What is the maximum concentration of NaCl for life in the oceans to be possible?
Was it fortuitous that the concentration was originally no higher, in which case the evolution of life on Earth would not have occurred?
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There are organisms called halophiles which thrive in conditions of high salt concentration - and, indeed, require high salt concentrations in order to grow. According to Wikipedia:
Halophiles can be found in water bodies with salt concentration more than five times greater than that of the ocean ...
Since the earliest life on Earth originated in the oceans over 3.5 billion years ago, it is likely that it was adapted by evolution to suit the salinity of the oceans as they were then. If the salinity of the oceans had been significantly different then life would have evolved in a different direction.
First thing: the oceans were (probably) much less saline when life first formed. EDIT Well, so much for my ancient primary school data - with thanks to trond hansen for pointing out that
Initial salinity of the oceans was 1.5–2× the modern value and remained high throughout the Archean in the absence of long-lived continental cratons required to sequester giant halite beds and brine derived from evaporating seawater. Marine life was limited to microbes (including cyanobacteria) that could tolerate the hot, saline early ocean.
Living species slowly adapted as salinity, as well as atmospheric and dissolved gas levels, changed over epochs.
Next - there's a maximum salt concentration that water can support in solution. After that it precipitates out, so there's your effective upper bound.
And, dare I say it, "Life, ummm, finds a way."