Mixotrophy everywhere on land and in water: thegrand écarthypothesis

Abstract

There is increasing awareness that many terrestrial and aquatic organisms are not strictly heterotrophic or autotrophic but rather mixotrophic. Mixotrophy is an intermediate nutritional strategy, merging autotrophy and heterotrophy to acquire organic carbon and/or other elements, mainly N, P or Fe. We show that both terrestrial and aquatic mixotrophs fall into three categories, namely necrotrophic (where autotrophs prey on other organisms), biotrophic (where heterotrophs gain autotrophy by symbiosis) and absorbotrophic (where autotrophs take up environmental organic molecules). Here we discuss their physiological and ecological relevance since mixotrophy is found in virtually every ecosystem and occurs across the whole eukaryotic phylogeny, suggesting an evolutionary pressure towards mixotrophy. Ecosystem dynamics tend to separate light from non-carbon nutrients (N and P resources): the biological pump and water stratification in aquatic ecosystems deplete non-carbon nutrients from the photic zone, while terrestrial plant successions create a canopy layer with light but devoid of non-carbon soil nutrients. In both aquatic and terrestrial environments organisms face a grand ecart (dancer's splits, i.e., the need to reconcile two opposing needs) between optimal conditions for photosynthesis vs. gain of non-carbon elements. We suggest that mixotrophy allows adaptation of organisms to such ubiquist environmental gradients, ultimately explaining why mixotrophic strategies are widespread.

Does evolutionary history determine specificity in broad ecological interactions?

“…and Paphiopedilum spp., and T. calospora, another frequent orchid associate, are not clear. Many photosynthetic orchid species are capable of extracting fungal energy as adults, a condition referred to as partial mycoheterotrophy (Gebauer, Preiss, & Gebauer, 2016;Selosse, Charpin, & Not, 2017), and some species have evolved to utilize fungal carbon exclusively (Selosse, Bocayuva, Kasuya, & Courty, 2016;Selosse et al, 2017). These fungi are generally thought to act as saprotrophs in forest environments (Rasmussen, 1995;Roberts, 1999), although Tulasnella asymmetrica is a fungus known to be facultatively ectomycorrhizal and exploited by mycoheterotrophic liverworts, as well (Bidartondo, Bruns, Weiß, Sérgio, & Read, 2003;Oberwinkler, Cruz, & Suárez, 2017).…”

Universal entrainment mechanism controls contact times with motile cells

“…Phagotrophic selectivity is complex, and surprisingly common even amongst microbial species historically considered exclusive autotrophs (e.g. some green microalgae [42]). It certainly depends on many factors including chemical cues and cell surface properties.…”

“…For swimming microorganisms, many interactions hinge on close contact. These include fundamental processes like nutrient uptake [24][25][26][27][28][29][30][31]; viral and fungal infection of microorganisms of ecological and commercial importance [32][33][34], eukaryotic fertilisation [35]; and grazing, which happens on natural preys [30,[36][37][38] as well as marine microplastics [39,40], and is recently being discovered as a fundamental behaviour in many strains of motile green algae until recently regarded as exclusive phototrophs [34,41,42]. With the exception of complex feeding currents in ciliates like Vorticella [43,44] or Paramecium [45,46], the window of opportunity for these microbial interactions to take place will depend on a finite contact time T .…”

Use of glucose biosensors to measure extracellular glucose exudation by intertidal microphytobenthos in southern Tasmania

“…The small decreases seen in the antibiotic treated communities/cultures probably came from both heterotrophic uptake by diatoms and possibly antibiotic-resistant bacteria. Active heterotrophic uptake of glucose in the dark by diatoms has been demonstrated in many studies (Palmisano et al 1985, Rivkin and Putt 1987, Tuchman 1996 and may, in fact, be the norm rather than the exception (Mitra et al 2016, Selosse et al 2017. This is the first study we know of to use glucose micro biosensors to quantify MPB extracellular glucose concentration and exudation.…”