Daniela Marrale

Biochemical composition of sedimentary organic matter and bacterial distribution in the Aegean Sea: trophic state and pelagic–benthic coupling

Abstract Biochemical composition of sedimentary organic matter (OM), vertical fluxes and bacterial distribution were studied at 15 stations (95–2270 m depth) in the Aegean Sea during spring and summer. Downward fluxes of labile OM were significantly higher in the northern than in the southern part and were higher in summer than in spring. Primary inputs of OM were not related to sedimentary OM concentrations, which had highest values in summer. Sedimentary chlorophyll-a concentrations were similar in the northern and southern parts. Carbohydrates, the main component of sedimentary OM, were about 1.2 times higher in the southern part than in the northern, without significant temporal changes. Total proteins were higher in summer and about double in the northern part. Sedimentary proteins appeared more dependent upon the downward flux of phytopigment than of proteins. Sedimentary OM was characterised by a relatively large fraction of soluble compounds and showed better quality in the northern part. The lack of a depth-related pattern in sedimentary OM and the similar concentrations in the two areas suggest that differences in sedimentary OM quality in the Aegean basin are dependent on system productivity; the bulk of sedimentary OM is largely conservative. Sedimentary bacterial density was about double in the northern part and higher in spring than in summer, but bacterial size was about three times higher in summer, resulting in a larger bacterial biomass in summer. Bacterial density was coupled with total and protein fluxes, indicating a rapid bacterial response to pelagic production. Bacterial biomass was significantly correlated with sedimentary protein and phytopigment concentrations, indicating a clear response to accumulation of labile OM in the sediments. In all cases bacteria accounted for

Organic matter composition of the continental shelf and bathyal sediments of the Cretan Sea (NE Mediterranean)

Abstract The seasonal, spatial and bathymetric changes in the distribution of chloroplastic pigments (Chl a, phaeopigments and CPE), TOC, TON, ATP, bottom water nutrient content and the main biochemical classes of organic compounds (lipids, proteins and carbohydrates) were recorded from May 1994 to September 1995 over the continental margin of northern Crete. The concentration of chloroplastic pigment equivalents (CPE) was always low, dropping dramatically along the shelf-slope gradient. Microbial activity (ATP) also dropped sharply beyond the continental shelf following a distribution pattern similar to TOC and TON. Lipid, protein and carbohydrate concentrations, as well as biopolymeric carbon were comparable to those reported for other more productive areas, however, the quality of the organic matter itself was rather poor. Thus, carbohydrates, the dominant biochemical class, were characterised by being highly (80–99%) refractory, as soluble carbohydrates represented (on annual average) only 6% of the total carbohydrate pool. Protein and lipid concentrations strongly decreased with depth, indicating depletion of trophic resources in the bathyal zone. Proteins appeared to be the more degradable compounds and indeed the protein to carbohydrate ratios were found to decrease strongly in the deeper stations. Organic matter content and quality decreased both with increasing distance from the coast and within the sediment. All sedimentary organic compounds were found to vary between sampling periods, with the changes being more pronounced over the continental shelf. The different temporal patterns of the various components suggest a different composition and/or origin of the OM inputs during the different sampling periods. The amount of material reaching the sediments below 540 m is extremely low, suggesting that most of the organic material is decomposed and/or utilised before reaching the sea floor. In conclusion, the continental shelf and bathyal sediments of the Cretan Sea can be considered, from a trophic point of view, as two different subsystems.

Heterotrophic nanoflagellates, bacteria, and labile organic compounds in continental shelf and deep-sea sediments of the Eastern Mediterranean

A bstractThe abundance and biomass of heterotrophic nanoflagellates were examined in continental and deep-sea sediments of the Cretan Sea (Eastern Mediterranean); at depths of 40 to 1540 m. Nanoflagellate distribution was compared to the composition of sedimentary organic matter and bacterial standing stocks to investigate trophic interactions and factors potentially affecting distribution. Quantitative estimates were obtained using different samplers for testing whether the box corer is as effective as the multiplecorer for bacterial and protozoan population estimates. The sediments of the deep Cretan Sea appeared extremely deficient in organic nutrients, and were composed mostly (more than 90%) of detritus. Labile organic compounds (such as lipids, proteins, and soluble carbohydrates) were present at extremely low concentrations, decreasing with water depth. Refractory and structural carbohydrates were the dominant biochemical class. The decrease in food quality with depth was associated with a strong decline of the RNA:DNA ratio. Benthic bacteria were constrained by food availability, and reacted to different organic matter inputs (especially total carbohydrates) at different depths. Large size bacteria were significantly correlated with the amounts of proteins and chloroplastic pigments. Heterotrophic nanoflagellate distribution in the continental shelf and deep-sea sediments of the Cretan Sea was controlled by available food sources (i.e., labile organic compounds and bacteria). Flagellate density was significantly correlated with the concentration of food indicators (chlorophyll a, soluble carbohydrates, and lipids), and to bacterial number and biomass. Despite the oligotrophy of the system, flagellate densities were high (40–119 × 103 g−1) and dominated by small cells (3 to 6 μm in length). These results, coupled with the high nanoflagellate to bacterial biomass ratio (up to 0.27 at 40 m depth), suggest that benthic nanoflagellates may contribute significantly to the direct transfer of detrital carbon and bacterial biomass to the metazoan component of the food web in the Cretan Sea.

Small-Scale Distribution of Bacteria, Enzymatic Activities, and Organic Matter in Coastal Sediments.

The small-scale distribution of several structural (bacterial abundance, phytopigment, total and soluble protein, and carbohydrate content) and functional ecological variables (enzymatic activities, frequency of dividing cells) was investigated in coastal sediments during a spring bloom. For bacterial abundance, enzymatic activity, and organic matter determinations, samples were collected at 5-m depth from a sediment surface delimited by a 42 x 42-cm frame, divided into 49 squares. In order to test the influence of the bottom microtopography on the investigated variables, the size of this frame was defined to cover the distance between two subsequent ripples. As indicated by Fishers index, benthic bacteria, enzymatic activities, proteins, carbohydrates, and their soluble fractions exhibited an aggregate distribution. Sampling size (i.e., sample unit of 36 cm2) was appropriate for all variables, except for chlorophyll a and frequency of dividing cells that displayed a contagious distribution. To estimate the reliability of the current sampling strategy, we compared the mean values from three randomly selected sample units with the average value of the entire sediment surface (i.e., 49 samples). For all variables reported in this study, the use of three replicates was representative of the mean values of the sampled area with a confidence limit within +/-20%. Bacterial population sizes did not correlate with their potential food sources (e.g., phytopigments, proteins, and carbohydrates), or with enzymatic activities, suggesting the presence of possible time lags between organic inputs and microbial response. Chlorophyll a during the spring bloom displayed much higher concentrations than phaeopigments and correlated with carbohydrates. Chlorophyll a distribution was autocorrelated and displayed a large patch size (1,134 cm2). Phaeopigments and proteins accumulated in the central depression of the ripple-mark structure, apparently depending upon a passive accumulation due to the reduced current action. In contrast, microphytobenthic biomass and bacterial numbers were highest in the two opposite ripples, suggesting that different driving forces operate selectively on the living components.

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

Abstract 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.

Bacteria and organic matter dynamics during a bioremediation treatment of organic-rich harbour sediments.

We studied the dynamics of bacteria and organic matter in the Ancient Port of Genoa (Italy) during a bioremediation treatment of sediment (during summer-autumn 1998) in an area characterised by continuous sewage discharge. A strong increase in total benthic bacterial density (TBN) was recorded at the end of the study, from 14 x 10(8) to 58-172 x 10(8) cell g(-1) in different parts of the treated area. The TBN increase was linked to organic matter depletion, from more than 40 to less than 20 mg x g(-1). In order to highlight the main ecological mechanisms involved in bioremediation, a laboratory experiment based on both water and sediment from the basin studied was carried out. We observed an increase in TBN during the first 20 days and a decrease in sediment organic matter (up to about 20%). Increases of organic matter (about 2-fold) and TBN (from 21 to 33 x 10(9) cell l(-1)) occurred in the overlying water, suggesting a strong association between the sediments and water column processes. Hydrolytic activities, which double in the sediment and increase up to a 300-fold in the water, are consistent with the decrease in sediment organic matter and with the water fraction dynamics.

Seasonal changes and biochemical composition of the labile organic matter flux in the Cretan Sea

Abstract Downward fluxes of labile organic matter (lipids, proteins and carbohydrates) at 200 (trap A) and 1515 m depth (trap B), measured during a 12 months sediment trap experiment, are presented, together with estimates of the bacterial and cyanobacterial biomasses associated to the particles. The biochemical composition of the settling particles was determined in order to provide qualitative and quantitative information on the flux of readily available organic carbon supplying the deep-sea benthic communities of the Cretan Sea. Total mass flux and labile carbon fluxes were characterised by a clear seasonality. Higher labile organic fluxes were reported in trap B, indicating the presence of resuspended particles coming from lateral inputs. Particulate carbohydrates were the major component of the flux of labile compounds (on annual average about 66% of the total labile organic flux) followed by lipids (20%) and proteins (13%). The biopolymeric carbon flux was very low (on annual average 0.9 and 1.2 gC m−2 y−1, at trap A and B). Labile carbon accounted for most of the OC flux (on annual average 84% and 74% in trap A and B respectively). In trap A, highest carbohydrate and protein fluxes in April and September, corresponded to high faecal pellet fluxes. The qualitative composition of the organic fluxes indicated a strong protein depletion in trap B and a decrease of the bioavailability of the settling particles as a result of a higher degree of dilution with inorganic material. Quantity and quality of the food supply to the benthos displayed different temporal patterns. Bacterial biomass in the sediment traps (on average 122 and 229 μgC m−2 d−1 in trap A and B, respectively) was significantly correlated to the flux of labile organic carbon, and particularly to the protein and carbohydrate fluxes. Cyanobacterial flux (on average, 1.1 and 0.4 μgC m−2 d−1, in trap A and B, respectively) was significantly correlated with total mass and protein fluxes only in trap A. Bacterial carbon flux, equivalent to 84.2 and 156 mgC m−2 y−1, accounted for 5–6.5% of the labile carbon flux (in trap A and B respectively) and for 22–41% protein pool of the settling particles. These results suggest that in the Cretan Sea, bacteria attached to the settling particles represent a potential food source of primary importance for deep-sea benthic communities.

Sediment organic matter and meiofauna community response to long-term fish-farm impact in the Ligurian Sea (Western Mediterranean)

Quantitative and qualitative changes in meiofauna community structure were investigated to assess the impact of a fish farm, which was operating continuously for 15 years (La Spezia Gulf, W Mediterranean). Sediment samples were collected in June, July, September, October 2000 and February 2001 for the analysis of phytopigments (chlorophyll-a and phaeopigments), the biochemical composition of organic matter (proteins, carbohydrates and lipids) and related to meiofaunal parameters. Sediment organic matter reached extremely high concentrations beneath the fish cages when compared to the control. Particularly lipids, carbohydrates and chlorophyll-a were significantly higher in fish-farm sediments. On a long-term basis meiofauna displayed adaptations in sediments beneath the cages resulting in an increase of density. Organic impact on meiofaunal community structure was evident in terms of an increase of the nematodes to copepods (Ne/Co) and nauplius to copepods (Na/Co) ratios in fish farm sediments. Cumaceans and kinorhynchs were encountered in control sediments, but disappeared in fish-farm samples. These data suggest that meiofauna is a sensitive tool for evaluating the effects of organic enrichment in fish farm impacted areas.

Community experiments using benthic chambers : Microbial significance in highly organic enriched sediments

Abstract The structure of the benthic microbial loop was studied in order to understand heterotrophic pathways in the suboxic sediments of the Rapallo Harbour in autumn, 1996. Sediments were characterized by the large accumulation of organic detritus (17.2 – 21.4 μg chloroplastic pigment equivalents (CPE)g−1; carbohydrates and proteins: 7.8–16.7 and 6.7–7.5 mg g−1). Due to the high organic load, benthic bacteria and protozoa displayed extremely high densities (1.4 × 109 cells g−1 and 26.9 × 105 cells g−1). Meiofauna, protozoa and bacteria showed an approximate biomass ratio of 1:2:20. the presence of large amounts of organic matter appeared to determine a shift of the benthic size structure toward the increasing dominance of the smaller components of the benthic food webs. These data indicate that the sediments of the Rapallo Harbour were dominated by microbial biomasses to a larger extent than in non-food limited environments, characterized by a lower organic contamination. On the results presented in th...