WINTER PICOPLANKTON DISTRIBUTIONS AND DIVERSITY OVER A THREE-YEAR STUDY OFF THE NORTHWESTERN ANTARCTIC PENINSULA
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2024
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Abstract
High latitude-driven seasonal shifts govern the northwestern Antarctic Peninsula (NAP), which is experiencing some of the most rapid climatic warming trends on Earth. The ecological seascape faced by picoplankton has transitioned to a shorter and milder sea ice season. Among the microbiological consequences reported in the NAP is the shift in dominant phytoplankton populations, benefiting smaller sized species during the spring. Scarce Antarctic winter studies have suggested that low chlorophyll a levels correlate with pico and nanoplankton but these observations have been restricted to surface waters and have been limited in geographical scope. A picoplankton-focused survey was conducted off the NAP during three consecutive winters (2012 − 2014). The water column (15 − 750 m) featured Winter Water, Transitional Weddell Water and Upper Circumpolar Deep Water. Interannual variability in sea ice conditions influenced shallowing of the mixed layer over the period studied (104 ± 28 to 79 ± 49 m). Flow cytometry was used to quantify phototrophic picoeukaryotes (PPE) and bacterioplankton abundances and distribution from 300 hydrographic samples. A metabarcoding analysis accompanied 230 samples, to investigate the taxonomic composition of picoeukaryotes, including heterotrophic picoeukaryote taxa (HPE).
The low chlorophyll a levels observed each winter correlated with persistent populations of pelagic PPE (R20122 = 0.86, R20132 = 0.63, R20142 = 0.62), regardless of sea ice cover (ROW2 = 0.48, RSI2 = 0.65). PPE abundances were in the 0.5 − 3.9 x 103 cell·mL-1 range, while observed bacterioplankton estimates were two orders of magnitude higher (0.8 − 3.5 x 105 cell·mL-1), both consistent with previous polar reports. PPE concentrations decreased with depth − nevertheless they were regularly detected beyond the mixed layer, in the upper 100 m and rarely even at 200 m. Bacterioplankton abundances were more stable in the upper 100 m in years with higher sea ice while a decreasing depth gradient was more abrupt in 2014 (i.e., in 2012 the bacterioplankton levels only decreased 4% from surface to 100 m, compared to 14% in 2013). This last year of the winter study (2014) showed a subsurface (50 m) maximum in mean prokaryoplankton abundance with a decrease of 25% at 100 m. Total picoplankton abundance increased each winter, coincident with diminished sea ice cover during the same time period. The co-occurrence of PPE and bacterioplankton varied and was positively influenced by sea ice.
The taxonomic identities of the picoeuaryote populations focused on the 0.22 - 3.0 μm seawater fraction. The 18S rRNA amplicon metabarcoding results revealed the winter picoeukaryote community to be more diverse under conditions of lower sea ice. Obligate phototrophic taxa included Mamiellophyceae and Pelagophyceae. Among mixotrophic taxa, Dinoflagellata and Ciliophora were observed regardless of sea ice cover/open water conditions, while heterotrophic taxa were heavily biased towards parasitic picoeukaryotes, previously reported to be abundant in the Gerlache Strait in the west Antarctic Peninsula mesopelagic during winter and present at surface and in summer. Across the three winters surveyed, a single Bathycoccus-affiliated operational taxonomic unit (OTU) was observed to be the most abundant PPE in the upper mixed layer, even in sea ice-covered waters. The dominance of this prasinophyte coincided with seasonal patterns reported for Mamiellales in Arctic waters. Micromonas spp. have been regularly detected in spring and summer Antarctic Peninsula molecular and pigment-based surveys, nevertheless, Micromonas spp. appear to be outcompeted by Bathycoccus sp. in Southern Ocean winter waters, suggesting the prasinophyte seasonal pattern could occur in both polar ocean systems. Syndiniales-affiliated OTUs dominated the HPE community throughout the water column and the surveyed area. From the 3043 OTUs of the amplicon library, 37% were affiliated to parasitic groups. The prevalence of the parasitic Syndiniales spp. was evident throughout the water column and across the regional survey. All together, these findings substantiate the importance of picoeukaryotes to the late winter ecology of the NAP, and open questions regarding the role of phototrophic picoeukaryotes and parasitic Syndiniales in the nutrient and energy transfer of a rapidly changing winter environment.
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Biological oceanography, Microbiology, Ecology, Antarctica, Bacterioplankton, Eukaryote picoplankton, Microbial ecology, Molecular biology, Southern Ocean
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150 pages
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