Martin Tobias has left his roles as CEO and chairman of Imperium Renewables, according to the biodiesel company.

In an announcement that gave no explanation for the former Microsoft manager's departure, the company said Friday that John Plaza, president and founder of Imperium Renewables, would act as an interim CEO while current director Nancy Floyd, also a managing director at Nth Power, would become the chairwoman.

Company officials declined to comment on the leadership change.

Rick Kment, a biofuels analyst at research firm DTN, said the announcement is another sign of the pressure that has been building up in the biofuel industry over the last several months, what with profit reductions and concerns about biofuels' viability.

"This really doesn't surprise me and I would foresee more of these [transitions] as stockholders really focus on profits at the end of the year," he said. "It has been a significantly poor second half of the year for profits in the biofuels industry."

The news comes after Imperium raised $214 million in February, including $113 million in private-equity investment and $101 million in debt financing (see Imperium Raises $214M). The company also filed for a $345 million initial public offering in May, but hasn't yet made its stock market debut.

Also in the announcement Friday, Seattle-based Imperium said its 100-million-gallon-per-year biodiesel plant in Grays Harbor, Wash., is operating at full capacity and is experiencing "solid customer demand" for its fuel.

The company also has converted its original biodiesel facility in Seattle into a research and development lab focused on "next-generation" feedstocks such as jatropha and algae, according to the announcement.

Biofuel companies have seen their margins squeezed in the past few months as prices for the crops used to make the fuels have gone up while biofuels prices have gone down (see Ethanol Margins Suffer and Ethanol's Tough Times Continue). A number of biofuel companies have canceled proposed plants (see Ethanol Margins Suffer, Biofuels Get Funding as an Ethanol Plant Gets Canceled, E3 Plant Craps Out, Another Ethanol Plant Gets Canceled and Biofuel Forecast Buoys a Bit).

Entrepreneurs are trying different approaches for dealing with the hungry period.

Some ethanol companies, such as VeraSun Energy Corp. (NYSE: VSE), appear to have settled on an acquisition strategy (see VeraSun to Buy Better Margins). And Xethanol Corp. (AMEX: XNL) last week announced it was diversifying into other renewable sectors, including wind power, solar power, energystorage energy infrastructure, energy efficiency, waste recycling, agricultural processes and biomass gasification for electricity production.

All the pressure also has brought additional tension into the balance between short-term profits and long-term feasibility, such as research and development efforts, Kment said.

But while he expects to see more CEO changes soon, Kment said the pool of biofuel CEOs and managers is relatively small -- so we're likely to see the same faces again.

"I would not be surprised to see several of these people moving around to different companies in the next year or two," he said. "Because it is a relatively captive industry, it almost seems like it might be like the situation at the NFL, where you have the same players moving around to different teams."

Kment also said he thinks Imperium is unlikely to go public under the current market conditions.

"We've really seen a significant decline in support for publicly traded biofuels at this point," he said, referring to low share prices for biofuel companies (see Ethanol Stocks Keep Falling). "At the same time, if the industry moves around and changes significantly in the next few months, [an IPO] could be a possibility."