Marine microalgae sequester as much CO₂ into carbohydrates as terrestrial plants. Polymeric carbohydrates (i.e., glycans) provide carbon for heterotrophic organisms and constitute a carbon sink in ...the global oceans. The quantitative contributions of different algal glycans to cycling and sequestration of carbon remain unknown, partly because of the analytical challenge to quantify glycans in complex biological matrices. Here, we quantified a glycan structural type using a recently developed biocatalytic strategy, which involves laminarinase enzymes that specifically cleave the algal glycan laminarin into readily analyzable fragments. We measured laminarin along transects in the Arctic, Atlantic, and Pacific oceans and during three time series in the North Sea. These data revealed a median of 26 ± 17% laminarin within the particulate organic carbon pool. The observed correlation between chlorophyll and laminarin suggests an annual production of algal laminarin of 12 ± 8 gigatons: that is, approximately three times the annual atmospheric carbon dioxide increase by fossil fuel burning. Moreover, our data revealed that laminarin accounted for up to 50% of organic carbon in sinking diatom-containing particles, thus substantially contributing to carbon export from surface waters. Spatially and temporally variable laminarin concentrations in the sunlit ocean are driven by light availability. Collectively, these observations highlight the prominent ecological role and biogeochemical function of laminarin in oceanic carbon export and energy flow to higher trophic levels.
We analyzed size-specific dry mass, sinking velocity, and apparent diffusivity in field-sampled marine snow, laboratory-made aggregates formed by diatoms or coccolithophorids, and small and large ...zooplankton fecal pellets with naturally varying content of ballast materials. Apparent diffusivity was measured directly inside aggregates and large (millimeter-long) fecal pellets using microsensors. Large fecal pellets, collected in the coastal upwelling off Cape Blanc, Mauritania, showed the highest volume-specific dry mass and sinking velocities because of a high content of opal, carbonate, and lithogenic material (mostly Saharan dust), which together comprised ~80% of the dry mass. The average solid matter density within these large fecal pellets was 1.7 g cm⁻³, whereas their excess density was 0.25 ± 0.07 g cm⁻³. Volume-specific dry mass of all sources of aggregates and fecal pellets ranged from 3.8 to 960 micrograms mm⁻³, and average sinking velocities varied between 51 and 732 m d⁻¹. Porosity was >0.43 and >0.96 within fecal pellets and phytoplankton-derived aggregates, respectively. Averaged values of apparent diffusivity of gases within large fecal pellets and aggregates were 0.74 and 0.95 times that of the free diffusion coefficient in sea water, respectively. Ballast increases sinking velocity and, thus, also potential O2 fluxes to sedimenting aggregates and fecal pellets. Hence, ballast minerals limit the residence time of aggregates in the water column by increasing sinking velocity, but apparent diffusivity and potential oxygen supply within aggregates are high, whereby a large fraction of labile organic carbon can be respired during sedimentation.
Vertical carbon fluxes between the surface and 2500
m depth were estimated from
in situ profiles of particle size distributions and abundances me/asured off Cape Blanc (Mauritania) related to deep ...ocean sediment traps. Vertical mass fluxes off Cape Blanc were significantly higher than recent global estimates in the open ocean. The aggregates off Cape Blanc contained high amounts of ballast material due to the presence of coccoliths and fine-grained dust from the Sahara desert, leading to a dominance of small and fast-settling aggregates. The largest changes in vertical fluxes were observed in the surface waters (<250
m), and, thus, showing this site to be the most important zone for aggregate formation and degradation. The degradation length scale (
L), i.e. the fractional degradation of aggregates per meter settled, was estimated from vertical fluxes derived from the particle size distribution through the water column. This was compared with fractional remineralization rate of aggregates per meter settled derived from direct ship-board measurements of sinking velocity and small-scale O
2 fluxes to aggregates measured by micro-sensors. Microbial respiration by attached bacteria alone could not explain the degradation of organic matter in the upper ocean. Instead, flux feeding from zooplankton organisms was indicated as the dominant degradation process of aggregated carbon in the surface ocean. Below the surface ocean, microbes became more important for the degradation as zooplankton was rare at these depths.
Saharan dust input and seasonal upwelling along North-West Africa provide a model system for studying microbial processes related to the export and recycling of nutrients. This study offers the first ...molecular characterization of prokaryotic particle-attached (PA; >3.0 μm) and free-living (FL; 0.2-3.0 μm) players in this important ecosystem during August 2016. Environmental drivers for alpha-diversity, bacterial community composition, and differences between FL and PA fractions were identified. The ultra-oligotrophic waters off Senegal were dominated by Cyanobacteria while higher relative abundances of Alphaproteobacteria, Bacteroidetes, Verrucomicrobia, and Planctomycetes (known particle-degraders) occurred in the upwelling area. Temperature, proxy for different water masses, was the best predictor for changes in FL communities. PA community variation was best explained by temperature and ammonium. Bray Curtis dissimilarities between FL and PA were generally very high and correlated with temperature and salinity in surface waters. Greatest similarities between FL and PA occurred at the deep chlorophyll maximum, where bacterial substrate availability was likely highest. This indicates that environmental drivers do not only influence changes among FL and PA communities but also differences between them. This could provide an explanation for contradicting results obtained by different studies regarding the dissimilarity/similarity between FL and PA communities and their biogeochemical functions.
Production, oxygen uptake, and sinking velocity of copepod fecal pellets egested by "Temora longicornis" were measured using a nanoflagellate ("Rhodomonas" sp.), a diatom ("Thalassiosira ...weissflogii"), or a coccolithophorid ("Emiliania huxleyi") as food sources. Fecal pellet production varied between 0.8 pellets ind⁻¹ h⁻¹ and 3.8 pellets ind⁻¹ h⁻¹ and was significantly higher with "T. weissflogii" than with the other food sources. Average pellet size varied between 2.2 ×10⁵ µm³ and 10.0 ×10⁵µm³. Using an oxygen microsensor, small-scale oxygen fluxes and microbial respiration rates were measured directly with a spatial resolution of 2 µm at the interface of copepod fecal pellets and the surrounding water. Averaged volume-specific respiration rates were 4.12 fmol O₂ µm⁻³ d⁻¹ 2.86 fmol O₂ µm⁻³ d⁻¹, and 0.73 fmol O₂ µm⁻³ d⁻¹ in pellets produced on "Rhodomonas" sp.," T. weissflogii", and "E. huxleyi", respectively. The average carbon-specific respiration rate was 0.15 d⁻¹ independent on diet (range: 0.08- 0.21 d⁻¹) Because of ballasting of opal and calcite, sinking velocities were significantly higher for pellets produced on" T. weissflogii" (322 ± 169 m d⁻¹) and "E. huxleyi" (200 ± 93 m d⁻¹) than on "Rhodomonas" sp. (35 ± 29 m d⁻¹). Preservation of carbon was estimated to be approximately 10-fold higher in fecal pellets produced when "T. longicornis" was fed "E. huxleyi" or" T. weissflogii" rather than "Rhodomonas" sp. Our study directly demonstrates that ballast increases the sinking rate of freshly-produced copepod fecal pellets but does not protect them from decomposition.
It is known that
Microsetella norvegica
feed on phytoplankton and provide an important link to higher trophic levels in Arctic fjords, such as fish sprat (
Sprattus sprattus
) and three-spined ...stickleback (
Gasterosteus aculeatus
). It has recently been suggested that
M. norvegica
may also contribute substantially to carbon flux attenuation during periods of high abundance. However, we still know very little about how seasonal variations in abundance and vertical distribution of
M. norvegica
impact the efficiency of the biological carbon pump in Arctic fjords. We investigated the role of
Microsetella norvegica
, a small harpacticoid copepod, for particulate organic carbon flux attenuation via aggregate feeding in a subarctic fjord. We quantified the vertical distribution and abundance of
M. norvegica
, phytoplankton, and marine snow simultaneously with a Digital Autonomous Video Plankton Recorder in Porsangerfjord, northern Norway, between August 2013 and November 2014. We estimated the highest abundance of
M. norvegica
as 4.86x10
6
individuals m
-2
in October. Our results suggest that
M. norvegica
preferred diatoms over both marine snow and the prymnesiophyte
Phaeocystis pouchetii
during euphotic bloom conditions. However, during oligotrophic conditions when phytoplankton were scarce,
M. norvegica
switched to marine snow as a food source.
M. norvegica
has the potential to explain 1.4% and 0.29% of the total carbon flux attenuation in October and November, respectively. These results suggest that small copepods feed on settling detritus when no alternative food is available. Detritus feeding by
M. norvegica
may have an ecological impact during the polar night, both via direct carbon flux attenuation, but also as secondary producers in periods with low primary production. Currently small copepods such as
M. norvegica
are not included in carbon budgets or large-scale modelling, but considering their potentially high abundance they may represent an important but overlooked pathway in both the carbon cycle and trophic level interactions.
Plankton imaging systems supported by automated classification and analysis have improved ecologists' ability to observe aquatic ecosystems. Today, we are on the cusp of reliably tracking plankton ...populations with a suite of lab‐based and in situ tools, collecting imaging data at unprecedentedly fine spatial and temporal scales. But these data have potential well beyond examining the abundances of different taxa; the individual images themselves contain a wealth of information on functional traits. Here, we outline traits that could be measured from image data, suggest machine learning and computer vision approaches to extract functional trait information from the images, and discuss promising avenues for novel studies. The approaches we discuss are data agnostic and are broadly applicable to imagery of other aquatic or terrestrial organisms.
The relative abundance of individual archaeal membrane
lipids, namely of glycerol dialkyl glycerol tetraethers (GDGTs) with
different numbers of cyclopentane rings, varies with temperature, which
...enables their use as a paleotemperature proxy index. The first
GDGT-based index in marine sediments called TEX86
is believed to reflect mean annual sea surface temperature (maSST). The
TEX86L is an alternative temperature
proxy for “low-temperature” regions (<15 ∘C), where the original TEX86 proxy
calibration shows a larger scatter. However,
TEX86L-derived temperatures still
display anomalous estimates in polar regions. In order to elucidate the
potential cause of the disagreement between the
TEX86L estimate and SST, we analyzed
GDGT fluxes and TEX86L-derived
temperatures in sinking particles collected with time-series sediment traps
in high-northern- and high-southern-latitude regions. At 1296 m depth in the
eastern Fram Strait (79∘ N), a combination of various transporting
mechanisms for GDGTs might result in seasonally different sinking velocities
for particles carrying these lipids, resulting in strong variability in the
TEX86L signal. The similarity of flux-weighted TEX86L temperatures from
sinking particles and surface sediments implies an export of GDGTs without
alteration in the Fram Strait. The estimated temperatures correspond to
temperatures in water depths of 30–80 m, where nitrification might occur,
indicating the favorable depth habitat of Thaumarchaeota. In the Antarctic
Polar Front of the Atlantic sector (50∘ S),
TEX86L-derived temperatures displayed
warm and cold biases compared to satellite-derived SSTs at 614 m depth, and
its flux-weighted mean signal differs from the deep signal at 3196 m.
TEX86L-derived temperatures at 3196 m
depth and the surface sediment showed up to 7 ∘C warmer
temperatures relative to satellite-derived SST. Such a warm anomaly might be
caused by GDGT contributions from Euryarchaeota, which are known to dominate
archaeal communities in the circumpolar deep water of the Antarctic Polar
Front. The other reason might be that a linear calibration is not
appropriate for this frontal region. Of the newly suggested SST proxies
based on hydroxylated GDGTs (OH-GDGTs), only those with OH-GDGT–0 and
crenarchaeol or the ring index (RI) of OH-GDGTs yield realistic temperature
estimates in our study regions, suggesting that OH-GDGTs could be applied as
a potential temperature proxy in high-latitude oceans.
Vertical particle fluxes are responsible for the transport of carbon and biogenic material from the surface to the deep ocean, hence understanding these particle fluxes is of climate relevance. ...Sediment traps operated in Fram Strait in the framework of the Arctic long-term observatory FRAM provide an estimate of vertical particle fluxes in a region of high CO2 uptake. Until now the source area (catchment area) of trapped particles is unclear; however, lateral advection of particles is supposed to play an important role. This study presents a Lagrangian method to backtrack the origin of particles for two Fram Strait moorings equipped with sediment traps in 200 m and 2300 m depth by using the time-dependent velocity field of a high-resolution, eddy-resolving ocean-sea ice model. Our study shows that the extent of the catchment area is larger the deeper the trap and the slower the settling velocity. Chlorophyll-a concentration as well as sea ice coverage of the catchment area are highest in the summer months. The high sea ice coverage in summer compared to winter can possibly be related to a weaker across-strait sea level pressure difference, which allows more sea ice to enter the then well-stratified central Fram Strait where moorings are located. Hence, highest vertical particle fluxes may be expected in late summer and autumn. Furthermore, a backward sea ice tracking approach shows that the origin and age of sea ice drifting through Fram Strait, partly responsible for vertical particle fluxes, varies strongly from year to year, pointing to a high variability in the composition of particles trapped in the moorings.
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