Angry environmental activists say they have more than 70,000 signatures and plan to flood a WGBH trustees meeting next week calling for the ouster of conservative lightning-rod billionaire David Koch from the board — but the public TV and radio station for now is standing by the longtime PBS mega-donor.

“David Koch has essentially dedicated himself — and tens of millions of dollars — to deliberately mislead the public about climate change,” claimed Emily Southard of environmental group Forecast the Facts. “That’s completely incompatible with an organization like WGBH, which is dedicated to public education.”

Koch, a prominent Mitt Romney fundraiser last year, has donated $18.6 million to WGBH since 1982 and $10 million specifically to the science program NOVA, according to Koch Companies spokeswoman Melissa Cohlmia. He’s also one of the station’s 32 trustees.

His steady, heavy contributions could cause WGBH to commit acts of “journalistic self-censorship” and influence programming, the petition claims.

“We want WGBH to be making decisions on how to best serve the public interest without having to deal with these conflicts of interest,” said Brad Johnson of Forecast the Facts.

About 50 activists plan to picket the Brighton TV station next Wednesday and deliver the petition to the board of trustees in a five-minute presentation during their meeting, Southard said.

WGBH vice chairwoman Maureen Ruettger said, “We will listen to it just like anything else. You have to listen to both sides.”

WGBH spokesman Michael Raia dismissed the idea that Koch influences programming.

“Absolutely not,” agreed trustee Terrie F. Bloom. “They are very, very careful about that sort of thing.”

Jason Talbot, who just ended a stint as co-chairman of WGBH’s community advisory board, said, “For me, personally, it’s business and, to me, funding is very difficult to come by. If I was to choose between taking funding from a guy like that and not having NOVA, I would prefer to have NOVA.”

Koch’s reps said he believes in “an open, honest and science-based debate” about climate change, and won’t step down.

“We can unequivocally state that David Koch has never interfered with or tried to influence WGBH’s programming decisions,” Cohlmia said. “Further, he has no intention to resign from the board of WGBH.”

Koch did resign earlier this year from the board at New York’s PBS affiliate, WNET, which had run a critical documentary that partly focused on him, then took the unusual step of airing his rebuttal after the broadcast.

Meanwhile, Johnson said the group will target individual trustees if next week’s meeting fails to get results, and encourage students to lobby professors who sit on the board.