Two new diphenylketones (1 and 2), a new xanthone (3), and a known xanthone analogue (4) were isolated and identified from Talaromyces islandicus EN-501, an endophytic fungus obtained from the fresh collected marine red alga Laurencia okamurai. Their structures were elucidated on the basis of NMR spectroscopic and X-ray crystallographic analysis. The joint isolation of benzophenones and xanthones from the same fungal strain supports the biogenesis of xanthones via a benzophenone intermediate. It is worth mentioning that xanthones 3 and 4 have a methyl group at C-6 and C-2, respectively, which is uncommon compared with typical xanthones usually having a methyl group at C-8. Compounds 1–4 exhibited potent antioxidative activities against DPPH (1,1-diphenyl-2-picrylhydrazyl) and ABTS (2,2′-azino-bis(3-ethylbenzothiazoline-6-sulphonate) radicals with IC₅₀ values ranging from 0.58 to 6.92 μg/mL, which are stronger than that of the positive controls BHT (butylated hydroxytoluene) and ascorbic acid. Compounds 1, 3, and 4 also showed inhibitory activities against several pathogenic bacteria.

The co-cultivation of two or more different microbial strains in one culture vessel was supposed to be a viable experimental approach for enhancing the diversity of the compounds produced. Two new meroterpenoid derivatives, chermebilaenes A (1) and B (2), together with three known sesquiterpenoids, sesquicaranoic acid B (3), cyclonerodiol (4) and bisabol-l-on-13-säuremethylester (5), were characterized from a co-culture of the marine-derived fungal isolates of Penicillium bilaiae MA-267 and Penicillium chermesinum EN-480. Neither fungus produced these compounds when cultured alone under the same conditions. Compound 1 represents an unprecedented acorane-type sesquiterpene hybridized with an octadecadienoic acid skeleton. The structures were elucidated on the basis of spectroscopic analysis, and the absolute configurations were assumed on the basis of acidic hydrolysis combined with modified Mosher’s method and electronic circular dichroism (ECD) calculations. Compound 1 showed potent inhibitory activities against Ceratobasidium cornigerum and Edwardsiella tarda.