The role of mixotrophic protists in the biological carbon pump

Abstract

Abstract. The traditional view of the planktonic food web describes consumption of inorganic nutrients by photoautotrophic phytoplankton, which in turn supports zooplankton and ultimately higher trophic levels. Pathways centred on bacteria provide mechanisms for nutrient recycling. This structure lies at the foundation of most models used to explore biogeochemical cycling, functioning of the biological pump, and the impact of climate change on these processes. We suggest an alternative new paradigm, which sees the bulk of the base of this food web supported by protist plankton communities that are mixotrophic -combining phototrophy and phagotrophy within a single cell. The photoautotrophic eukaryotic plankton and their heterotrophic microzooplankton grazers dominate only during the developmental phases of ecosystems (e.g. spring bloom in temperate systems). With their flexible nutrition, mixotrophic protists dominate in more-mature systems (e.g. temperate summer, established eutrophic systems and oligotrophic systems); the more-stable water columns suggested under climate change may also be expected to favour these mixotrophs. We explore how such a predominantly mixotrophic structure affects microbial trophic dynamics and the biological pump. The mixotroph-dominated structure differs fundamentally in its flow of energy and nutrients, with a shortened and potentially more efficient chain from nutrient regeneration to primary production. Furthermore, mixotrophy enables a direct conduit for the support of primary production from bacterial production. We show how the exclusion of an explicit mixotrophic component in studies of the pelagic microbial communities leads to a failure to capture the true dynamics of the carbon flow. In order to prevent a misinterpretation of the full implications of climate change upon biogeochemical cycling and the functioning of the biological pump, we recommend inclusion of multi-nutrient mixotroph models within ecosystem studies.

Bridging the Gap between Knowing and Modeling Viruses in Marine Systems—An Upcoming Frontier

“…Other recent studies, however, are questioning the paradigm where bacteria are central in the mechanisms for nutrient recycling, by pointing instead to the importance of mixotrophic organisms (Berge et al, 2016;Caron, 2016;Ward and Follows, 2016), although the question on the importance mixotrophs has previously been raised (Hall et al, 1993;Arenovski et al, 1995;Fenchel, 2008). According to this new paradigm, communities of mixotrophic protist plankton support the bulk of the base of the microbial food webs, having some fundamental differences in the flow of energy and nutrients from the bacteria-dominated microbial food web paradigm (Mitra et al, 2014). This mixotroph-dominated structure implies a shorter and potentially more efficient circuit from nutrient regeneration to primary production, explaining that primary production may be supported by bacterial production.…”

“…Mixotrophy is of particular interest given that mixotrophic organisms also have a significant impact on bacterial populations. Despite being known as important player in the microbial food webs, only recently mixotrophs have received attention in modeling studies (Ward et al, 2011;Mitra et al, 2014;Berge et al, 2016;Ward and Follows, 2016). The inclusion of these "perfect beats" combining several modes of nutrition requires additional inclusion of complexity into models, as shown by Flynn and Mitra (2009); stoichiometric implication must be considered by the model, by which the C-to-nutrient ratios must be explicitly expressed for the mixotrophs and food source, otherwise the model is doomed to fail to reflect the reality of the trophic interactions, both at the mixotrophs level as on the ecosystem level.…”

“…However, the potential impact of viruses on the flow of energy and matter seems to be too important for their exclusion. Ultimately, the inclusion of viruses may lend additional insights into more enduring topics such as the link or sink problem in the carbon cycling (Vichi et al, 2004;Fenchel, 2008), as well as on more recent topics such as the role of mixotrophs (Mitra et al, 2014) and the relationship between the limitation of nutrients and viral infection (Béchette et al, 2013).…”

“…• The classification of DOM into a larger number of categories and the identification of different processes controlling different fractions of DOM (Keller and Hood, 2013;Hasumi and Nagata, 2014); • The inclusion of P, critically important for viral replication (Wilson et al, 1996;Monier et al, 2012), but also consider multiple nutrients (organic and inorganic), as they have a significant influence on the physiologic processes of phytoplankton cells and limit their growth; • A latent period between infection and lysis, proportions of lysogenic vs. lytic virus infection under different nutrient availability (Béchette et al, 2013), and going beyond the type-I interactions between viruses and hosts and the exponential decay of viruses; • Taking upon the ERSEM model-type approach with multielement cycles, variable stoichiometry of both living entities and OM, and several functional groups; • Move toward the new paradigm in food webs where mixotrophy has a central role (Mitra et al, 2014;Berge et al, 2016).…”

Modeling Plankton Mixotrophy: A Mechanistic Model Consistent with the Shuter-Type Biochemical Approach

“…Despite the potential significance of mixotrophy in aquatic systems and the existence of mathematical models describing this trophic mode, few ecological/biogeochemical models consider it explicitly (e.g., Mitra et al, 2014). Those models that do include mixotrophs demonstrate the potential for mixotrophy to significantly impact the flow of energy and nutrients in the system.…”