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

Diane K Stoecker,Peter J Lavrentyev

doi:10.3389/fmars.2018.00292

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 most of bacterivory. Planktonic ciliates with acquired phototrophy form the bulk of microzooplankton biomass in both the ultra-oligotrophic 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 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 year round data indicate that are available show that mixotrophic plankton persist in the water column during the long polar winter when irradiance is low or absent and ice cover reduces further 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.

Highlights

Mixotrophy, defined as the combination of phagotrophy and photosynthesis in an individual cell, is increasingly recognized as an important trophic mode among planktonic protists, both “phytoplankton” and “microzooplankton,” in the global ocean but in polar waters mixotrophy has not received much attention
In this review we primarily focus on the following contrasting regions of the Arctic: the deep oligotrophic Canada Basin of the Arctic Ocean, the relatively shallow and productive Barents and Kara shelf seas, and the main entrance for the Atlantic water into the Arctic Ocean, the Fram Strait (Figure 1)
Many mixotrophic flagellates are found in Arctic waters, but there is only one reported field investigation (Sanders and Gast, 2012) of mixotrophy from Arctic Seas

Summary

Introduction

Mixotrophy, defined as the combination of phagotrophy and photosynthesis in an individual cell, is increasingly recognized as an important trophic mode among planktonic protists, both “phytoplankton” and “microzooplankton,” in the global ocean (reviewed in Caron, 2017; Leles et al, 2017; Selosse et al, 2017; Stoecker et al, 2017) but in polar waters mixotrophy has not received much attention. This mixed mode of nutrition is so wide-spread that the above dichotomy may be no longer suitable for an accurate description of the role of unicellular eukaryotes in the ocean (Flynn et al, 2013). Modeling indicates that mixotrophy can increase mean organism size and trophic transfer, potentially resulting in increased production at upper levels in the food web (Mitra et al, 2014; Ward and Follows, 2016).

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Conclusion