Compounds from the marine environment exhibit a wide variety of biological activities, and thus hold much promise as potential drugs. The halichondrins, isolated from the Kaikoura sponge Lissodendoryx sp. are no exception to this, demonstrating potent anticancer activity. Novel cytotoxic compounds have also been isolated from the Chatham Rise sponge Lamellomorpha strongylata. Knowledge of the cellular origins of such compounds is desirable, in order to establish if the sponge or associated micro-organisms are producing the compounds of interest. Siderophores are also important molecules, which are produced on demand by bacteria in order to obtain sufficient iron necessary for their growth. Knowledge of the biosynthesis of these compounds has potential for the control of undesirable bacteria, such as the anthrax-causing pathogen Bacillus anthracis. Cell separation studies have been carried out on Lamellomorpha strongylata, locating a swinholide in sponge-associated filamentous bacteria and theonellapeptolides in sponge-associated unicellular bacteria. A microscopic analysis of dissociated cells from Lissodendoryx sp. was also undertaken. The structures of four new halichondrins (3.13 - 3.16), isolated from Lissodendoryx sp., have been determined from spectral data. All of these compounds are very similar to known B series halichondrins, with differences occurring only beyond carbon 44. As biological activity has been shown to be derived from the portion of the molecule between carbons 1 and 35, they all retain good activity in the P388 assay as expected. A new siderophore, petrobactin sulfonate (4.2), was characterised, along with three cyclic imide siderophore derivatives (4.3 - 4.5). Petrobactin sulfonate is the first marine siderophore containing a sulfonated 3,4-dihydroxy aromatic ring. The structures were elucidated from spectral data, resulting in a revision of the NMR assignments of petrobactin.