Phagotrophy by the picoeukaryotic green alga Micromonas: implications for Arctic Oceans

Abstract

Photosynthetic picoeukaryotes (PPE) are recognized as major primary producers and contributors to phytoplankton biomass in oceanic and coastal environments. Molecular surveys indicate a large phylogenetic diversity in the picoeukaryotes, with members of the Prymnesiophyceae and Chrysophyseae tending to be more common in open ocean waters and Prasinophyceae dominating coastal and Arctic waters. In addition to their role as primary producers, PPE have been identified in several studies as mixotrophic and major predators of prokaryotes. Mixotrophy, the combination of photosynthesis and phagotrophy in a single organism, is well established for most photosynthetic lineages. However, green algae, including prasinophytes, were widely considered as a purely photosynthetic group. The prasinophyte Micromonas is perhaps the most common picoeukaryote in coastal and Arctic waters and is one of the relatively few cultured representatives of the picoeukaryotes available for physiological investigations. In this study, we demonstrate phagotrophy by a strain of Micromonas (CCMP2099) isolated from Arctic waters and show that environmental factors (light and nutrient concentration) affect ingestion rates in this mixotroph. In addition, we show size-selective feeding with a preference for smaller particles, and determine P vs I (photosynthesis vs irradiance) responses in different nutrient conditions. If other strains have mixotrophic abilities similar to Micromonas CCMP2099, the widespread distribution and frequently high abundances of Micromonas suggest that these green algae may have significant impact on prokaryote populations in several oceanic regimes.

Functional Redundancy Facilitates Resilience of Subarctic Phytoplankton Assemblages toward Ocean Acidification and High Irradiance

“…The dominance of picoeukaryotes (mainly M. pusilla) under LL and diatoms under HL can also explain the counterintuitively rising Chla:C ratios with increasing light (Table 1). Previous studies have shown low Chla:C ratios in M. pusilla resulting from high energy demand imposed by motility (Halsey et al, 2014) and its mixotrophic nature (McKie-Krisberg and Sanders, 2014). The lower Chla:C under LL in the initial part of the experiment thus may primarily reflect ecological shifts within the species assemblages, rather than photo-physiological effects.…”

Strong Seasonality of Marine Microbial Eukaryotes in a High-Arctic Fjord (Isfjorden, in West Spitsbergen, Norway)

“…Furthermore, mixotrophy may facilitate survival during the dark winter period, as suggested for phototrophic flagellates in high-latitude lakes (60,61). The Arctic Micromonas ecotype, which is dominant and widespread in Arctic waters (58,(62)(63)(64), was recently shown to be capable of phagotrophy (65). Living cells of the Arctic Micromonas ecotype have been detected around Spitsbergen even during the polar night (15,40).…”

Winter mixing impacts gene expression in marine microbial populations in the Gulf of Aqaba

“…These results suggest that, similar to cyanobacteria, Mamiellales are metabolically active at the aphotic depth with a large fraction of genes assigned to anabolic processes. The upregulation of phagocytosis at 440 m suggests mixotrophy, which is a strategy applied by Micromonas under light or nutrient limitation (McKie-Krisberg & Sanders 2014). Yet, comparable to the results obtained for cyanobacteria at this depth, we detected mostly low expression of organic-compound transporters in Mamiellales (Table S4).…”