Symbiosis: Structure and Functions, Ecological and Evolutionary Role, Sélosse 2000

(La Symbiose : Structures et Fonctions, Rôle Écologique et Évolutif)

Book review by Ehsan Kayal

What is symbiosis? How is it defined? What does it involve? And how did it come to be? These are some of the questions French Biologist Marc-André Sélosse explores in this book.

It is not simple to define “symbiosis,” which differs between the English and French languages. Sélosse focuses on the narrower French definition, which involves “long-term coexistence of two different organisms throughout their life with reciprocal benefits” [Sélosse 2000: 22]. In English, the term “symbiosis” refers not only to mutualism (reciprocal benefits), but also to parasitism or commensalism (one partner benefit without impacting the other partner). One important aspect of the study of mutualistic symbiosis is exploring reciprocal physiological exchanges that happen between symbionts (organisms involved in symbiosis). Such exchanges are facilitated by morphological changes in one, the other or both partners.

Sélosse offers readers a glimpse of the breadth of symbiotic relationships found in nature. For example, there are bacteria living inside worms around deep-sea vents in the ocean that use chemical energy seeping from the earth to create organic compounds that nourish their worm hosts. The author also highlights the case of lichen, which appears to be a single species, but is actually formed by a symbiosis between fungi and algae. Sélosse focuses especially on symbiosis involving mycorrhiza, which are the “symbiotic organs formed by a root and a fungus” [Sélosse 2000: 140], and on root nodules, the “symbiotic organs of legumes, often around roots, containing bacteria of the Rhizobiaceae family responsible for fixing nitrogen” [Sélosse 2000: 140].

Leguminous plant hosts provide their Rhizobia symbionts with carbon, while protecting these bacteria from harmful oxygen, in exchange for nitrogen. The efficiency of these symbiosis is such that the “rhizobiaceae-containing root nodules fix as much atmospheric nitrogen per year as the fertilizer industry” [Sélosse 2000: 54]. Similarly, plants with mycorrhizal associations supply their symbionts with carbon in exchange for water, phosphate and other nutrients furnished by the fungi. Studies have linked “an increasing diversity of mycorrhiza” with “increasing diversity of plants” [Sélosse 2000: 67].

Symbiosis can short-circuit the mineralization-immobilization cycle (conversion of inorganic compounds to organic compounds by micro-organisms or plants) by bringing together partners that thrive on each other’s by-products. This results in a “concentration of resources” [Sélosse 2000: 49] that allows organisms to conquer new ecological niches as exemplified by lichen and corals, but also playing a key role in the evolution of the soil that promotes vegetation successions [Sélosse 2000: 60]. For example, lichen is the first to colonize bare rock, where it produces citric acid and holds water, slowing altering the rock in the early stage of soil creation. Rhizobial symbioses then pitch in by fixing nitrogen, otherwise absent in a rocky substrate.

The book also flips some of the common understandings of organismal biology. For instance, many herbivores lack digestive enzymes to break down plant material. Rather, they feed on the population of microorganisms of their rumen, or the “pocket situated upstream of the stomach in ruminants that harbors a symbiotic microflora” [Sélosse 2000: 141]). The rumen is a large organ, whose volume can represent “8 to 15% of total body weight” [Sélosse 2000: 32]. That makes those microorganisms (protists, fungi and bacteria) the “true” herbivores, while the ruminants are secondary consumers.

The older traces of symbiosis predate the origin of the eukaryotic cell (cell with a nucleus enclosed within a nuclear envelope), where the mitochondrion (the energy-producing organelle of the cell) are descendants of a likely unique endosymbiotic event occurring some two billion years ago. In that event, an anaerobic archaean (third kingdom of life composed of unicellular organisms lacking a nucleus; the other two kingdoms are eukaryotes and bacteria) captured a facultative aerobic type of bacteria, which became mitochondria. Similarly, the acquisition by some eukaryotic cells of a chloroplast originating from cyanobacteria gave way to photosynthetic plants.

In some sense, “no organism lives alone, and each carries a symbiotic cortege without which one cannot understand neither the physiology nor the ecological success of the organism” [Sélosse 2000: 134].