Abstract
Rising temperatures and changing winds drive the expansion of the highly productive polynyas (open water areas surrounded by sea ice) abutting the Antarctic continent. Phytoplankton blooms in polynyas are often dominated by the haptophyte Phaeocystis antarctica, and they generate the organic carbon that enters the resident microbial food web. Yet, little is known about how Phaeocystis blooms shape bacterial community structures and carbon fluxes in these systems. We identified the bacterial communities that accompanied a Phaeocystis bloom in the Amundsen Sea polynya during the austral summers of 2007–2008 and 2010–2011. These communities are distinct from those determined for the Antarctic Circumpolar Current (ACC) and off the Palmer Peninsula. Diversity patterns for most microbial taxa in the Amundsen Sea depended on location (e.g., waters abutting the pack ice near the shelf break and at the edge of the Dotson glacier) and depth, reflecting different niche adaptations within the confines of this isolated ecosystem. Inside the polynya, P. antarctica coexisted with the bacterial taxa Polaribacter sensu lato, a cryptic Oceanospirillum, SAR92 and Pelagibacter. These taxa were dominated by a single oligotype (genotypes partitioned by Shannon entropy analysis) and together contributed up to 73% of the bacterial community. Size fractionation of the bacterial community [<3 μm (free-living bacteria) vs. >3 μm (particle-associated bacteria)] identified several taxa (especially SAR92) that were preferentially associated with Phaeocystis colonies, indicative of a distinct role in Phaeocystis bloom ecology. In contrast, particle-associated bacteria at 250 m depth were enriched in Colwellia and members of the Cryomorphaceae suggesting that they play important roles in the decay of Phaeocystis blooms.
Highlights
Phytoplankton blooms account for a significant fraction of marine primary production
It was decided to incorporate these data in order to derive first hints regarding the reproducibility of bacterial community compositions that accompany Phaeocystis blooms in the Amundsen Sea polynya (ASP) and to gain early insights into the bacterial taxa that may associate with Phaeocystis colonies and other particles
During the 2010–2011 austral summer, surface water samples were collected at 18 sites across the ASP (Figure 1): 3 sites along the Dotson glacier, 2 sites covered by pack ice at the outer fringe of the polynya near the shelf break and 13 sites across a bloom of Phaeocystis antarctica (>10 μg Chl a.L−1, O2saturation at >400%, pCO2 100–250 ppm) in the open waters of the polynya
Summary
Phytoplankton blooms account for a significant fraction of marine primary production. Such blooms occur in the open ocean [e.g., by the cyanobacterium Trichodesmium (Capone et al, 1997, 2005) or the diatoms Hemiaulus and Rhizoselenia (Subramaniam et al, 2008)] as well as in the coastal ocean (e.g., Karenia, Pseudonitzschia), where they can be a nuisance for aquaculture and fisheries. The haptophyte Phaeocystis is a ubiquitous marine phytoplankter that causes blooms in coastal seas. Species contained in this genus have typical geographic distributions with P. pouchetii dominating in the Arctic Ocean, P. globosa in temperate coastal seas and P. antarctica occupying diverse niches in the Southern Ocean, respectively (Schoemann et al, 2005). Phaeocystis blooms significantly impact local carbon, nutrient and sulfur cycles (Van Boekel and Stefels, 1993; Yager et al, 2012) and can disturb ecosystems (Chen et al, 1999)
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