Chemical and biochemical parameters of cultured diatoms and bacteria from the Adriatic Sea as possible biomarkers of mucilage production.

Bacteria and diatom strains from the Adriatic Sea were investigated, under standard and altered environmental conditions, for carbohydrate production and for the presence of specific biomarkers. Algae from P-depleted cultures showed an increase in extracellular carbohydrate production, a significantly lower chlorophyll a content and unchanged total lipid levels. However, the fatty acid composition of algal cultures was severely affected by low P levels, in that, total saturated and monounsaturated fatty acids increased and total polyunsaturated fatty acids decreased. Marine heterotrophic bacteria resulted enriched by 4 to 6 orders of magnitude in mucilage samples respect to surrounding seawater, unlike other groups of bacteria such as the non-halophylic heterotrophs. The major fatty acids detected in bacteria were 16:0 and 18:1n-7; the uneven fatty acids 17:0i, 17:0 and 17:1 also constituted an important component of various strains and, as a result, the total monounsaturated fraction represented the main component of total fatty acids. All the mucilage samples analysed shared the same general fatty acid composition features with a high amount of saturated components, especially 16:0; typical marine polyunsaturated fatty acids, such as 20:5n-3 and 22:6n-3, were found at very low levels. With regard to the sterol composition, the analysed algal species and bacteria showed that different compounds prevailed in the different species, and under P-deprivation sterol distribution resulted differently affected in the various algal species. In mucilage samples an overall prevalence of cholesterol was observed and, among 4alpha-methylsterols, constantly present, dinosterol prevailed in all samples. Vibrational IR spectroscopic analyses confirmed the main results obtained with the GC analysis: a higher unsaturation degree in nutrient replete diatom cultures than in P-depleted ones, a lower amount of P-containing compounds in the latter, bacterial lipid profiles with a high amount of free carboxylic acids and/or ketones and a low unsaturation degree and, finally, mucilage samples with a very low unsaturation degree. All these results allowed some speculations on the involvement of the various microbial and phytoplankton components in mucilage genesis.