The acceleration of dissolved cobalt's ecological stoichiometry due to biological uptake, remineralization, and scavenging in the Atlantic Ocean
The stoichiometry of biological components and their influence on dissolved distributions have long been of interest in the study of the oceans. Cobalt has the smallest oceanic inventory of inorganic micronutrients and hence is particularly vulnerable to influence by internal oceanic processes in...
| Main Authors: | , , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2017-10-01
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| Series: | Biogeosciences |
| Online Access: | https://www.biogeosciences.net/14/4637/2017/bg-14-4637-2017.pdf |
| Summary: | The stoichiometry of biological components and their influence on dissolved
distributions have long been of interest in the study of the oceans. Cobalt
has the smallest oceanic inventory of inorganic micronutrients and hence is
particularly vulnerable to influence by internal oceanic processes including
euphotic zone uptake, remineralization, and scavenging. Here we observe not
only large variations in dCo : P stoichiometry but also the acceleration of
those dCo : P ratios in the upper water column in response to several
environmental processes. The ecological stoichiometry of total dissolved
cobalt (dCo) was examined using data from a US North Atlantic GEOTRACES
transect and from a zonal South Atlantic GEOTRACES-compliant transect
(GA03/3_e and GAc01) by Redfieldian analysis of its
statistical relationships with the macronutrient phosphate. Trends in the
dissolved cobalt to phosphate (dCo : P) stoichiometric relationships were
evident in the basin-scale vertical structure of cobalt, with positive dCo : P
slopes in the euphotic zone and negative slopes found in the ocean interior
and in coastal environments. The euphotic positive slopes were often found to
accelerate towards the surface and this was interpreted as being due to the
combined influence of depleted phosphate, phosphorus-sparing (conserving)
mechanisms, increased alkaline phosphatase metalloenzyme production (a zinc
or perhaps cobalt enzyme), and biochemical substitution of Co for depleted
Zn. Consistent with this, dissolved Zn (dZn) was found to be drawn down to
only 2-fold more than dCo, despite being more than 18-fold more abundant in
the ocean interior. Particulate cobalt concentrations increased in abundance
from the base of the euphotic zone to become ∼ 10 % of the
overall cobalt inventory in the upper euphotic zone with high stoichiometric
values of ∼ 400 µmol Co mol<sup>−1</sup> P. Metaproteomic
results from the Bermuda Atlantic Time-series Study (BATS) station found
cyanobacterial isoforms of the alkaline phosphatase enzyme to be prevalent in
the upper water column, as well as a sulfolipid biosynthesis protein
indicative of P sparing. The negative dCo : P relationships in the ocean
interior became increasingly vertical with depth, and were consistent with
the sum of scavenging and remineralization processes (as shown by their dCo : P
vector sums). Attenuation of the remineralization with depth resulted in the
increasingly vertical dCo : P relationships. Analysis of particulate Co with
particulate Mn and particulate phosphate also showed positive linear
relationships below the euphotic zone, consistent with the presence and
increased relative influence of Mn oxide particles involved in scavenging.
Visualization of dCo : P slopes across an ocean section revealed hotspots of
scavenging and remineralization, such as at the hydrothermal vents and below
the oxygen minimum zone (OMZ) region, respectively, while that of an estimate
of Co* illustrated stoichiometrically depleted values in the mesopelagic and
deep ocean due to scavenging. This study provides insights into the coupling
between the dissolved and particulate phase that ultimately creates Redfield
stoichiometric ratios, demonstrating that the coupling is not an
instantaneous process and is influenced by the element inventory and rate of
exchange between phases. Cobalt's small water column inventory and the
influence of external factors on its biotic stoichiometry can erode its
limited inertia and result in an acceleration of the dissolved stoichiometry
towards that of the particulate phase in the upper euphotic zone. As human
use of cobalt grows exponentially with widespread adoption of lithium ion
batteries, there is a potential to affect the limited biogeochemical inertia
of cobalt and its resultant ecology in the oceanic euphotic zone. |
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| ISSN: | 1726-4170 1726-4189 |