Other scenarios are possible, however, as EPS may also enhance the buoyancy of particles ( Riedel et al., 2006 Assmy et al., 2013 Belt et al., 2018). During the melting season, the high concentrations of extracellular polymeric substances (EPS) they have produced while inhabiting the ice ( Ewert and Deming, 2013) favor their agglomeration ( Riebesell et al., 1991) and therefore their descent to the seafloor and contribution to sediments. In sea ice, microalgae are largely concentrated near the ice–water interface within the skeletal layer of the congelation ice, where light is sufficient, the water is relatively rich in nutrients at the start of the bloom season, and physico-chemical conditions (e.g., pH, salinity, temperature) are relatively stable. The contribution of sympagic algae to total primary production varies depending on the season and the region (from <1% to 60% e.g., Loose et al., 2011 Dupont, 2012 Fernández-Méndez et al., 2015), but these ice algae represent a crucial food source for the marine food web ( Søreide et al., 2010). For both types of microalgae, light availability is the principal factor controlling bloom onset (e.g., Cota, 1985 Lavoie et al., 2005 Lee and Whitledge, 2005 Popova et al., 2010 Campbell et al., 2016), while nutrient availability is the key factor determining its magnitude, duration, and taxonomic composition. Primary production in the Arctic Ocean is characterized by short-lived, springtime blooms of sympagic (ice-associated) algae and phytoplankton, which are the major sources of autochthonous organic carbon for the Arctic food web ( Horner and Schrader, 1982 Gosselin et al., 1997 Pabi et al., 2008 Wassmann et al., 2011). A succession of bacterial stressors during Arctic ice melt helps to explain the efficient export of sea-ice algal material to the seabed. We thus suggest that protection of sinking sympagic material from bacterial degradation early in a melt season results from low bacterial activity due to salinity stress, while later in the season, algal production of bactericidal compounds induces bacterial mortality. Following snow melt, however, and saturating levels of photosynthetically active radiation in June, we observed enhanced ice-algal production of bactericidal compounds (free palmitoleic acid up to 4.8 mg L –1). Short-term stress reflected by cis-trans isomerase activity was observed only in samples of sinking particles collected early in the time series.
Benner lab project pisces series#
The viability of sympagic-associated bacteria was strong in May (only approximately 10% mortality of total bacteria) and weaker in June (average mortality of 43% maximum of 75%), with instances of elevated mortality in sinking particle samples across the time series (up to 72%).
Benner lab project pisces free#
We also measured cis-trans isomerase activity, known to indicate rapid bacterial response to salinity stress in culture studies, as well as free fatty acids known to be produced by algae as bactericidal compounds. We applied a method not previously used in polar regions-quantitative PCR coupled to the propidium monoazide DNA-binding method-to evaluate the viability of bacteria associated with sympagic and sinking algae. We further tested this hypothesis by analyzing samples of sea ice and sinking particles collected from May 18 to June 29, 2016, in western Baffin Bay as part of the Green Edge project. Salinity stress due to melting ice has been suggested to account for such low bacterial activity. During sea-ice melt in the Arctic, primary production by sympagic (sea-ice) algae can be exported efficiently to the seabed if sinking rates are rapid and activities of associated heterotrophic bacteria are limited.
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