Q. Devresse et al.: Eddy-enhanced primary production sustains heterotrophic microbial activities
5203
ated. We converted the cell abundance of the different au-
totrophic pico- and nanoplankton populations into biomass
assuming 43 fg C per cell for Prochlorococcus, 120£fg C per
cell for Synechococcus, 500fgC per cell for eukaryotic pi-
coplankton, and 3.100 fg C per cell for eukaryotic nanoplank-
ton based on Hernändez-Hernändez et al. (2020). We re-
ort the autotrophic pico- and nanoplankton biomass as the
sum of eukaryotic pico- and nanoplankton and cyanobacteria
(Prochlorococcus and Synechococcus) biomass. The abun-
dance of eukaryotic pico- and nanoplankton and cyanobac-
teria (Prochlorococcus and Synechococcus) can be found in
Table $2.
2.3 Microbial activities
carbon. Then, 100uL of 2N NaOH and 15 mL secintillation
cocktail were added. All samples were counted the follow-
ing day in a liquid scintillation analyser (Packard Tri-Carb,
model 1900 A).
Primary production (PP) of organic carbon was calculated
according to Gargas (1975):
12
pp (umo1 CL a7!) _ a2 x DI °C X1.05xkı x kı DD
al
where al and a2 are the activities (DPM: disintegrations per
minute) of the added solution and the sample corrected for
the dark sample, respectively, and DI!?C is the concentration
(umol L7'h of dissolved inorganic carbon (DIC) in the sam-
ple. DIC concentration was calculated from total alkalinity
using the R package seacarb (Gattuso et al., 2020). Total al-
kalinity of the seawater was acquired through the open-cell
titration method (Dickson et al., 2007). The value 1.05 is a
correction factor for the discrimination between !?C and !1C,
as the uptake of the !*C isotope is 5 % slower than the uptake
of ?C, kı is a correction factor for sub-sampling (bottle vol-
ume / filtered volume), and k, is the incubation time (a7.
Total primary production (PProrT; umol C L7! d7!) was de-
rived from the sum of PPpoc and PPpoc according to
Primary production (PP) was determined from !*C incorpo-
:ation according to Nielsen (1952) and Gargas (1975). Poly-
carbonate bottles (Nunc EasYFlask, 75 cm”) were filled with
260 mL prefiltered (mesh size of 200 um) sample and spiked
with 50uL ofa — 11 uCi NaH!*CO7 solution (Perkin Elmer,
Norway). A total of 200 uL was removed immediately after
spiking and transferred to a 5 mL scintillation vial for deter-
mination of added activity. Then, 50uL of 2N NaOH and
4mL seintillation cocktail (Ultima Gold AB) were added.
Duplicate samples from the top three depths at selected sta-
tions (Table S1) were incubated in 12h light and 12h dark at
22°C, which was the average temperature of the upper 100 m
depth (22 + 3°C) along the transect. The incubator was set
to reproduce three light levels — 1200-1400, 350, and 5uE -
with high values representing surface irradiance at the time
of sampling. The incubation length was chosen for two rea-
sons. First, we expected low productivity of the open ocean
3hytoplankton community due to low biomass and low nutri-
ent concentrations at the start of the incubation. Under these
conditions, short-term incubations of only a few hours may
underestimate PP because carbon assimilation by algal cells
may be too low to discriminate against !*C adsorption as de-
(ermined in blank dark incubation (Engel et al., 2013). More-
over, the release of freshly assimilated carbon into the DOM
pool has a timescale of several hours because of the equi-
libration of the tracer and because metabolic processes of
organic carbon exudation follow those of carbon fixation in-
side the cell (Engel et al., 2013). Incubations were stopped by
filtration of a 70 mL sub-sample onto 0.4 um polycarbonate
filters (Nuclepore). Particulate primary production (PPpoc)
was determined from material collected on the filter, while
che filtrate was used to determine dissolved primary produc-
‚on (PPpoc). Al filters were rinsed with 10 mL sterile fil-
:ered (<0.2 um) seawater and then acidified with 250 uL 2N
HCl to remove inorganic carbon (Descy et al., 2002). Filters
were transferred into 5 mL scintillation vials, and 4 mL scin-
tillation cocktail (Ultima Gold AB) was added. To determine
PPpDoc, 4 mL of filtrate was transferred to 20 mL scintillation
vials and acidified with 100uL IN HCl. Scintillation vials
were left open in the fume hood for 14h to remove inorganic
PPrTor = PPpoc + PPpoc-
(2)
The percentage of extracellular release (PER; %) was calcu-
lated as
PP
PER = (Zipoc) x 100.
PPrortT
(3
Bacterial biomass production (BP) rates were measured
through the incorporation of labelled leucine (°H) (spe-
cific activity 100 Ci mmol7!; Biotrend) using the microcen-
trifuge method (Kirchman et al., 1985; Smith and Azam,
1992). Duplicate samples and one killed control (1.5 mL
sach) were labelled using °H-leucine at a final concentration
of 20nmolL7!. BP was determined down to 800 m depth,
and, for practical reasons, we chose an incubation temper-
ature of 14°C as an average over this depth interval. How-
ever, in this paper, only data from the top 100m depth are
shown, and BP rates were corrected for the difference be-
tween incubation and in situ temperature (Eq. 4). All sam-
ples were incubated for 6 h in the dark with headspace. Con-
trols were poisoned with trichloroacetic acid. All samples
were measured on board with a liquid scintillation analyser
(Packard Tri-Carb, model 1900 A). H-leucine uptake was
converted to carbon units by applying a conversion factor of
1.55kg C mol! Jeucine (Simon and Azam, 1989).
BP rates from incubations at 14°C were converted to BP
rates at 22 °C following the equation from Löpez-Urrutia and
Morän (2007):
BP»5°C = BP14°C X 1.906
(4\
https://doi.ore/10.5194/bg-19-5199-2027
Bioseosciences. 19. 51995219. 2022
