Marked spatiotemporal variations in small phytoplankton structure in contrasted waters of the Southern Ocean (Kerguelen area)

Abstract

In the Southern Ocean, diatom blooms have attracted a lot of attention, while other small nonsilicified phytoplankton groups have been less studied. Here, small phytoplankton (< 20 μm, including small diatoms and nonsilicified small phytoplankton) are focused on in two contrasting areas: the productive Kerguelen plateau and its surrounding low productivity waters. To assess the diversity and spatial structuration of phytoplankton, discrete plankton samples (0-300 m layer) of two size fractions (< 20 and 20-100 μm) were analyzed with 18S rDNA amplicon sequencing in late summer. Phytoplankton seasonal succession was described using flow cytometry, pigments, and environmental data, from two previous cruises (during the onset and decline of the diatom bloom). In the mixed layer, small nonsilicified phytoplankton represented less than 10% of chlorophyll a (Chl a) during the onset and late diatom bloom on the plateau, but they increased on and off the plateau after the bloom (53-70% of Chl a). Phaeocystis antarctica was relatively abundant at all stations after the bloom, but other small phytoplanktonic groups featured marked differences on and off the plateau. Higher NH + 4 concentrations on the plateau appeared to stimulate the presence of Micromonas, while Pelagophytes were enhanced off the plateau. A diverse assemblage of small diatoms was also promoted off the plateau, where silicate concentration was still high. Interestingly, P. antarctica represented up to 25% of all reads at 300 m depth off the plateau in the larger size fraction suggesting a significant contribution to carbon export through aggregation in low productive waters.

Seasonal dynamics of prokaryotes and their associations with diatoms in the Southern Ocean as revealed by an autonomous sampler

“…The aim of the present study was to provide a high‐resolution description of the microbial community composition during an entire season in the Southern Ocean. Given the dominant contribution of diatoms to bulk phytoplankton biomass during the productive season in our study region (Lasbleiz et al ., 2016; Rembauville et al ., 2017; Irion et al ., 2020), we further aimed to explore the associations between prokaryotic taxa and diatom species. We addressed these objectives using samples collected with an autonomous sampler and fixed with mercuric chloride in surface waters of the central Kerguelen Plateau, where natural iron fertilization induces massive spring phytoplankton blooms.…”

Trophic states regulate assembly processes and network structures of small chromophytic phytoplankton communities in subtropical coastal ecosystem

“…As previous study described (Pujari et al, 2019), certain SCP community may be missing due to the bias of the primers used for PCR ampli cation. Besides, the discrepancy of region that make certain SCP discovered in other studies (Bolaños et al, 2020;Irion et al), such as Haptophyceae, Pelagophyceae, and Cyanophyceae, to be inevitably left out in the Beibu Gulf.…”

“…Anthropogenic activities and climate changes impose severe effects on aquatic coastal ecosystems with the advent and expansion of coastal aquaculture, urban, agricultural, and industrial runoff (Escalas et al, 2019). As an important component in aquatic coastal ecosystems, marine small chromophytic phytoplankton (SCP) that de ned as cells within the size range of 0.2-20μm, including both picophytoplankton and nanophytoplankton, have an advantage over larger cells in nutrient uptake due to their higher surface to volume ratio (Irion et al, 2020;Irwin et al, 2006). They are vital contributors of primary production for its photosynthetic component and perform a range of ecological roles in carbon…”

“…Modern molecular methods have revolutionized our knowledge of assessment of marine SCP communities and diversity. For example, 18S rRNA (for pico-eukaryotes) (Irion et al;Staay et al, 2000) represent useful tools for ecologists to study diversity. Besides, functional gene markers, especially rbcL, which encodes the large subunit (L) of ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) involved carbon assimilation (Nan et al, 2016;Xu and Tabita, 1996), have been extensively used and provide increased resolution for assessments of biodiversity on SCP community across different coastal environments, including the bay of Bengal (Pujari et al, 2019), Western Iberia (Ferreira et al, 2019), Western Mediterranean (Paches et al, 2019) and Northern South China Sea .…”

Trophic states regulate assembly processes and network structures of small chromophytic phytoplankton communities in subtropical coastal ecosystem

“…Anthropogenic activities and climate changes impose severe effects on aquatic coastal ecosystems with the advent and expansion of coastal aquaculture, urban, agricultural, and industrial runoff (Escalas et al, 2019). As an important component in aquatic coastal ecosystems, marine small chromophytic phytoplankton (SCP) that de ned as cells within the size range of 0.2-20μm, including both picophytoplankton and nanophytoplankton, have an advantage over larger cells in nutrient uptake due to their higher surface to volume ratio Irion et al, 2020;Irwin et al, 2006). They are vital contributors of primary production for its photosynthetic component and perform a range of ecological roles in carbon xation, nutrient cycling and regulating overall food-web dynamics (Bolaños et al, 2020;Capblancq, 1990;Falkowski et al, 1998).…”