While Turkish leaders have slammed the German authorities for not showing solidarity in the fight against terrorism, German politicians have criticized Turkey over human rights and press-freedom issues.
At the time of being arrested, he was a reported for Die Welt, a German newspaper.
German officials view the list from Ankara as "absurd" and "ridiculous".
Magdalena Freudenschuss, who lives with their two children in Berlin, tells RTL Television that Peter Steudtner "does this kind of work because he believes in non-violence and believes in ways to make this world and our societies a better place".
In a statement, Turkey's foreign ministry said they have kept Germany's charge d'affaires in Ankara informed of Peter Steudtner's case, adding that "the independent Turkish judiciary must be trusted".
Merkel's spokesman Steffen Seibert said Wednesday that in the current context, any progress on Turkey's bid to join the European Union was "not on the cards".More news: French defense chief de Villiers quits over clash with Macron
"Certainly we couldn't have accepted the deal", Germany's foreign ministry said. At 22 billion euros ($25 billion) in deliveries of mostly auto parts and chemical products, Turkey ranked in 15th place past year, BGA said.
Turkish deputy prime minister Mehmet Simsek said on Thursday the reports were untrue.
Foreign Minister Sigmar Gabriel, a day after his ministry summoned Turkey's ambassador, interrupted his holidays and returned to Berlin to deliver his unusually strong comments towards President Recep Tayyip Erdogan.
Steudtner "was no Turkey expert - he never wrote about Turkey, he had no contacts in the political establishment. and never appeared as a critic", Gabriel told reporters. "They are trying to earn points for themselves", said Ibrahim Kalin, the presidential spokesman.
"Until now there was guidance for certain groups but we're saying that now applies to all German citizens, not just for those with certain jobs".
German authorities say 22 German citizens were held in Turkey in the subsequent crackdown, nine of whom remain in custody. "There is no such thing". But these six are human rights activists and, moreover, one is a German.More news: Street Fighter V adds Abigail from Final Fight on July 25
Experts say these stinging measures will strongly impact tourism and investment in Turkey.
German Justice Minister Heiko Maas also weighed in on behalf of the detained activists.
"I can't see how we as the German government can continue to guarantee corporate investments in Turkey if there is the threat of arbitrary expropriation for political reasons". He added that the German minister's "ill-tempered" remarks could neither be associated with a serious state like Germany nor the friendship between the two countries. So far this year, bookings from Germany have accounted for about 10 percent of Turkey's tourists.
Germany's top diplomat also said Berlin would discuss with its European partners whether or not to stop the flow of EU pre-accession funds to Turkey.
Germany on Thursday announced it was considering pulling back from investments in Turkey as part of a range of steps in response to Ankara's refusal to address concerns about the imprisonment of a German human rights activist.
The Turkish foreign ministry said statements by the spokesmen for the German chancellor and foreign ministry on Wednesday constituted "diplomatic rudeness" and said the judiciary can not be instructed or counselled by anyone.More news: Agency says OPEC compliance with output cut fell in June