A new sesterterpene, sesteralterin (1), four new meroterpenes, tricycloalterfurenes A-D (2-5), and a known meroterpene, TCA-F (6), were obtained from the culture extract of an Alternaria alternata strain (k21-1) isolated from the surface of the marine red alga Lomentaria hakodatensis. The structures and relative/absolute configurations of these compounds were identified by spectroscopic analyses, mainly including 1D/2D NMR, ECD, and mass spectra and quantum chemical calculations. Compound 1 represents the first nitidasane sesterterpene naturally produced by fungi, and 2-5 feature a tetrahydrofuran unit rarely occurring in tricycloalternarenes. Compounds 1-6 were assayed for inhibition of the growth of four marine plankton and one marine alga-pathogenic bacterium.