From aqueous to gaseous. Charophytes on the move.


One of the most important evolutionary events in the history of life, which shaped our global environment as we see it today, can be dated back to the mid-Paleozoic era between 460 and 360 million years ago (Crane & Kenrick 1997, p. 33). The transition of aquatic plant life to terra firma enabled the commencement of terrestrial life.

Phylogenetic studies have linked the ancestral origin of land plants to a charophycean green algae. Charophycean green algae share several similarities with land plants (Crane & Kenrick 1997, p. 34). Both plants and charophyceans are multicellular, eukaryotic, photosynthetic autotrophs. Both groups also share similarities in the structure of their cell walls, which contain cellulose. Chloroplasts containing the photosynthetic pigments chlorophyll a and b are also evident in both groups (Reece & Campbell 2012, p. 600).


So how did charophytes make the transition onto land? Charophycean algae inhabited shallow waters around the banks of temporary and permanent freshwater lakes, ponds and streams. These areas were subject to fluctuations in water levels and resultant desiccation when water levels dropped dramatically (Caisova & Gabka 2009, p. 4). Natural selection allowed certain algae to survive dry periods when not submerged in water through the development of sporopollenin. Sporopollenin is a polymer casing which surrounds exposed zygotes and prevents desiccation and preservation of spores. This adaptation allowed ancestral charophyceans to eventually live permanently above the water line (Crane & Kenrick 1997, p. 33).

Atmospheric conditions above the water line were far more favourable than those beneath. Most importantly there was direct sunlight, which allowed unfiltered photons to power photosynthesis. Increased CO2 levels also allowed for greater energy production, as plants convert CO2 into glucose for energy during photosynthesis. (Reece & Campbell 2012, p. 601). There was also more nutrient rich soil to help growth aided by mychorrizae. These combined factors and adaptations allowed ancestral charophyceans to make the evolutionary jump from an aqueous to gaseous medium and live permanently on land (Bennici 2008, p. 214).


References:


Bennici, A. 2008, "Origin and early evolution of land plants: Problems and considerations", Communicative & Integrative Biology, vol. 1, no. 2, pp. 212-218.

Caisová, L. & Gabka, M. 2009, "Charophytes (Characeae, Charophyta) in the Czech Republic: Taxonomy, autecology and distribution", Fottea, vol. 9, no. 1, pp. 1-43.

Crane, P.R. & Kenrick, P. 1997, "The origin and early evolution of plants on land", Nature, vol. 389, no. 6646, pp. 33-39.

Reece, J.B. & Campbell, N.A. 2012, Campbell biology, 9th (Australian version) edn, Pearson Australia, Frenchs Forest, N.S.W.

BBSRC Media 2017, How plants crawled out of the sea. [Online video]

Available at: https://youtube/OyCyIOEyL20?t=85 [Accessed 17 March 2018].

Comments

  1. Tasmin17 March 2018 at 15:41

    Very interesting. I was just wondering if sporopollenin was first observed in the charophytes, or whether it was observed perhaps in the immediate ancestor to the charophytes?

    ReplyDelete

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