Elon Musk stepped up his criticism of Bitcoin as environmentally unsustainable while also signaling support for another digital coin and saying he remains a believer in cryptocurrency.

The Tesla Inc. chief executive officer said in a tweet Thursday he worries about “massive use” of coal and other carbon-intensive energy to generate electricity needed to mine digital currency. That followed an earlier Twitter post of a chart from the University of Cambridge showing Bitcoin’s electricity consumption has skyrocketed this year.

It marked the second day he’s criticized crypto mining and follows a decision to suspend Tesla car purchases using Bitcoin, a move that roiled digital currency markets over the past 24 hours.

But Musk also signaled his support for another digital currency Thursday in another tweet that said he is working with Dogecoin developers to “improve system transaction efficiency.”

Cryptocurrency champion

Musk has been an early corporate champion of adopting digital currencies and has helped spur the popularity of Dogecoin, a cryptocurrency started as a joke in 2013.

In February, Tesla said it had purchased US$1.5 billion in Bitcoin and planned to accept it as a payment, triggering a rally in its own stock as well as the currency. But the about-face on Wednesday sent the value of it and other cryptocurrencies sliding.

Bitcoin remained not far from the lowest levels of the day after Musk’s latest tweet. The largest cryptocurrency fell 0.4 per cent to US$49,128 as of 6:58 a.m. in Hong Kong on Friday, after dropping 9.5 per cent on Thursday, according to composite pricing from Bloomberg. Dogecoin was up 10 per cent in the past hour, according to CoinGecko.com.

Tesla shares rose 0.3 per cent in postmarket trading in New York after closing down 3.1 per cent to US$571.69. The stock is down about 19 per cent this year.