Mixotrophic basis of Atlantic oligotrophic ecosystems

Abstract

Oligotrophic subtropical gyres are the largest oceanic ecosystems, covering >40% of the Earth's surface. Unicellular cyanobacteria and the smallest algae (plastidic protists) dominate CO 2 fixation in these ecosystems, competing for dissolved inorganic nutrients. Here we present direct evidence from the surface mixed layer of the subtropical gyres and adjacent equatorial and temperate regions of the Atlantic Ocean, collected on three Atlantic Meridional Transect cruises on consecutive years, that bacterioplankton are fed on by plastidic and aplastidic protists at comparable rates. Rates of bacterivory were similar in the light and dark. Furthermore, because of their higher abundance, it is the plastidic protists, rather than the aplastidic forms, that control bacterivory in these waters. These findings change our basic understanding of food web function in the open ocean, because plastidic protists should now be considered as the main bacterivores as well as the main CO 2 fixers in the oligotrophic gyres.

Mixotrophic haptophytes are key bacterial grazers in oligotrophic coastal waters

“…The mixotrophic mode of nutrition is considered to be particularly beneficial when inorganic nutrients are limiting (Nygaard and Tobiesen, 1993;Rothhaupt, 1996;Flö der et al, 2006). It has been demonstrated that mixotrophic algae, in particular those o5 mm in size, are major grazers of picoplankton in oligotrophic marine systems, being regularly responsible for 450% of the total bacterivory (Unrein et al, 2007;Zubkov and Tarran, 2008;Hartmann et al, 2012). These estimates were obtained using completely different methodologies, strongly suggesting a global significance of mixotrophy in all marine systems (Gasol et al, 2008).…”

A global perspective on marine photosynthetic picoeukaryote community structure

“…The success of these organisms could be because of mixotrophic behaviour (that is, an ability to supplement their phototrophic physiology by preying on bacterioplankton). Indeed, recent evidence from natural environments suggests that PPEs can contribute greatly to bacterivory (Zubkov and Tarran, 2008;Sanders and Gast, 2011;Hartmann et al, 2012). The nutritional flexibility offered by mixotrophy gives a significant competitive advantage over both purely phototrophic and aplastidic cells under different light (depth) and nutrient regimes.…”

“…This focus is largely because of the increased recognition of PPEs as key contributors to phytoplankton biomass and primary production across various marine pelagic ecosystems (Li, 1994;Worden et al, 2004;Richardson and Jackson, 2007;Cuvelier et al, 2010;Jardillier et al, 2010;Grob et al, 2011). Moreover, PPEs have recently been shown to be key bacterivores controlling bacterioplankton abundance in both temperate (Zubkov and Tarran, 2008) and oligotrophic gyre (Hartmann et al, 2012) waters. Furthermore, oceanic nitrogen:phosphorus (N:P) ratios are likely to be governed by plankton biogeography because of the varying nutrient metabolism of different groups (Weber and Deutsch, 2010).…”

Some Mixotrophic Flagellate Species Selectively Graze on Archaea

“…Currently, there is no doubt of the ubiquity of mixotrophy and its significance in the functioning of planktonic systems. Under oligotrophic conditions, phototrophic flagellates can account for up to 80% of total bacterial grazing (4,6,7).Predation by protists is among the primary mortality factors of prokaryotes in planktonic communities and thus is an important selective pressure. It becomes a structuring factor of the abundance, morphology, composition, and activity of bacterial assemblages (8, 9).…”

“…Currently, there is no doubt of the ubiquity of mixotrophy and its significance in the functioning of planktonic systems. Under oligotrophic conditions, phototrophic flagellates can account for up to 80% of total bacterial grazing (4,6,7).…”