Nature and lability of northern Adriatic macroaggregates

Abstract

The key organic constituents of marine macroaggregates (macrogels) of prevalently phytoplankton origin, periodically occurring in the northern Adriatic Sea, are proteins, lipids and especially polysaccharides. In this article, the reactivity of various macroaggregate fractions in relation to their composition in order to decode the potentially »bioavailable« fractions is summarized and discussed. The enzymatic hydrolysis of the macroaggregate matrix, using α-amylase, β-glucosidase, protease, proteinase and lipase, revealed the simultaneous degradation of polysaccharides and proteins, while lipids seem largely preserved. In the fresh surface macroaggregate samples, a pronounced degradation of the α-glycosidic bond compared to β-linkages. Degradation of the colloidal fraction proceeded faster in the higher molecular weight (MW) fractions. N-containing polysaccharides can be important constituents of the higher MW fraction while the lower MW constituents can mostly be composed of poly- and oligosaccharides. Since the polysaccharide component in the higher MW fraction is more degradable compared to N-containing polysaccharides, the higher MW fraction represents a possible path of organic nitrogen preservation. Enzymatic hydrolysis, using α-amylase and β-glucosidase, revealed the presence of α- and β-glycosidic linkages in all fractions with similar decomposition kinetics. Our results indicate that different fractions of macroaggregates are subjected to compositional selective reactivity with important implications for macroaggregate persistence in the seawater column and deposition.

Keywords: lability; marine macroaggregates; northern Adriatic.

Figure 1

FT-IR spectra of (A) macroaggregate matrix, surface sample (black bold line) and water column sample (black dotted line); and (B) macroaggregate interstitial water colloids, surface sample (blue bold line) and water column sample (blue dotted line).

Figure 2

C_org./N_tot. (A) and C_org./P_tot. (B) molar ratios ultrafiltrate retentates (UF/0) and permeates (UF/F) using a nominal molecular weight cutoff of 30–10 (UF1), 10–5 (UF2) and <5 kDa (UF3) at the start, after 1 week and after 4 weeks of the degradation experiment.

Figure 3

Concentration changes of carbohydrates and proteins during various enzyme hydrolyses (6 hours at 26 °C).

Figures 4

FT-IR spectra of the (A) surface and (B) water column macroaggregate matrices, and after (i) α-amylase + β-glucosidase, (ii) protease + proteinase K hydrolysis: carbohydrate bands (region ~1150–900 cm⁻¹), protein bands (region 1654–1635 cm⁻¹), lipid bands (region 2950–2850 cm⁻¹) and inorganic (mineral) components (region <1000 cm⁻¹).

Figures 5

FT-IR spectra of the surface macroaggregate matrix and aqueous phase of experimental slurries after α-amylase + β-glucosidase and protease + proteinase K hydrolysis.

Figure 6

FT-IR spectra of the surface macroaggregate matrix (sampled at the beginning of the degradation experiment; t₁—black line), and after lipase (after 3 weeks at 26 °C, t₁—blue line) hydrolysis: carbohydrate bands (region ~1150–900 cm⁻¹), protein bands (region 1654–1635 cm⁻¹), lipid bands (region 2950–2850 cm⁻¹) and inorganic (mineral) components (region <1000 cm⁻¹).