SHORT COMMUNICATION
Microphytobenthic biomass, diversity and exopolymeric substances in a shallow dystrophic coastal lagoon Francesca Di Pippo1, Paolo Magni2, Roberta Congestri3*
Di Pippo F, Magni P, Congestri R. Microphytobenthic biomass, diversity and exopolymeric substances in a shallow dystrophic coastal lagoon. J Mar Microbiol. 2018;2(1): 06-12. ABSTRACT OBJECTIVE: This study is aimed to provide a first insight into the microphytobenthos (MPB) species composition, biomass and exopolymeric substances (EPS) of the organic enriched Cabras lagoon (Western Mediterranean Sea, Italy), due to the lack of information on the presence and distribution of microphytobenthic assemblages in this system, although their importance has been shown in other organic-enriched coastal lagoons. METHODS: Surface sediment samples were collected at three sites of the lagoon (C1, C2, C3), differing in sediment granulometry, on 4 occasions, over one year. MPB biomass, expressed as chlorophyll a and organic matter, was evaluated as well as the quantity of two operationally distinct EPS fractions. Sampled communities were also characterized in light and confocal microscopy to assess the spatial distribution and composition of microphytobenthic species in relation with sediment characteristics. Alcian Blue cytochemical staining of EPS samples was also used to ascertain, during light microscopy observations, the presence of acidic groups in EPS residues.
INTRODUCTION
T
he Cabras lagoon (Sardinia, Italy, Mediterranean Sea) is a shallow transitional system renowned for its naturalistic (Ramsar Convention on Wetlands, Natura 2000 network for EU Habitat directive) and economic importance (e.g. artisanal fisheries). Anthropogenic pressure due to massive nutrient loading, reduction of freshwater input from upland, modifications of the inlets and other man-made interventions have reduced the water exchange with the adjacent Gulf of Oristano, leading to hypoxic and anoxic conditions in near-bottom waters, especially in summer. These dystrophic events caused major loss of the biological resources of the lagoon (1). To assess the ecological quality of the lagoon, numerical models have been developed to predict the evolution of both hydrological and ecological variables within the lagoon system under different meteorological forcing (2). In parallel, investigations of the physical and chemical characteristics of the sediments and the macrobenthic assemblages have shown a close link between the distribution of organic-C bounding fine sediments, benthic macroinvertebrates, and the water residence times computed from the model (1). In this context, detailed analysis of sediment dynamics are particularly important because the partitioning and transport of fine sediments can strongly influence the redistribution and accumulation of large amounts of organic matter, and consequently the spatio-temporal distribution of benthic assemblages and the trophic status and functioning of the lagoon (1,3). Recent investigations indeed demonstrated the effect of confinement, organic enrichment and saprobity levels on benthic community distribution in this shallow water body (4). Primary producers are considered to be an important source of organic matter in the Cabras lagoon (3). Nevertheless, studies on the presence, distribution and diversity of microphytobenthos (MPB) in the Cabras lagoon and more in general in these shallow lagoon systems are still lacking although the importance of the highly variable, biological component has
RESULTS AND CONCLUSIONS: MPB assemblages developed in biofilms comprising both eukaryotic and prokaryotic members. Communities were dominated by diatom species, mainly of the genera Navicula and Nitzschia, associated with cyanobacteria and green algae in most samples. Microorganisms were distributed in patches of different thickness and embedded in a polymeric matrix, characterized by both neutral and acidic polysaccharidic residues. Spatial and temporal patterns in species composition and biomass were also observed, likely due to the habitat heterogeneity within each site. Sediment properties seemed to influence the diatom assemblages. The muddy nature of sediments at site C2 enabled the development of epipelic forms, such as Surirellales diatoms, known for their high motility through the sediment while sandy bottoms at site C1 and C3 supported the growth of episamnic species (i.e. among the genera Amphora, Cocconeis and Cymbella), that are able to adhere to the grains with stalks, tubes and/or apical pads. Cyanobacteria dominated the extremely variable (salinity, water temperature, light and water availability) site C2 over the other MPB members, likely due to their physiological versatility and capacity of producing protective exopolymers. Key Words: Microphytobenthos; Phototrophic biofilms; EPS; Cyanobacteria; Coastal lagoon
been reported for different organic-enriched coastal systems (5,6). MPB plays a role in modulating the exchange of nutrients between the sediments and the water column (7) and it is an important food source (8). Benthic phototrophic primary producers may occur, at the sediment surface, in the form of biofilms where cyanobacteria and microalgae are embedded in a common matrix composed of exopolymeric substances (EPS) (9,10). In the biofilms, filamentous cyanobacteria along with colonial and tube dwelling diatoms tend to entangle forming a network which also traps sediment particles, thereby producing tough and coherent structures that ultimately contribute to sediment stabilization (9). In this study, a quali-quantitative approach based on the use of variety of descriptive and analytical techniques was used to provide the first insight into the spatio-temporal variations in taxonomic composition, biomass and exopolymeric substances of the microphytobenthic assemblages in relation to main environmental cues at three sampling sites of the Cabras lagoon, over one year.
MATERIALS AND METHODS Sampling Three sites were selected along the salinity and saprobity gradients, based on past research carried out in the Cabras lagoon system (1,3,4,11-16). Detailed site description has been already reported [4]. Briefly, the northern site, C1, with a mean depth of 1.5 m, is close to the main freshwater tributary (‘Mare e Foghe’ river) and characterized by sandy sediments and halophytic vegetation (Phragmites sp.) C2 site (mean depth: 0.2 m) is a confined area, connected to the main basin by a narrow inlet, closed during the warm season, located in the satellite ‘Sali e Pauli’ pond, it presents muddy sediments and halophytic vegetation (Salicornia sp.). The southern site, C3 (mean depth: 1.0 m) is close to the cannel connecting the lagoon to the Gulf of Oristano and presents muddy-sand sediments and abundant submerged vegetation, including Ruppia sp.
1 IRSA-CNR, Water Research Institute - National Research Council and CNR–IAMC, National Research Council, Institute for Coastal Marine Environment, Italy; 2CNR– IAMC, National Research Council, Institute for Coastal Marine Environment, Italy; 3Department of Biology, Laboratory of Biology of Algae, University of Rome, Italy.
Correspondence: Dr. Roberta Congestri, Department of Biology, Laboratory for Biology of Algae, University of Rome, Italy. Telephone +39-0672595989, e-mail:
[email protected] Received: January 30, 2018, Accepted: February 15, 2018, Published: February 23, 2018 This open-access article is distributed under the terms of the Creative Commons Attribution Non-Commercial License (CC BY-NC) (http:// creativecommons.org/licenses/by-nc/4.0/), which permits reuse, distribution and reproduction of the article, provided that the original work is properly cited and the reuse is restricted to noncommercial purposes. For commercial reuse, contact
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J Mar Microbiol Vol 2 No 1 February 2018
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Congestri et al Field surveys were carried out in 2009-2010 period. A total of 72 sediment samples were taken on four sampling campaigns at the three study sites, in September 2009, March, July and September 2010. At each site and sampling occasion, water temperature, salinity, dissolved oxygen (DO) were measured, before and after sediment collection, using a salinometer WTW LF 197 and a portable oxymeter WTW Oxil 197. The mean value of the two measurements were then used for data analyses. Sediment samples were collected using a manual core tube (40 cm long and 5.5 cm in diameter) gently pushed into the sediments by hand, the core surface layers (3-5 mm) were then carefully extruded and sliced off. Six sample replicates at each site were randomly collected (6).
LABORATORY ANALYSIS Each sediment replicate was homogenized and subsamples were obtained to quantify photosynthetic pigments, water content (WC), organic matter (OM) and EPS. WC was computed after drying in a oven the sediment samples, 50°C for 24 h, and then expressed as percentage of the wet weight; OM content was measured by loss on ignition (LOI) at 500°C for 3 h from a subsample of 1 g and expressed as DW percentage. Chlorophyll a (Chl-a), used as an estimation of the MPB biomass, and pheopigment degradation products, were extracted from wet sediments (ca. 1 g) using 90 % acetone after 24 h of darkness at 4°C, samples were centrifuged (3000 rpm, 10 min) and the extracts spectrophotometrically analyzed. Pigment concentration values were obtained according to Lorenzen’s method (17) as described in (18), where the volume of water is substituted by grams, g, of sediment DW. Two operationally defined EPS fractions were extracted (19) from lyophilized sediment samples incubated in 400 μL distilled water for 1 h at 30°C. The extract was centrifuged (5 min, 6000 g) and the water-extractable EPS fraction determined. This represents the EPS portion loosely bound to the sediments. The pellet was subsequently incubated with 500 μL 0.1 M Na2EDTA for 16 h at room temperature. The obtained extract was then centrifuged (5 min, 6000 g) and the EDTA-extractable EPS fraction assayed. This fraction is tightly bound to the sediments. Both fractions were then quantified by the phenol-sulphuric acid method, using glucose as reference (20). The existence of associations between all the measured variables in this study was assayed by the Pearson product-moment correlation (significance levels p
