Mixotrophic Plankton in the Polar Seas: A Pan-Arctic Review

Abstract

Polar marine ecosystems are characterized by low water temperatures, sea ice cover, and extreme annual variation in solar irradiance and primary productivity. A review of the available information from the Arctic suggests that mixotrophy (i.e., the combination of photosynthetic and phagotrophic modes of nutrition in one cell) is wide spread among plankton. In the central Arctic Ocean (AO) in summer, mixotrophic flagellates such as Micromonas and Dinobryon can account for much of bacterivory. Planktonic ciliates with acquired phototrophy form the bulk of microzooplankton biomass in both the ultraoligotrophic deep basins of AO and its productive shelf seas. With the exception of the diatom bloom in the marginal ice zone, mixotrophic ciliates often dominate total chlorophyll in the mixed layer in summer taking advantage of the 24-h insolation. Their relatively high growth rates at low temperatures indicate that they are an important component of primary and secondary production. The key Arctic copepod species preferentially feed on chloroplast-bearing ciliates, which form an important link in the planktonic food web. The limited available year round data indicate that mixotrophic plankton persist in the water column during the long polar winter when irradiance is low or absent and ice cover further reduces light penetration. These observations suggest that at high latitudes an alternative food web based on mixotrophy may dominate the pelagic lower food web during much of the year.

Sampling bias misrepresents the biogeographical significance of constitutive mixotrophs across global oceans

“…Biases among records of CMs may also be particularly high owing to differential efforts at sampling CMs of different sizes. It is not common practice to identify to species level in plankton surveys, especially among the smaller flagellate cells that are coincidentally mixotrophic (Sanders & Gast, ; Stoecker & Lavrentyev, ). Incubation experiments have demonstrated ingestion of prey by these small CMs (< 20 µm), whereas taxonomic identification using traditional diagnostic approaches has been difficult owing to few distinctive features for these species.…”

“…Some co-occurring protist species (including competitors, prey and predators) are also known mixotrophs, although many are not often recorded because they are not themselves HAB species. Indeed, of the 80 CM species included within our analysis, 51 species were dinophytes, despite the importance of mixotrophy having been well recognized in other taxonomic groups (Gast et al, 2014;Stoecker & Lavrentyev, 2018;Unrein et al, 2014).…”

“…Biases among records of CMs may also be particularly high owing to differential efforts at sampling CMs of different sizes. It is not common practice to identify to species level in plankton surveys, especially among the smaller flagellate cells that are coincidentally mixotrophic Stoecker & Lavrentyev, 2018).…”

No evidence of phago-mixotropy in Micromonas polaris, the dominant picophytoplankton species in the Arctic

“…Arctic phytoplankton communities experience extreme environmental conditions such as nutrient limitation, exposure to a long period of darkness (polar winter) and changes in light levels under the ice caused by the variation in snow coverage and ice thickness [15]. In such a highly variable context, it has been suggested that phago-mixotrophy (ability to combine photosynthesis and bacterivory) could be a common trophic strategy among Arctic protists [16]. At the scale of the global ocean, phago-mixotrophy is an important, but until recently underestimated, process for energy and nutrient transfer (e.g.…”

“…Phago-mixotrophy would be another advantageous trait that could contribute to the success of M. polaris in the Arctic. Under prolonged periods of darkness or low irradiance, phago-mixotrophs could survive, despite reduced or even null rates of photosynthesis, by supplementing their carbon requirements through phagocytosis [16, 35, 36]. Under oligotrophic conditions, phago-mixotrophy can also supply the cell with limiting nutrients [37].Micromonas has been previously hypothesized to be a phago-mixotroph in laboratory and field experiments [38][39][40][41].…”