TERMS OF EMPLOYMENT

Exhibit 10.1

February 12, 2007

Carlos Riva

c/o Celunol Corp.

55 Cambridge Parkway, 8^th Floor

Cambridge, MA 02142

Dear Mr. Riva:

Reference is made to that certain Agreement and Plan of Merger and Reorganization of even date herewith by and among Diversa Corporation, a Delaware corporation (“we” or “Parent”), Celunol Corp., a Delaware corporation (the “Company”), Concord Merger Sub, Inc., a Delaware corporation and a wholly-owned subsidiary of Parent and William Lese, as the Company Stockholders’ Representative (the “Merger Agreement”), providing for the merger of Merger Sub with and into the Company, with the Company being the surviving corporation and continuing as a wholly-owned subsidiary of Parent (the “Merger”).

As an inducement to the execution of the Merger Agreement by Parent and the Company, and in furtherance of satisfaction of the condition to closing set forth in Section 7.4(a) of the Merger Agreement, we hereby agree, and request that you agree, by execution of the acknowledgement below, to negotiate, in good faith and in an expeditious manner, the terms of an employment agreement between you and Parent in form and substance reasonably acceptable to you and Parent, including, without limitation, whether a grant of Parent’s restricted stock or restricted stock units should replace or be substituted for all or any portion of the grant of an option to purchase one million (1,000,000) shares of Parent’s common stock referenced in the Employment Agreement (as defined below). The parties also acknowledge that their mutual goal