Mucilage microcosms

Abstract

In the summers of 2000, 2001 and 2002, large amounts of sticky mucilaginous material aggregated to form masses of impressive dimensions over large areas of the Adriatic Sea, particularly in its northern part. Aggregates differing in size were sampled by SCUBA divers and submitted to chemical (nutrient and organic matter concentrations) and biological analysis (virus, bacteria and phytoplankton abundances and bacterial metabolism). Suspended and sinking mucilaginous aggregates were biota-rich environments where the abundance of planktonic organisms and the concentration of nutrients were orders of magnitude higher than in the surrounding seawater. The embedded phytoplankton was mostly composed of diatoms, but the dinoflagellate Gonyaulax fragilis, previously reported in association with the aggregates, was also present. A variety of processes occurred within the mucilaginous aggregates which resulted in the transformation of the organic matter composition and lability and contributed to a partial degradation of mucilage. For an efficient degradation of mucilage, several conditions are necessary: high bacterial abundance and activity and an efficient recycling of nutrients within the aggregates. Most of these conditions, appear to change depending on the type and age of the aggregate. During the first phase of aggregation (cobwebs and ribbons), bacterial activities addressed the degradation of organic matter, particularly that of the nitrogen fraction. The degradation products were rapidly taken up by bacteria, supporting an increase in their abundance and production. In aged mucilage (clouds), the degradation processes decreased and the bacterial metabolism suggested the presence of new organic labile compounds probably due to phytoplankton production. On the basis of our results, stringers, generally considered the first step of the aggregation process, seemed to be the result of a mechanical disruption of other types of aggregates.

Viruses, prokaryotes and biochemical composition of organic matter in different types of mucilage aggregates

“…Prokaryote and viral abundance as well as values of PCP and enzymatic activities in aggregates varied within the range reported by Herndl (1988) and Del Negro et al (2005) in the Adriatic Sea, and were comparable also with values reported from highly productive coastal benthic systems , Mei & Danovaro 2004.…”

“…Studies on natural and experimentally produced aggregates have shown that prokaryote growth decreased from early colonized to aged aggregates, suggesting that PCP and degradation rates might depend upon the age of the aggregates (Alldredge & Gotschalk 1990, Müller-Niklas et al 1994, Unanue et al 1998, Grossart & Ploug 2000, 2001, Grossart et al 2003. Recently, Del Negro et al (2005) found higher prokaryote abundance, C production and enzymatic activities in early forming aggregates (cobwebs and ribbons) than in aged mucilage (clouds).…”

“…Prokaryotic C production (PCP) was determined by separately for ambient seawater and all types of aggregates. The same 3 H-leucine concentration was utilized previously (Del Negro et al 2005), thus allowing a proper comparison with literature data. After incubation, all samples were added with 100% trichloroacetic acid (TCA; final concentration, 5%) and processed as described by Smith & Azam (1992), before scintillation counting.…”

“…These authors also related differences in shape, size (from a few millimeters to several meters), position and persistence in the water column, and effects on the benthos to different stages of the mucilage life span. Moreover, there is evidence that different marine aggregates display differences in their biochemical composition and microbial abundance and activities (Azam & Long 2001, Kiørboe 2001, Del Negro et al 2005). …”

Composition and function of mucilage macroaggregates in the northern Adriatic

“…Macroaggregate degradation is supported by high bacterial abundance and activity, and by an efficient recycling of nutrients, which seem dependent on aggregate type and age (Del Negro et al 2005). Bacterial extracellular enzymes are important catalysts in the degradation of macroaggregate OM to DOM, significantly influencing the marine biogeochemical cycling of organic carbon and other elements that limit microorganism growth.…”