Three times as much mercury has been found in mountain lions in the Santa Cruz Mountains than in their inland brethren, and the likely culprit is coastal fog, a first-of-its-kind study by UC Santa Cruz has found.

The fog is apparently pulling mercury out of the ocean and dripping it over the coastal mountains, a potentially lethal problem for cougars because it bioaccumulates in their fat and could eventually contribute to their demise, the study, published Tuesday in the journal Scientific Reports, concluded.

Mercury poisoning is known to cause memory and motor coordination problems and has been shown in studies in the Florida Everglades to cause reproductive problems in pumas. Mercury can come from natural sources, but researchers said most of the mercury currently in the atmosphere comes from burning coal.

“What makes it a cause for concern is that this could be repeating itself in foggy coastal regions across the world,” said Chris Wilmers, an associate professor of environmental studies at UC Santa Cruz and a co-author of the study. “Elsewhere, where you have seen high levels of mercury were either ocean-derived or stream-derived, (but) this is a completely terrestrial food chain.”

The study tested the hair and whiskers of dozens of mountain lions, young and old, living on the coast and in inland areas, including the Klamath, Sierra Nevada and Cascade ranges, with a few samples taken from museums for comparison. Lab analysis found an average of three times more methylmercury in the Santa Cruz mountain lions than in cougars living in less-foggy inland regions.

But mountain lions weren’t the only study subjects that bolstered the fog hypothesis. The scientists also tested deer, the cougar’s primary prey, and lichen, a mossy plant that the deer eat. The deer on the coast had twice as much methylmercury, but the lichen from the Santa Cruz area had four times as much of the chemical than its inland examples.

“The lichen is important because mercury in lichen can only come from the atmosphere,” said Peter Weiss-Penzias, an environmental toxicologist at UC Santa Cruz and the lead author of the study. “Deer and puma could get their mercury from other sources (mainly from consuming fish), but because the lichen shows the same pattern, it strengthens our argument, or hypothesis.”

Weiss-Penzias was quick to point out that the levels were not high enough to harm the deer, and humans do not appear to be in any danger because they do not eat mountain lion meat. It is nevertheless a problem, he said, because — just like swordfish, tuna and other apex predators in the sea — the mercury bioaccumulates in the cougars over time.

“As global mercury levels increase, coastal food webs may be at risk to the toxicological effects of increased methylmercury burdens,” the report said.

Wilmers, the founder of the Puma Project, which has tracked about 40 pumas in the Santa Cruz Mountains since 2008, said the hair and whiskers of one cougar that died a few years ago of unknown causes had levels of mercury well above the 21 micrograms per gram threshold considered life-threatening in mammals and humans.

Two females in the study had levels high enough to limit the viability of their offspring, he said.

Weiss-Penzias said the findings illustrate a little-known, and complicated, chemical transformation that could have a major negative impact on the natural world if it isn’t controlled.

The mercury, he said, wafts into the atmosphere from coal-burning plants, and a great deal of it eventually lands in the ocean. The chemical then sinks to a depth — about 1,000 feet — where there is very little oxygen. At that point, he said, a little understood biological process begins. The bacteria that exists deep in the ocean convert mercury atoms into methylmercury, a more volatile, fat-soluble, form of the chemical.

“The form that the mercury is in really matters in terms of how it gets into living systems,” said Weiss-Penzias.

The newly formed methylmercury comes to the surface in the upwellings of deep water that are typical along the California coast. That’s when the coastal fog takes over. It has a unique ability that rain does not to pick up methylmercury and carry it over land, where it is spread in water droplets. It is then absorbed by lichen, which deer eat. Mountain lions eat the deer.

Weiss-Penzias said he got the idea to study what was in the fog when he was biking to work one day in 2010 and wondered what was in the fog droplets that were clouding his glasses.

“There were some papers from the Arctic in the 2007 time period that suggested methylmercury could come out of the ocean and impact coastal ecosystems, but no one had ever looked at this in coastal fog before,” he said.

He published papers in 2012 and 2016 that documented methylmercury in the fog and speculated that it could have an effect on the coastal ecosystem.

“This paper shows that, yeah, it probably does,” he said. “This is the first report of its kind.”

The study comes after a 2018 study that showed “extremely low levels of genetic diversity” in many mountain lion populations in California, including the north and central coasts.

Wilmers said the Santa Cruz coastal population is isolated on three sides by highways, water and urban areas, giving it almost no access to the Hamilton Range and other open spaces. The isolation can lead to inbreeding, which dilutes the cats’ gene pool and can lead to health and reproductive problems.

There are as many as 5,000 pumas in California, according to state Department of Fish and Wildlife estimates, but their future viability could be in danger if too many problems, like poor genetic health, high mortality rates from car strikes and mercury poisoning are combined.

Weiss-Penzias said he plans to duplicate this study in other foggy areas to see if the results match.

Peter Fimrite is a San Francisco Chronicle staff writer. Email: pfimrite@sfchronicle.com Twitter: @pfimrite