Contrasting effects of probable nitrogen limitation and its absence on mercury speciation in the south Atlantic ocean as indicators of microbial mercury transformations

Igor Živković (Speaker), Bratkic, A. (Contributor), Milena Horvat (Contributor), Malcolm Woodward (Contributor), Matthew Humphreys (Contributor), Mladen Šolić (Contributor), Natalia Bojanić (Contributor)

Activity: Talk or presentationTalk or presentation at a conference

Description

Microbial transformations of methylated mercury (MeHg) and dissolved
gaseous mercury (DGM) at the lowest marine trophic levels are
still not well understood. This is especially important in oligotrophic
and nutrient-limited seas, where microbial food web dominate over
classical food web. To better understand the effects of probable nitrogen
limitation and its absence (N-limitation and no-N-limitation) on the
abundance of Hg species, waters of South Atlantic Ocean along 40°S
parallel were investigated in 2011/2012 during GEOTRACES cruise
JC068. Our statistical analysis of the results focuses on the examination
of the effects of N-limitation on relations between different mercury
fractions (total mercury – THg, MeHg, and DGM) and nutrients
and environmental parameters. In order to test the nutrient-limitation
hypothesis, we have compared our results with previous analysis from
the Central Adriatic Sea.
Non-metric multi-dimensional scaling showed discrete separation of
data obtained under N-limitation and no-N-limitation conditions; this
separation is mostly emphasized for DGM data. Mann-Whitney test
showed significant differences in DGM and MeHg concentrations under
N-limitation and no-N-limitation.
Principal component analysis (PCA) has revealed two contrasting
clusters. First cluster is grouped around DGM and nutrients under
no-N-limitation. Only under no-N-limitation, DGM was significantly
related to nutrients and environmental parameter, which might be
an indication of metabolism-dependent Hg transformations. DGM
is significantly correlated with dissolved inorganic carbon (DIC) under
no-N-limitation, indicating that organic matter remineralization
might be the driving force for DGM production in the open ocean.
In addition, DGM concentrations can be estimated within ±20% from
the measured values in 70% of data under no-N-limitation from DIC
and nitrate concentrations using multiple linear regression analysis.
DGM shows negative correlation with seawater temperature under
no-N-limitation, and positive correlation under N-limitation conditions.
The same trends are found in the Central Adriatic Sea under the absence
and presence of probable phosphorus limitation, respectively,
indicating nutrient and temperature dependent Hg reduction.
The second PCA cluster is grouped around MeHg, temperature and
fluorescence under N-limitation. This might indicate that net Hg methylation
in the water column is related with microbial methylation under
N-limitation when microbial growth is impeded. This is contrary from
the analysis in the Central Adriatic Sea where the activity of heterotrophic
bacteria seemed responsible for most of net Hg methylation in
seawater under the absence of probable phosphorus limitation during
winter. Combined, these results indicate the presence of at least two
different Hg methylation pathways in open seawater.
Period12 Sep 2019
Event titleInternational Conference on Mercury as a Global Pollutant 2019
Event typeConference
Conference number14
Degree of RecognitionInternational