Aeroterrestrial Coleochaete (Streptophyta, Coleochaetales) models early plant adaptation to land.
OBJECTIVE
The streptophyte water-to-land transition was a pivotal, but poorly understood event in Earth history. While some early-diverging modern streptophyte algae are aeroterrestrial (living in subaerial habitats), aeroterrestrial survival had not been tested for Coleochaete, widely regarded as obligately aquatic and one of the extant green algal genera most closely related to embryophytes. This relationship motivated a comparison of aeroterrestrial Coleochaete to lower Paleozoic microfossils whose relationships have been uncertain.


METHODS
We tested the ability of two species of the experimentally tractable, complex streptophyte algal genus Coleochaete Bréb. to (1) grow and reproduce when cultivated under conditions that mimic humid subaerial habitats, (2) survive desiccation for some period of time, and (3) produce degradation-resistant remains comparable to enigmatic Cambrian microfossils.


RESULTS
When grown on mineral agar media or on quartz sand, both species displayed bodies structurally distinct from those expressed in aquatic habitats. Aeroterrestrial Coleochaete occurred as hairless, multistratose, hemispherical bodies having unistratose lobes or irregular clusters of cells with thick, layered, and chemically resistant walls that resemble certain enigmatic lower Paleozoic microfossils. Whether grown under humid conditions or air-dried for a week, then exposed to liquid water, aeroterrestrial Coleochaete produced typical asexual zoospores and germlings. Cells that had been air-dried for periods up to several months maintained their integrity and green pigmentation.


CONCLUSIONS
Features of modern aeroterrestrial Coleochaete suggest that ancient complex streptophyte algae could grow and reproduce in moist subaerial habitats, persist through periods of desiccation, and leave behind distinctive microfossil remains.
Department of Botany, University of Wisconsin, Madison, 53706-1381, USA. lkgraham@wisc.edu
Linda E
LE
Graham
Patricia
P
Arancibia-Avila
Wilson A
WA
Taylor
Paul K
PK
Strother
Martha E
ME
Cook
D000222:Adaptation, Physiological; D005075:Biological Evolution; D002473:Cell Wall / Q000502:physiology; D003890:Desiccation; D005580:Fossils; D006358:Hot Temperature; D012100:Reproduction, Asexual; D057949:Streptophyta / Q000166:cytology / Q000254:growth & development / Q000502:physiology*; D013997:Time Factors; D014867:Water / Q000502:physiology
D014867:Water