Abstract
Abstract Sea ice covers approximately 5% of the ocean surface and is one of the most extensive ecosystems on the planet. The microbial communities that live in sea ice represent an important food source for numerous organisms at a time of year when phytoplankton in the water column are scarce. Here we describe the distributions and physiology of sea ice microalgae in the poorly studied Amundsen Sea sector of the Southern Ocean. Microalgal biomass was relatively high in sea ice in the Amundsen Sea, due primarily to well developed surface communities that would have been replenished with nutrients during seawater flooding of the surface as a result of heavy snow accumulation. Elevated biomass was also occasionally observed in slush, interior, and bottom ice microhabitats throughout the region. Sea ice microalgal photophysiology appeared to be controlled by the availability of both light and nutrients. Surface communities used an active xanthophyll cycle and effective pigment sunscreens to protect themselves from harmful ultraviolet and visible radiation. Acclimation to low light microhabitats in sea ice was facilitated by enhanced pigment content per cell, greater photosynthetic accessory pigments, and increased photosynthetic efficiency. Photoacclimation was especially effective in the bottom ice community, where ready access to nutrients would have allowed ice microalgae to synthesize a more efficient photosynthetic apparatus. Surprisingly, the pigment-detected prymnesiophyte Phaeocystis antarctica was an important component of surface communities (slush and surface ponds) where its acclimation to high light may precondition it to seed phytoplankton blooms after the sea ice melts in spring.
Highlights
Microalgal biomass was relatively high in sea ice in the Amundsen Sea, due primarily to well developed surface communities that would have been replenished with nutrients during seawater flooding of the surface as a result of heavy snow accumulation
The mean microalgal biomass in sea ice within our study region (17.3 ± 17.8 mg chlorophyll a (Chl a) m-2), which consisted primarily of stations sampled in the Amundsen Sea during summer, was comparable to the circumpolar summer mean (12.9 ± 16.5 mg Chl a m-2) calculated from historical data in the Antarctic Sea Ice Processes and Climate (ASPeCt)-Bio database (Meiners et al, 2012)
Like the ASPeCt-Bio database, peak Chl a biomass during our study was distributed in microhabitats throughout the ice column
Summary
Over the course of an annual cycle, the sea ice that forms on the surface of polar oceans extends over an area of 15-22 × 106 km2 This enormous surface area ranks sea ice as one of the most expansive ecosystems on Earth, covering approximately 4.1–6.1% of the surface area of the global ocean (Arrigo, 2014). The number of observations of sea ice microbial communities has increased in recent years, there are still large areas around the Antarctic for which few samples are available These include the south Pacific and south Indian oceans and the Amundsen Sea (Meiners et al, 2012).
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