4.2 Role of different biological carriers in the export of TE.
In the following, we consider the 9 elements (P, Cd, Ba, Mo, Cu, Ni, V,
Y, Mn) for which Fxs are positive throughout the season
(Figure 7). Among these, 7 elements (P, Mo, Cd, Cu, Ni, V, Mn) have
known biological functions and can therefore be directly associated with
biological carrier phases. Overall, this is confirmed by the seasonal
dynamics of their Fxs that presented 1 or 2 maxima
corresponding to the cups that collected sinking material during the
first (cups# 3, 4, 5) or second (cups# 9, 10) export event. To go a
step forward, we took advantage of the detailed description of
biological matter export provided by microscopic observations
(Rembauville et al. 2015a) in the same cup material. An important aspect
was to quantify the carbon content of exported diatoms and all types of
fecal pellets. Microscopic observations revealed that diatoms dominated
the phytoplankton community (Blain et al. 2020), and that 12 different
taxa contributed significantly (>1% of total biomass) to
both the surface carbon biomass and carbon export. However, the
concentrations of TE with a biological role certainly varied throughout
the season in surface waters due to intense uptake and remineralisation
as observed for Fe above the Kerguelen plateau (Blain et al. 2008; Bowie
et al. 2015). Similarly, TE quota are likely to vary over time in
surface diatoms, with consequences on TE composition of the fecal
pellets. The absence of data on seasonal changes in TE concentrations in
the water column and the large uncertainty of the TE transfer efficiency
between phytoplankton and zooplankton led us to make a rather
conservative choice of only three biological carriers, vegetative cells,
spores and fecal pellets. Additionally, we considered the total particle
mass, POC, PON, POCdiat, and used CaCO3as a tracer of calcifying organisms.
We first investigated the role of these different biological carriers
using PCA (Figure S3) based on Fxs and the different
biological carriers mentioned above. However, this approach did not
prove informative on the association of a given TE with a biological
carrier, except for V, which was strongly associated with vegetative
cells and/or calcifying organisms exported during the second bloom. The
strong association of Mn with the first bloom, as revealed by the PCA,
is not meaningful, because Fxs of Mn is high only in cup
#3 albeit the export of this bloom is collected by cups #4 and #5 as
well (Blain et al. 2020). For Ba and Y, the PCA does not provide any
clues on their association with a particular biological carrier.
Co-linearity between the different biological descriptors may have
hampered the emergence of more significant relationships for other
elements.
We have therefore analysed the data set using a different statistical
tool, the Partial Least Square Regression (PLSR), also referred to as
Projection of Latent Structure Regression (Abdi 2010). This method
considers a set of predictors (X) and descriptors (Y) and extracts a
single set of scores from both simultaneously. The method can be seen as
a simultaneous PCA on X and Y which achieves the best relationships
between X and Y. The method is efficient even when the variables are
possibly correlated and when the number of variables is large compared
to the number of observations. This method has been successfully applied
to determine the ecological vectors associated with sinking carbon flux
(Rembauville et al. 2015), to predict the partitioning of carbon within
plankton assemblages based on bio-optical properties (Rembauville et al.
2017) or to link biological diversity and carbon fluxes (Guidi et al.
2016). To apply PLSR we considered the total export flux of the 15
elements (descriptors) and the different biological vectors (predictors)
mentioned above, and we considered Ti as an overall predictor of
lithogenic material. It is important to note that with this approach the
search for relationships between elements and the lithogenic carrier
phase does not require the use of an elemental ratio. To summarize the
results of the PLSR analysis we present the projections of both
descriptors and predictors in a three-dimensional space defined by the
three first latent variables (Figure 8) which represent 57.5%, 22.1%
and 8.6 % of the covariance, respectively. The three corresponding 2D
dimensional projections in the latent vectors space are provided in
Figure S4. Three different groups of TE emerge from this analysis.
TEs associated with the lithogenic carrier phase . The PLSR,
clearly identifies a group of TEs (Al, Zr, Cr, Fe, Th, Co, Mn and Y) for
which the seasonal dynamics are strongly related to the lithogenic
carrier phase, represented by Ti. This result is in line with the
conclusions of the PCA and TExs analysis (Figure 6). The
PLSR provides novel information for the groups Cd, P, Ba and V, Ni, Mo,
Cu.
TEs associated with fecal pellets and diatom spores. The export
of Cd, P and Ba was strongly associated with POCfp and
to a lesser extend to POCsp. For Ba, this result is not
surprising considering that particulate Ba is largely found as
authigenic mineral barite in the ocean (Dehairs et al. 1980), formed by
precipitation from dissolved Ba in low oxygen environments. Such anoxic
microenvironments are typically found in fecal pellets (Alldredge and
Cohen 1987; Ploug 2001) or in aggregates like marine snow which in our
study contained large quantities of spores (Blain et al. 2020). Strong
correlations of Cd and P export fluxes have already been observed with
sediment traps deployed in the upper water column (> 1500m)
whereas this relationship vanished at greater depth (Ho et al. 2011;
Conte et al. 2019). In the present study, the export of Cd was mainly
driven by spores during the first bloom and by fecal pellets throughout
the season, while vegetative cells and calcifying organisms present
during the second bloom played a minor role for Cd export. Cd, but also
Co, can substitute for Zn in the carbonic anhydrase (CA) enzyme, Cd-CA
and Zn-CA, respectively (Morel et al. 2020). This has been demonstrated
for diatoms under low Zn conditions (Lane and Morel 2000). Cd-CA is
present in Thalassiosira antarctica , Chaetoceros dichaeta ,Proboscia alata and Proboscia inermis (Morel et al. 2020),
species that were well represented in our sediment traps (Blain et al.
2020). Interestingly T. antarctica and C. dichatea are
small and spore forming diatoms which dominated during the first bloom,
while the genus Proboscia contains large diatoms exported as
vegetative cells that thrived during the second bloom. Cd utilisation by
different diatoms in surface waters could explain the seasonal variation
of Cd export in the sediment traps. Cd can also be coincidentally taken
up by the divalent transporter under Fe-limited conditions (Lane et al.
2008; Horner et al. 2013). At the beginning of the season, the reservoir
of Zn and Fe was large above the Kerguelen plateau (Wang et al. 2019),
but the rapid development of the massive bloom of small diatoms could
lead to a rapid decrease in Zn to levels at which the substitution of Zn
by Cd in CA occurred and/or Cd being taken up by the divalent
transporter. No strong signal of particulate Cd was associated with the
second bloom suggesting that the substitution of Zn by Cd in CA or
divalent transport uptake are not dominant processes at the end of the
productive season, either due to increased Zn or Fe concentrations
provided by remineralisation after the first bloom or due to lower
requirements of large diatom cells which do not need Cd for CA
activities. Although calcifying organisms including coccolithophorids,
present during the second bloom, have high Cd requirements (Ho et al.
2003; Sunda 2012), their contribution was likely hidden behind the large
fluxes associated with fecal pellets.
TEs associated with lithogenic and biological carrier phases.V, Mo, Cu and Ni export fluxes are both driven by lithogenic and
biological carrier phases. This is a consequence of both their
significant contribution to Kerguelen basalt composition (Table 2) and
their biological role in microorganisms. Using a similar approach to
that used for Cd, we examine the seasonal dynamics of the export of
these four metals by first summarizing a few recent insights on their
biological role for microorganisms relevant for our study. We then
discuss how these observations can provide clues to understand the
seasonal dynamics of their export.
The main non-lithogenic V export event coincided with the flux of large
vegetative diatoms and calcifying organisms after the second bloom
(Figure 6). Due to the similar seasonal patterns of these biological
carrier phases, it is not possible, based on PLSR, to make a clear
preferential association with either of them. The current knowledge on
the biological role of V is mainly related to diatoms, thus our
discussion on the temporal changes of V export focuses on this
phytoplankton group. V is a cofactor of haloperoxidase enzymes (VHPO)
that produce organo-halogens (Moore et al. 1996; Murphy et al. 2000;
Hill and Manley 2009). Haloperoxidase activity by diatoms could alter
the quorum sensing of prokaryotes and therefore protect diatoms against
algicidal prokaryotes (Amin et al. 2012). In contrast to the seasonal
dynamics of all other elements, the export of V associated to the
biological fraction was higher during the second than during the first
bloom (Figure 7). Seasonal observations of diatom and prokaryotic
communities in the surface layer revealed compositional changes and
strong associations (positive and negative) between diatom species and
prokaryotic taxa (Liu et al. 2020). Positive associations could result
from interactions based on the exchange of metabolites between diatoms
and prokaryotes for resource acquisition, but negative associations are
more difficult to interpret. The seasonal dynamics of non-lithogenic
particulate V, if related to VHPO activity, could suggest that some
diatoms efficiently reduce the growth of targeted prokaryotic taxa with
algicidal activity in the phycosphere.
The prevalence of non-lithogenic particulate V during the second bloom
could be related to seasonal changes of the bioavailability of Fe.
Haloperoxidase can contain Fe-heme as prosthetic group instead of V.
Fe-heme containing enzymes could dominate the haloperoxidase activity of
diatoms when the bioavailable Fe stock is high such as at the beginning
of the season. However, as biological uptake during the first bloom
consumed a large part of the bioavailable Fe, haloperoxidase activity of
diatoms dominating during the second bloom may have switched to VHPO,
which requires the uptake of vanadate, an anion that is always present
at non-limiting concentrations in seawater. V can also be found in
nitrogenase (nif ) involved in the fixation of dinitrogen
(N2) where it substitutes Mo. A recent study illustrated
that Mo/Fe containing nif genes are overexpressed by prokaryotic
communities on marine particles (Debeljak et al. 2021). Therefore, the
association of V or Mo with vegetative diatoms could partly be explained
by N2 fixing prokaryotes attached to particles and their
downward transport could be a biological carrier phase for Mo and V.
The dominant biological carrier phase for Cu was different to that of V
and Mo. Cu export was mainly related to diatom spores and to a lesser
extend to fecal pellets whereas no clear association with vegetative
cells and CaCO3 was observed (Figure 8 and Figure S4).
Cu is a co-factor of a large number of oxidative enzymes involved in
different metabolic pathways including Fe acquisition (Maldonado and
Price 2001) and nitrogen cycling (Kuypers et al. 2018). Another
noticeable feature of these Cu proteins is that most of them are located
outside eukaryotic cells or in the periplasm of prokaryotes (Silva and
Williams 2001). A possible consequence can be that Cu enzymes are prone
to rapid degradation and release of Cu following cell death. Fecal
pellets or spores could provide a protected environment during export,
which could explain our observations.
The biological carrier phases for Ni were mainly diatoms (spores or
vegetative cells) and fecal pellets had a minor role. Among the many
biological pathways, Ni is involved in the assimilation of urea
(Oliveira and Antia 1984) and is also the cofactor of an enzyme of the
superoxide dismutase (SOD) family which can substitute for Fe-superoxide
dismutase in low Fe environments (Dupont et al. 2010; Cuvelier et al.
2010). These requirements for Ni likely lead to high Ni quota of diatoms
relative to other phytoplankton groups (Twining et al. 2012). It was,
however, also noted that 50% of Ni contained in diatoms is associated
with the frustule with an unknown function. These Ni dependent enzymes
suggest that diatom spores, vegetative cells and fecal pellets that
contained mainly diatoms are all potential biological vectors of Ni
export. If true, the lack of a marked difference between the first and
the second bloom dominated by spores and vegetative cells, respectively,
is surprising. A larger contribution of diatoms to Ni export would be
expected during the second bloom for two reasons. First, the
assimilation of urea is likely only noticeable when the switch from
NO3- to
NH4+ uptake has occurred, thus after
the first bloom. Second, since Fe bioavailability was lower during the
second bloom, Fe-SOD is likely to be replaced by Ni-SOD. We suggest an
additional process to significantly contribute to the biological export
of Ni. Methanogenic Archaea utilise different enzymes belonging
to the hydrogenase, reductase or CO dehydrogenase families where Ni is
present as co-factor (Mulrooney and Hausinger 2003). MethanogenicArchaea have been detected in different marine particles like
marine snow or fecal pellets (Maarel et al. 1999) where they could
thrive within anoxic niches (Alldredge and Cohen 1987; Ploug 2001). A
time series of the composition of the particulate matter in the surface
layers would certainly provide new data required to decipher between
these different hypotheses.