Biochemical composition of pico-, nano- and micro-particulate organic matter and bacterioplankton biomass in the oligotrophic Cretan Sea (NE Mediterranean)

The biochemical composition of different particle size classes (pico-, nano- and micro-particulate matter) and the bacterioplankton biomass were studied over an annual cycle in the Cretan Sea (South Aegean Sea, NE Mediterranean; from 40 to 1540 m depth) to investigate the origin, composition and fate of the suspended particles and to quantify bacterioplankton contribution to organic carbon pools. The oligotrophy of this system was indicated by the extremely low particulate lipid, protein and carbohydrate concentrations (4-15 times lower than in more productive systems). The biopolymeric carbon (BPC as the sum of lipid, protein and carbohydrate carbon) accounted for 80-100% of POC, suggesting the autochthonous origin of the particles. The most evident characteristic of this oligotrophic environment was the dominance of the pico-particles through all seasons, accounting for 43-45% of total carbohydrates, proteins and lipids. The proximate composition of the organic particles revealed the dominance of carbohydrates in all size-classes and highest values of the protein to carbohydrate ratio in the pico-particulate fraction. The relative proportion of the pico-, nano- and micro-particulate carbohydrates, proteins and lipids varied seasonally. The increase in the average particle size from February to September 95, probably as a result of aggregation, appeared to be related to the 'thermal stability' of the water column. The analysis of the vertical distribution of the three size classes revealed an increase in the pico fraction and a decrease in the larger components with increasing depth suggesting that nano- and micro-particles were being degraded and fragmented in the deeper water layers. Bacterial densities ranged from 1.1 to 8.8 x 108 cells l-1. Bacterial biomass accounted on average for more than 56% (up to 74%) of BPC and was by far, the most important living component. Bacterial-N accounted for a large proportion (>90%) of the protein nitrogen pool, indicating that almost no particulate detrital N was available for heterotrophic metabolism. Therefore, it is likely that bacteria utilise other sources, such as DOM and inorganic nutrients, to support their growth. The lack in particle variability appeared to be responsible for the rather consistent size structure and biochemical composition of the suspended particulate organic matter in this system.