in the warm and oligotrophic Red Sea using stable isotope labeling and cavity ring-down spectroscopy (13C-CRDS). Additionally, we assessed the contribution of bacterial DIC fixation (TBDIC %) relative to the total DIC fixation (TotalDIC fixation). Our study demonstrated that TBDIC fixation increased the TotalDIC fixation from 2.03 to 60.45 µg C L-1 d-1 within the photic zone, contributing 13.18 % to 71.68 % with an average value of 33.95 ± 0.02 % of the photic layer TotalDIC fixation. The highest TBDIC fixation values were measured at the surface and deep (400 m) water with an average value of 5.23 ± 0.45 µg C L-1 d-1, and 4.95 ± 1.33 µg C L-1 d-1, respectively. These findings suggest that the non-photosynthetic processes such as anaplerotic DIC reactions and chemo-autotrophic CO2 fixation extended to the entire oxygenated water column. On the other hand, the % of TBDIC contribution to TotalDIC fixation increased as primary production decreased (R² = 0.45, p <0.0001), suggesting the relevance of increased dark DIC fixation when photosynthetic production was low or absent, as observed in other systems. Therefore, when estimating the total carbon dioxide production in the ocean, dark DIC fixation must also be accounted as a crucial component of the carbon dioxide flux in addition to photosynthesis. [ABSTRACT FROM AUTHOR]

Resolving the environmental drivers shaping planktonic communities is fundamental for understanding their variability, in the present and the future, across the ocean. More specifically, addressing the temperature-dependence response of planktonic communities is essential as temperature plays a key role in regulating metabolic rates and thus potentially defining the ecosystem functioning. Here we quantified plankton metabolic rates along the Red Sea, a uniquely oligotrophic and warm environment, and analysed the drivers that regulate gross primary production (GPP), community respiration (CR), and net community production (NCP). The study was conducted on six oceanographic surveys following a north–south transect along the Saudi Arabian coast. Our findings revealed that GPP and CR rates increased with increasing temperature (R2=0.41 and 0.19, respectively; p<0.001 in both cases), with a higher activation energy (Ea) for GPP (1.20±0.17 eV) than for CR (0.73±0.17 eV). The higher Ea for GPP than for CR resulted in a positive relationship between NCP and temperature. This unusual relationship is likely driven by the relatively higher nutrient availability found towards the warmer region (i.e. southern Red Sea), which favours GPP rates above the threshold that separates autotrophic from heterotrophic communities (1.7 mmol O2 m -3 d -1) in this region. Due to the arid nature, the basin lacks riverine and terrestrial inputs of organic carbon to subsidise a higher metabolic response of heterotrophic communities, thus constraining CR rates. Our study suggests that GPP increases steeply with increasing temperature in the warm ocean when relatively high nutrient inputs are present. [ABSTRACT FROM AUTHOR]

Cavity ring‐down spectroscopy (CRDS) is a highly sensitive laser technique that allows the analysis of isotopic signals and absolute concentration of individual molecular species in small‐volume samples. Here, we describe a protocol to quantify photosynthetic 13C‐uptake rates of marine phytoplankton by using the CRDS technique (13C‐CRDS‐PP). We validated our method by comparing the 13C‐PP rates measured between CRDS and isotope ratio mass spectrometry (IRMS) in samples with different carbon content (30–160 μgC). The comparison revealed that 13C‐CRDS‐PP rates were highly correlated with those obtained by IRMS (Spearman correlation coefficient, ρ = 0.95, p < 0.0001, n = 15), with a mean difference between the two estimates of ± 0.08 mgC m−3 h−1. Moreover, the slope of the relationship between CRDS and IRMS results was not significantly different from 1 (F = 0.03, p = 0.86), and the intercept did not differ from 0 (F = 1.4, p = 0.24), indicating that there was no bias in the CRDS relative to the IRMS‐based measurements. A separate analysis also showed that despite the difference in volume and carbon content between samples (40 ± 10 μgC and 160 ± 40 μgC, respectively), the 13C‐CRDS‐PP technique provides similar results (Mann–Whitney test, U = 30.5, p = 0.90, n = 8). In addition, 13C‐CRDS‐PP rates measured along the Red Sea (∼ 176 mgC m−2 d−1) agreed with 14C‐based PP rates previously reported for similar locations. Thus, this study evidenced that the 13C‐CRDS‐PP method is sensitive enough to quantify carbon fixation rates in oligotrophic regions. [ABSTRACT FROM AUTHOR]

Resolving the environmental drivers shaping planktonic communities is fundamental to understanding their variability, present and future, across the ocean. More specifically, resolving the temperature-dependence of planktonic communities in low productive waters is essential to predict the response of marine ecosystems to warming scenarios, as ocean warming leads to oligotrophication of the subtropical ocean. Here we quantified plankton metabolic rates along the Red Sea, a unique oligotrophic and warm environment, and analysed the drivers that regulate gross primary production (GPP), community respiration (CR) and the net community production (NCP). The study was conducted on six oceanographic surveys following a north-south transect along Saudi Arabian coasts. Our findings revealed that Chl-a specific GPP and CR rates increased with increasing temperature (R² = 0.41 and 0.19, respectively, P < 0.001 in both cases), with a higher activation energy (AE) for GPP (1.2 ± 0.17 eV) than for CR (0.73 ± 0.17 eV). The higher AE for GPP than for CR resulted in a positive relationship between NCP and temperature. This unusual relationship is likely driven by (1) the relatively higher nutrient availability found towards the warmer region (the South of the Red Sea), and which favours GPP rates above the threshold that separates autotrophic from heterotrophic communities (1.7 mmol O2 m-3 d-1). (2) Due to the arid nature, the basin lacks riverine and terrestrial inputs of organic carbon to subsidise a higher metabolic response of heterotrophic communities, thus constraining CR rates. Our study demonstrates that GPP increases steeply with increasing temperature in the warm ocean when relatively high nutrient inputs are present. [ABSTRACT FROM AUTHOR]

Due to the increasing constraints on using the radioactive isotope 14C to measure primary productivity (14C-PP), we determined the surface carbon fixation rates in the Mediterranean Sea, using the alternative stable isotope 13C method (13C-PP). Rates obtained (13C-POCp) were compared with simultaneous 14C-POCp measurements in samples of different volumes (72 mL and 1.2 L). We also tested the variation of the percentage of dissolved primary production (PER), to the total productivity using organic and inorganic filters (14C method). 13C-POCp rates ranged from 0.4, in the Ionian basin, to 1.5 mgC m−3 h−1 in the Ligurian region. These results agreed with those found with the 14C-PP in 1.2-L samples (two sample t -test, t = 1.035, df = 22, P = 0.31). However, we found that 14C-POCp rates derived from 72-mL incubations were 46% lower than those measured with 13C-PP. The discrepancy between large and small volume incubations was likely due to differences in the number of large phytoplankton cells within the community. PER values measured with silver membrane filters yielded similar results than those obtained using polycarbonate filters. Our findings showed that when the sample size is carefully chosen, the 13C-PP provide comparable results to 14C-PP even in waters of low productivity in the Mediterranean Sea. [ABSTRACT FROM AUTHOR]

The distribution of bioactive trace metals has the potential to enhance or limit primary productivity and carbon export in some regions of the world ocean. To study these connections, the concentrations of Cd, Co, Cu, Fe, Mo, Ni, and V were determined for 110 surface water samples collected during the Malaspina 2010 Circumnavigation Expedition (MCE). Total dissolved Cd, Co, Cu, Fe, Mo, Ni, and V concentrations averaged 19.0 ± 5.4 pM, 21.4 ± 12 pM, 0.91 ± 0.4 nM, 0.66 ± 0.3 nM, 88.8 ± 12 nM, 1.72 ± 0.4 nM, and 23.4 ± 4.4 nM, respectively, with the lowest values detected in the Central Pacific and increased values at the extremes of all transects near coastal zones. Trace metal concentrations measured in surface waters of the Atlantic Ocean during the MCE were compared to previously published data for the same region. The comparison revealed little temporal changes in the distribution of Cd, Co, Cu, Fe, and Ni over the last 30 years. We utilized a multivariable linear regression model to describe potential relationships between primary productivity and the hydrological, biological, trace nutrient and macronutrient data collected during the MCE. Our statistical analysis shows that primary productivity in the Indian Ocean is best described by chlorophyll a, NO3, Ni, temperature, SiO4, and Cd. In the Atlantic Ocean, primary productivity is correlated with chlorophyll a, NO3, PO4, mixed layer depth, Co, Fe, Cd, Cu, V, and Mo. The variables salinity, temperature, SiO4, NO3, PO4, Fe, Cd, and V were found to best predict primary productivity in the Pacific Ocean. These results suggest that some of the lesser studied trace elements (e.g., Ni, V, Mo, and Cd) may play a more important role in regulating oceanic primary productivity than previously thought and point to the need for future experiments to verify their potential biological functions. [ABSTRACT FROM AUTHOR]

The universal temperature dependence of metabolic rates has been used to predict how ocean biology will respond to ocean warming. Determining the temperature sensitivity of phytoplankton metabolism and growth is of special importance because this group of organisms is responsible for nearly half of global primary production, sustains most marine food webs, and contributes to regulate the exchange of CO2 between the ocean and the atmosphere. Phytoplankton growth rates increase with temperature under optimal growth conditions in the laboratory, but it is unclear whether the same degree of temperature dependence exists in nature, where resources are often limiting. Here we use concurrent measurements of phytoplankton biomass and carbon fixation rates in polar, temperate and tropical regions to determine the role of temperature and resource supply in controlling the large-scale variability of in situ metabolic rates. We identify a biogeographic pattern in phytoplankton metabolic rates, which increase from the oligotrophic subtropical gyres to temperate regions and then coastal waters. Variability in phytoplankton growth is driven by changes in resource supply and appears to be independent of seawater temperature. The lack of temperature sensitivity of realized phytoplankton growth is consistent with the limited applicability of Arrhenius enzymatic kinetics when substrate concentrations are low. Our results suggest that, due to widespread resource limitation in the ocean, the direct effect of sea surface warming upon phytoplankton growth and productivity may be smaller than anticipated. [ABSTRACT FROM AUTHOR]

Phytoplankton size structure is key for the ecology and biogeochemistry of pelagic ecosystems, but the relationship between cell size and maximum growth rate (μmax) is not yet well understood. We used cultures of 22 species of marine phytoplankton from five phyla, ranging from 0.1 to 106 μm3 in cell volume (Vcell), to determine experimentally the size dependence of growth, metabolic rate, elemental stoichiometry and nutrient uptake. We show that both μmax and carbon-specific photosynthesis peak at intermediate cell sizes. Maximum nitrogen uptake rate ( VmaxN) scales isometrically with Vcell, whereas nitrogen minimum quota scales as Vcell0.84. Large cells thus possess high ability to take up nitrogen, relative to their requirements, and large storage capacity, but their growth is limited by the conversion of nutrients into biomass. Small species show similar volume-specific VmaxN compared to their larger counterparts, but have higher nitrogen requirements. We suggest that the unimodal size scaling of phytoplankton growth arises from taxon-independent, size-related constraints in nutrient uptake, requirement and assimilation. [ABSTRACT FROM AUTHOR]