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An investigation into the effects of increasing salinity on photosynthesis in freshwater unicellular cyanobacteria during the late Archaean
Author(s) -
Herrmann Achim J.,
Gehringer Michelle M.
Publication year - 2019
Publication title -
geobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.859
H-Index - 72
eISSN - 1472-4669
pISSN - 1472-4677
DOI - 10.1111/gbi.12339
Subject(s) - photosynthesis , cyanobacteria , brackish water , salinity , biology , botany , ecology , bacteria , genetics
The oldest species of bacteria capable of oxygenic photosynthesis today are the freshwater Cyanobacteria Gloeobacter spp., belonging to the class Oxyphotobacteria . Several modern molecular evolutionary studies support the freshwater origin of cyanobacteria during the Archaean and their subsequent acquisition of salt tolerance mechanisms necessary for their expansion into the marine environment. This study investigated the effect of a sudden washout event from a freshwater location into either a brackish or marine environment on the photosynthetic efficiency of two unicellular freshwater cyanobacteria: the salt‐tolerant Chroococcidiopsis thermalis PCC 7203 and the cyanobacterial phylogenetic root species, Gloeobacter violaceus PCC 7421. Strains were cultured under present atmospheric levels (PAL) of CO 2 or an atmosphere containing elevated levels of CO 2 and reduced O 2 (eCO 2 rO 2 ) in simulated shallow water or terrestrial environmental conditions. Both strains exhibited a reduction in growth rates and gross photosynthesis, accompanied by significant reductions in chlorophyll a content, in brackish water, with only C. thermalis able to grow at marine salinity levels. While the experimental atmosphere caused a significant increase in gross photosynthesis rates in both strains, it did not increase their growth rates, nor the amount of O 2 released. The differences in growth responses to increasing salinities could be attributed to genetic differences, with C. thermalis carrying additional genes for trehalose synthesis. This study demonstrates that, if cyanobacteria did evolve in a freshwater environment, they would have been capable of withstanding a sudden washout into increasingly saline environments. Both C. thermalis and G. violaceus continued to grow and photosynthesise, albeit at diminished rates, in brackish water, thereby providing a route for the evolution of open ocean‐dwelling strains, necessary for the oxygenation of the Earth's atmosphere.