Internal nitrogen pools in thalli of Gracilaria tikvahiae McLachlan were examined in three experiments as a function of total nitrogen content of the thallus, nitrogen deprivation, and nitrogen resupply. Amino acids and proteins appeared to form the major nitrogen storage pools in G. tikvahiae, while DNA appeared to be relatively unimportant in this regard. Inorganic nitrogen in the forms of NH4+ and NO3− was found in the thalli; however, its contribution to the total nitrogen, pools was small. Within the protein pool, the phycoerythrin pigments appear important as a source of nitrogen when thalli are initially becoming nitrogen limited. In general, there was an inverse relationship between the levels of nitrogen and the carbohydrate content of the algal thalli.

Increasing concentrations of nitrogen fertilizer led to increased growth and internal nitrogen content of Gracilaria tikvahiae and decreased yields of agar. The thallus nitrogen content was highly correlated with percent protein and protein:carbohydrate ratios, and percent agar was highly correlated with percent carbohydrate. Agars extracted from thalli grown under nitrogen enriched conditions had higher melting temperatures and greater gel strengths than did agars extracted from less enriched thalli. The data suggest that the increased gel strengths found in these experiments may be due to the greater molecular size of the agar polymers, as indicated by the increased melting temperatures associated with greater gel strengths.

A strain of Gracilaria sp., G-16, which produces an agar with high gel strength was grown under controlled culture conditions at 3 different temperatures, 2 light quantum fluxes, 2 salinities, and under N enrichment and starvation. Best productivity occurred at 24 °C, followed by 32 °C, then 15°C. Higher light quantum fluxes and nitrogen enrichment also contributed to greater productivities. Salinity changes of 17%o versus 33%o showed minimal effect on productivity. Slight salinity effects occurred when other culture factors contributed to reduced growth. Agar content was typically greater in plants grown at 17%o than 33%o, and was generally lower in plants grown at 32 °C than 24 or 15 °C. There was some effect of thallus N levels on agar content. Agar gel strength was greater in the N enrichment treatments, and was also affected by both temperature and salinity. The highest agar gel strength was found in the 32 °C, N enriched, 33%o treatments. Lower salinities of 17%o led to reductions in agar gel strength, but more significantly, the N enrichment effect on increased gel strength disappeared.

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Protein extracts from 22 species of marine macroalgae from Florida and North Carolina were compared for their abilities to agglutinate sheep and rabbit erythrocytes. Protein extracts from 21 algal species agglutinated rabbit erythrocytes compared to 19 for sheep erythrocytes. However, agglutination by brown algal extracts was variable. The agglutination produced by protein extracts from Dictyota dichotoma could be blocked by addition of polyvinylpyrrolidone. Protein extracts from North Carolina macroalgae were also tested against five bacterial species. Three of these agglutinated bacterial cells. Ulva curvata and Bryopsis plumosa agglutinated all five species. Protein extracts from five species of Florida algae were tested for their effects on mitogenesis in mouse splenocytes and human lymphocytes. Gracilaria tikvahiae HBOI Strain G-5, Ulva rigida and Gracilaria verrucosa HBOI Strain G-16S stimulated mitogenesis in mouse splenocytes, while Gracilaria tikvahiae HBOI Strain G-3 stimulated mitogenesis in human lymphocytes.