Set the column format specifier:
http://astropy.readthedocs.org/en/v0.3/table/construct_table.html#format-specifier
>>> from astropy.table import Table
>>> t = Table([[1.0]], names=['a'])
>>> t['a'].format = '%.16e'
>>> t.write(sys.stdout, format='ascii')
a
1.0000000000000000e+00