What drives DMS/P/O concentration variability and cycling in Antarctic waters
and sea ice between seasons?
Introduction
Sea ice plays an important role in structuring marine ecosystems. The seasonality of ice
cover is a key component of the ocean-atmosphere system that governs biogeochemical and
physical processes and cycles (Arrigo, Mock et al. 2010; Thomas and Dieckmann 2010; Damm
2016). The surface and interior of seasonal ice houses communities of microbes, algae, and
primary producers. These communities are thought to provide a significant ecological stimulus
as seasonal melting occurs in the Southern Ocean marine system (Legendre 1992; Lizotte
2001; Arrigo, Mock et al. 2010).
Primary production in the sea ice is closely associated with the cycling and seasonal
variability of sulfur compound concentration in Antarctic Waters. Dimethylsulfide (DMS) is a
trace gas that plays a key role in the global sulfur cycle (Lovelock 1972; Mahajan 2015) and the
metabolism of different marine trophic groups. DMS also plays a key role in atmosphere
radiative balance and has the potential to alter the global radiation budget (Bastian 1987). DMS
is derived by the cleavage of dimethylsulfoniopropionate (DMSP) by bacteria or sea-ice algae in
the surface ocean (DMSPd) (Kiene 2000; Luce 2011) and intra-cellular cleavage by
phytoplankton of particulate DMSP (DMSPp) (Asher 2017). DMSP serves various physiological
functions as an anti-oxidant (Sunda 2002) and cryoprotectant (Kirst 1991). Less known than
DMS and DMSP (DMS/P), dimethylsulfoxide (DMSO), a product of DMS oxidation, also serves
a number of ecological and physiological functions (Asher 2017).
Scientists are working to better understand DMS, DMSP, and DMSO (DMS/P/O) and
how these compounds vary spatially and temporally. Focus has been placed on quantifying
underlying production and removal processes and examining how physical and biological
processes control the DMS/P/O cycle. Although this has proven difficult in the past,
development of new technology and experimental methods has improved the quantification of
underlying processes. Methods include incorporation of 13C into quantifying rates of DMSP
synthesis (Stefels 2009), radioactively labeling DMS/P to quantify net DMS yield from DMSP
consumption (Kiene 2000), and a stable isotope tracer method for tracking production and
composition of compounds in the sulfur cycle (Asher 2011).
Several studies have used these methods, in addition to other experimental approaches,
in the Southern Ocean. Not only does this ocean contain the world’s highest surface water
DMS concentrations (Lana 2011), it also has robust seasonal cycles in sea-ice cover, solar
irradiance, and mixed layer depth, resulting in variability of surface DMS/P/O concentrations as
a result of dynamic biology (Kiene 2007). Previous studies have emphasized the importance of
expanding our knowledge of DMS/P/O seasonal dynamics and stress the need for on-going
biogeochemical studies in the Southern Ocean, especially in the face of climate change. The
two studies discussed below dive deeper into DMS/P/O variability, describing cycling patterns,
and how physical and biological processes control these cycles, especially during seasonal
transitions in coastal Antarctic waters and sea ice.
Article #1
Damm et al. 2016 measured concentrations of DMS/P reservoirs within Antarctic sea
ice, sea-ice slush layers, and brine samples from late winter to early spring. Their study aimed
to describe the relationship of DMS production with in situ sea-ice physical parameters,
nutrients, and chlorophyll α concentrations. They also explored the connection between the
DMS/P cycle and physical and biological processes in annual pack ice during the transitional
period from winter to spring. To do so, they collected ice core, sea-ice brine, seawater, and
zooplankton (krill) samples from six locations, or ice stations, off East Antarctica during the
SIPEX-2 voyage. Core samples were divided into two groups: (1) permeable sea ice and (2)
sea ice with an impermeable layer on the surface (impermeable sea ice). Analysis of the
samples included use of several methods measuring salinity, temperature and chlorophyll α,
nitrate, phosphate, and DMS/P concentrations. The results of their study are summarized
below.
Salinity profiles followed a C-shape and were generally homogeneous, with one maxima
near the surface and another at the bottom of the ice. Chlorophyll α vertical profiles within sea
ice depended on the physical properties of the sea ice (i.e. total ice thickness and snow depth).
Most of the ice cores had one or two chlorophyll maxima at varying depths. Increased
chlorophyll α was seen close to the surface of snow-covered pack ice where the incident solar
radiation available for photosynthesis was less attenuated.
The general pattern for nitrate and phosphate levels indicated highest levels in the
bottom layers and depleted levels near the sea-ice surface. The study recorded nitrate
concentrations in the water column which were approximately six times higher than
concentrations of sea ice. This was most likely a result of thick snow loading submerging the
ice and resulting in seawater flooding, repeatedly resupplying nutrients to the microbial
community near the surface of the ice.
Concentrations of nitrate measured from impermeable ice were particularly low, while
phosphate was in excess. Here, the ratio between nitrate and phosphate deviated from the
Redfield ratio (16:1), most likely due to an additional phosphate source in the sea ice,
suggesting nitrate was the limiting nutrient. Initial nitrate availability within the system triggered
algal growth, leading to chlorophyll α accumulation. This demonstrated that near surface nitrate
depletion is a cyclic progress. Slower downward transport of nutrients resulted in the
impermeable ice microbial community having a lower biomass relative to softer ice or slush.
Stations with partly permeable ice experienced flushing events migrating nutrients
downward due to brine drainage. Nutrients were inhibited when the sea ice became permeable,
resulting in conditions favorable for algal growth. Since the system is nitrate limited, depletion
will eventually restrict chlorophyll α within the sea-ice matrix. Nitrate depletion did not limit
chlorophyll α growth at stations with completely permeable sea ice and maxima were seen at
the ice-water interface. Nutrients at these stations were provided through two mechanisms,
downward transport in brine channels and/or direct exchange and convection along the icewater interface.
By measuring DMS/P in sea-ice, brine, slush samples, and under-ice sea water,
degradation of DMSP was traced to gain a better understanding of the cycling of DMSP in East
Antarctic sea ice during the winter-spring transition. Compared to spring/summer
concentrations, overall DMSP concentrations were lower and maxima localized in partially
impermeable, or frozen intermediate, surface layers of the ice in winter/spring. Sea-ice algae
growing at the ice-snow interface regulates osmotic stress by producing DMSP during freezing
and releasing DMSP/producing DMS during melting (Dickson 1986). Their results pointed to
pronounced seasonal/annual variation with respect to in situ DMSP production and cycling,
partly due to repeated freezing and melting events during the transitional period of winter/spring.
With increasingly permeable sea ice, enhanced slush formation, and brine convection,
the study expected to detect both DMS/P in the water column early on in their study. However,
concentrations of both remained below the detection limit. Rather than assuming insignificant
release of DMSP from the sea ice, the study explored masking by grazing zooplankton up
taking the DMSP. Zooplankton, krill in particular, are sloppy feeders and are known to facilitate
DMS transfer from the ocean to the atmosphere (Kasamatsu 2004). Measurements of stomach
and body part concentrations found elevated DMSP concentrations, indicating that krill were
grazing in and under the sea ice.
In summary, results from this study advanced the field by suggesting that DMSP
production between seasonal transitions is strongly coupled to conditions of snow and ice and
concentrations are spatially and temporally variable. Reoccurring freezing and melting events
during this transitional period, promoted DMSP production and release, respectively. DMSP
which was released from sea-ice algae and accumulated in the brine, was strongly dependent
on physical parameters of sea ice (i.e. thickness and permeability). This study also found that
frozen intermediate layers of sea-ice acting as a barrier for DMSP transport lead to high
concentrations of DMSP in slush layers on the surface, promoting production and efflux of DMS
into the atmosphere with melting. Lastly, the study found an unknown pathway in the
biogeochemical cycling of DMSP, consumption by grazing zooplankton.
Article #2
By using a combination of high-resolution DMS/P/O measurements, stable isotope tracer
experiments, and grazing assays, Asher et al. 2017 provided new insight into the seasonal
evolution and cycling of DMS/P/O along the Western Antarctic Peninsula (WAP), at the Palmer
Station. Four types of rate experiments were conducted to examine the underlying production
and consumption rates of DMS/P/O through various pathways. Net change in DMS/P
concentrations, under the presence of competitive inhibitors blocking the uptake of these
compounds, was measured to determine gross DMS production (GP). Stable isotope labeling
experiments were undergone to quantify specific DMS production pathways, essentially DMSP
cleavage and DMSO reduction. Similar experiments were undergone to measure rates of gross
DMS loss. Lastly, microzooplankton dilution and krill grazing experiments were conducted to
calculate net change in DMS, DMSPp/d, and chlorophyll α. DMSPp/d cleavage, DMSOd
reduction, DMS production due to microzooplankton, and DMS production due to krill grazing
rates were included to infer the total DMS production term. Gross DMS loss (biological and
photo-chemical) and removal from sea-air flux were included to infer the total DMS loss term.
The results of their study are summarized below.
Turnover rates from DMS GP experiments were significantly higher and more variable
than measurements from a previous Palmer Station study (Herrmann 2012). The observed
rates could be attributed to differences in inter-seasonal variability, phytoplankton biomass
dynamics, and sea ice cover between the two studies, which have shown to influence primary
production dynamics. The study also suggested that factors such as phytoplankton taxonomic
composition, mixed layer depth, and irradiance could explain the relatively low DMS/P
concentrations within the study site, relative to other Antarctic waters.
DMS sources and sink terms were relatively balanced throughout the majority of the
season, except for a massive diatom bloom in mid-December which most likely caused a peak
in DMS production/consumption terms. DMS production terms were measured from
algal/bacterial DMSPd cleavage and DMSPd reduction, with cleavage being the major source of
production. This indicated that DMSPd was the main precursor of DMS over the seasonal cycle
and elevated concentrations were the likely cause of increased Phaeocystis, or algae,
abundance. Microzooplankton dominated DMSP release and production. Presence of particular
phytoplankton taxa potentially influenced DMSP removal and production rates suggesting that
grazing is an important mechanism for DMS production in the WAP. Their grazing experiments
found that high densities of krill have the potential to accelerate DMS production directly
(grazing) and indirectly (fecal matter).
A poorly studied DSM production pathway, DMSOd reduction was not as large of a DMS
source contributor as seen in a previous study (Asher 2011) and environmental conditions or
presence of certain taxonomic groups of bacteria and algae determined the importance and
contribution of this pathway.
Rates of DMS removal through various pathways were measured. Sea-air flux and
photo-oxidation were relatively small DMS sinks. Gross DMS loss was most correlated with
bacterial production, suggesting that DMS/P were used as sources of bacterial sulfur and
energy (Zubkov 2002).
In summary, results from this study advanced the field by showing that total DMSP
accumulation is related to abundance of Phaeocystis and DMSPd is the main precursor of
DMS. The study also showed that grazing, especially by microzooplankton, is a considerable
source of DMS production while bacterial consumption controls DMS removal.
Conclusion
Both studies addressed different aspects of DMS/P/O cycling during seasonal transitions
in Antarctica. The first study focused on cycling within sea-ice and the second study on cycling
in Antarctic marine waters. Both studies documented seasonal variability in DMS/P/O
concentrations in sea-ice and marine waters. Damm et al. 2016 found highest concentrations of
DMSP in brine samples at the sea-ice surface which suggested that sea-ice surface algal
communities enhance DMSP production when released into brine during freezing and melting.
Both studies also addressed the effects of zooplankton grazing in the biogeochemical
cycling of DMS/P. Asher et al. 2017 found that DMSP cleavage from the dissolved pool and
release from microzooplankton grazing were the dominant sources of DMS while bacterial
uptake was the dominant source of DMS removal. Damm et al. 2016 also found low
concentrations of DMS/P in seawater under the ice even with brine drainage. This is likely a
result of in situ grazing by zooplankton which could act as a sink for DMSP production.
Only Asher et al. 2017 highlighted DMSO concentrations in Antarctic marine waters.
DMSO concentrations were well correlated with those of DMS and on a smaller scale, DMS
production rates were associated with DMSO reduction from the dissolved pool and krill grazing.
The compound remains understudied and future studies investigating the DMS/P cycle, should
consider incorporating DMSO.
Both studies provided several suggestions and recommendations for future studies,
technique exploration, and long-term monitoring. Further investigation should explore the role
of associated processes (i.e. bacterial lysis and nutrient recycling) that could potentially
influence the relationship and ratio between chlorophyll α and DMSP. Scientist should also
consider looking into temporal variability of seasonal DMS/P/O cycle concentrations and
production/consumption terms, focusing on the biogeochemical cycling of DMSP by
zooplankton. To further explore techniques, a study comparing stable isotope and radioisotope
labeled tracers, could shed light on the effectiveness of both methods. Looking at the bigger
picture, regional ocean models, including a sulfur specific biogeochemical module, should be
developed to assist researchers in long-term monitoring of certain areas. Asher et al. 2017
emphasized the importance of long-term monitoring of seasonal and inter-annual DMS/P/O
concentrations using automated analytical systems. In conjunction with advanced process
studies and on-going monitoring, this will allow scientist to study the effects of climate change
on the DMS/P/O cycle in rapidly changing polar marine waters.
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