microphyto corail 1

microphyto corail 1

(Parte 1 de 5)

Microbial photosynthesis in coral reef sediments (Heron Reef, Australia)

Ursula Werner*, Anna Blazejak1, Paul Bird2, Gabriele Eickert, Raphaela Schoon, Raeid M.M. Abed, Andrew Bissett, Dirk de Beer

Max Planck Institute for Marine Microbiology, Celsiusstrasse 1, 28359 Bremen, Germany

Abstract

We investigated microphytobenthic photosynthesis at four stations in the coral reef sediments at Heron Reef, Australia. The microphytobenthos was dominated by diatoms, dinoflagellates and cyanobacteria, as indicated by biomarker pigment analysis. Conspicuous algae firmly attached to the sand grains (ca. 100 m in diameter, surrounded by a hard transparent wall) were rich in peridinin, a marker pigment for dinoflagellates, but also showed a high diversity based on cyanobacterial 16S rDNA gene sequence analysis. Specimens of these algae that were buried below the photic zone exhibited an unexpected stimulation of respiration by light, resulting in an increase of local oxygen concen- trations upon darkening. Net photosynthesis of the sediments varied between 1.9 and 8.5 mmol O2 m 2 h 1 and was strongly correlated with Chl a content, which lay between 31 and 84 mg m 2. An estimate based on our spatially limited dataset indicates that the microphytobenthic production for the entire reef is in the order of magnitude of the production estimated for corals. Photosynthesis stimulated calcification at all investigated sites (0.2e1.0 mmol Ca2þ m 2 h 1). The sediments of at least three stations were net calcifying. Sedimentary N2-fixation rates (measured by acetylene reduction assays at two sites) ranged between 0.9 to 3.9 mmol N2 m 2 h 1 and were highest in the light, indicating the importance of heterocystous cyanobacteria. In coral fingers no N2-fixation was measurable, which stresses the importance of the sediment compartment for reef nitrogen cycling.

2007 Elsevier Ltd. All rights reserved.

Keywords: microphytobenthic photosynthesis; permeable sediments; calcification; N2-fixation; lightedark shift method; coral reefs; Australia; Heron Island

1. Introduction

Although situated in oligotrophic waters, shallow-water coral reefs are characterized by high biomasses and high community gross primary production rates (300e7000 g C m 2 y 1) (Kinsey, 1985; Crossland et al., 1991; Gattuso et al., 1998).

This means that energy and nutrients need to cycle efficiently within the reef ecosystem. Unconsolidated sediments often occupy large areas in shallow water coral reefs (Clavier and Garrigue,1999)andmaythereforebeanimportantcompartment for microphytobenthic primary production and for nutrient cycling. To contribute to the understanding of microbial processes in reef sediments we investigated microphytobenthic photosynthesis and its effect on calcification and we assessed sedimentary nitrogen fixation rates.

Sediment-living microphytobenthos (e.g. diatoms, dinoflagellates and cyanobacteria) may contribute significantly to reef ecosystem primary production (Charpy et al., 1998; Clavier and Garrigue, 1999; Heil et al., 2004) although high rates of primary production in these systems are often associated with corals (zooxanthellae), turf algae or larger benthic macroalgae (Kinsey, 1985; Gattuso et al., 1997; Gattuso et al., 1998). Per unit surface area the primary production rates of

* Corresponding author. Present address: Advanced Wastewater Management Centre, The University of Queensland, St. Lucia, Qld 4072, Australia.

E-mail addresses: u.werner@awmc.uq.edu.au (U. Werner), anna. blazejak@bgr.de (A. Blazejak), p.bird@csiro.au (P. Bird), geickert@ mpi-bremen.de (G. Eickert), rschoon@mpi-bremen.de (R. Schoon), rabed@ mpi-bremen.de (R.M.M. Abed), abisset@mpi-bremen.de (A. Bissett), dbeer@mpi-bremen.de (D. de Beer). 1 Present address: Federal Institute for Geosciences and Resources, Section Geomicrobiology, Stilleweg 2, 30655 Hannover, Germany. 2 Present address: CSIRO Long Pocket Laboratories, 120 Meiers Road, Indooroopilly, Qld 4068, Australia.

0272-7714/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2007.08.015

Available online at w.sciencedirect.com

Estuarine, Coastal and Shelf Science 76 (2008) 876e888 w.elsevier.com/locate/ecss microphytobenthos may be low in comparison to the rates of corals (Kinsey, 1985; Gattuso et al., 1998), but given the large areal extent of sediments the contribution of microphytobenthos and corals to reef primary production may be equally important on the whole reef ecosystem scale (Clavier and Garrigue, 1999).

Microphytobenthic photosynthesis may stimulate calcification, thereby influencing the cycling of inorganic carbon in the sediments and promoting sediment building and cementation. The calcification rate depends on the saturation state of the pore water with respect to carbonate and calcium. Calcifica- tion (or from right to left, the dissolution of CaCO3) can be written as

Calcification decreases dissolved inorganic carbon (DIC) by one and total alkalinity (TA) by two units. The release of

CO2 lowers the pH. The simplified reaction of photosynthesis (and from the right, aerobic respiration) is given in equation (2):

The uptake of CO2 by photosynthesis shifts the carbonate equilibrium towards carbonate and increases the pH; the up- take of nutrients during photosynthesis slightly increases TA. Thus, net photosynthesis increases the saturation state of the solution with respect to calcium carbonate and may stimulate calcification. Calcification may act as a buffer against a photosynthesis-driven pH increase, which is beneficial for phototrophic organisms, as most of them preferentially take up CO2, which is more abundant at lower pH (Zeebe and Gladrow, 2005).

On the other hand CaCO3 may dissolve during organic matter mineralization, which can be very active in coral reef sed- iments (Wild et al., 2004b; Werner et al., 2006a). Aerobic mineralization (equation 2 from the right to the left) promotes the dissolution of CaCO3 by the release of CO2, which lowers the pH and the saturation state of the solution with respect to calcium carbonate (Zeebe and Gladrow, 2005).

The oceanic water around coral reefs is typically depleted in nitrogen relative to phosphorous (Gattuso et al., 1998).

Thus, fixation of N2 from the atmosphere by N2-fixing reef organisms is of major importance for the nitrogen supply of coral reefs (Wilkinson et al., 1984; Larkum et al., 1988; Oneil and Capone, 1989). N2-fixation is performed exclusively by prokaryotes, with cyanobacteria and sulphate reducing bacte- ria being most important in marine environments. Therefore sediments, in which these organisms are abundant, could be important for the supply of nitrogen in the reef ecosystem.

We investigated microphytobenthic photosynthesis at

Heron Reef, Australia. Previously reported Chl a concentrations at Heron Reef were amongst the highest found in marine sediments (up to 1153 mg Chl a m 2)( Roelfsema et al., 2002; Heil et al., 2004). Our goal was to quantify microphytobenthic photosynthesis and N2-fixation rates and to investigate the impact of microphytobenthic photosynthesis on sedimentary calcification. We explored the heterogeneity of the microphytobenthic community by investigating the biomarker photopigments distribution and by investigating conspicuous epipsammic algae occurring at one station for marker pigments and for cyanobacterial 16S rDNA sequence diversity.

2. Material and methods 2.1. Study site

The study was carried out at Heron Island, Australia (23 270 S, 151 550 E). The island is located in the lagoonal platform reef Heron Reef, on the southern boundary of the Great Barrier Reef (Fig. 1a). Investigations were conducted at four stations (Fig. 1b). Two stations were close to the island at North Beach. North Beach 1 was located ca. 40 m offshore (water depth at low tide was 1 m) and North Beach 2 was situated close to the beach (ca. 0.5 m water depth at low tide). At the reef edge, where hydrodynamic forcing was strongest, the reef belt station was chosen (ca. 1 m water depth at low tide). The channel station (ca. 5 m water depth at low tide) was located outside the reef platform close to the reef edge, within the channel between Heron Reef and Wistari Reef. The sediment characteristics (analysed as described in Werner et al., 2006a) of North Beach 2, the reef belt and the channel station are given in Table 1.

2.2. Sampling

Sampling on the four stations was carried out over the course of 2 weeks in January 2002. Measurements that required fresh sediment or algal samples were carried out immediately after sampling. Preserved samples were analysed later. During the sampling period sea water temperature varied between 27 and 3 C and salinity ranged between 28 and 3. The top 10 cm of sediments were investigated. If not stated otherwise, the sediments were taken with plastic core liners with an inner diameter of 3.6 cm, and experiments were conducted at 28 C. An overview of the experiments and investigated parameter is given in Table 2.

2.3. Algal pigment analysis

Duplicate sediment cores (the same cores were prior used for microsensor analysis, see below) from North Beach 2, reef belt and the channel station were analysed for photopigment concentrations and composition. The sediments were sliced in 0.5 cm sections until 5 cm depth and in 1 cm sections from 5 to 10 cm depth. Sediments were stored frozen at 20 C and freeze dried before analysis. Sediment samples were sonicated (7 1 min) in 100% acetone and left for extraction for 24 h at 20 C. Before analysis the pre-filtered extract (Acrosdisc CR syringe filter; 0.45 m pore size, PALL, Gellman Laboratory) was diluted with water to a final concentration of 70% acetone (Buffan-Dubau and Carman, 2000).

At North Beach 2 conspicuous algae (ca. 100 m in diameter, surrounded by a hard wall) were abundant in the sediments

877U. Werner et al. / Estuarine, Coastal and Shelf Science 76 (2008) 876e888

(Fig. 2). We isolated fresh specimens of these algae and extracted photo pigments in triplicate samples (consisting each of ca. 150 specimens) as described above. However, different to the extraction of pigments from the bulk sediment samples, methanol was used as a extraction solvent for the isolated algae. Preliminary tests had shown that the extraction efficiency of methanol was higher in the samples of the isolated algae as compared to acetone, whereas the reverse was true for the bulk sediment samples. Photopigment extracts of the sediments and the isolated algae were subsequently separated on a Water HPLC (2690 Separation module), equipped with a Eurospher- 100 C18, 5 m Vertex column (Knauer Berlin, Germany), following the method by Wright et al. (1991). Absorption spectra of the separated compounds were measured on a Waters 996 Photo Diode Array detector; pigments were identified and quantified by comparison to pigment standards (DHI Waters and Environment, Denmark).

2.4. Microsensor measurements

We measured O2,C a2þ and pH pore water concentration profiles with microsensors in duplicate sediment cores taken at the four sampling sites. Measurements were performed in the laboratory immediately after sampling. The water layer on top of the sediment was gently stirred to develop a defined diffusiveboundarylayer (DBL).Measurements were performed in the light (PAR: 600 mmol photons m 2 s 1; lamp: KL 1500 electronic, Schott, Germany) and in the dark. 3 to 6 replicate profiles were measured per investigated parameter at each station. The O2 microsensors were amperometric Clark type electrodes with a guard (Revsbech, 1989), an actual sensing area of 5 m diameter and a 90% response time of less than 5 s. The microsensors were thick walled so that the tip diameter was approx. 300 m. The pH and Ca2þ electrodes were liquid ion-exchange membrane (LIX) glass microsensors as described indeBeer etal.(2000) witha90%responsetimeoflessthan1 s.

Net photosynthesis, diffusive oxygen uptake, calcification and decalcification rates were calculated from light and dark

O2 and Ca2þ profiles (Jørgensen and Revsbech, 1985; Kuehl et al., 1996), using Ficks first law:

where J is the flux, D0 is molecular diffusion coefficient of the solute in water (Li and Gregory, 1974) (corrected for temper-

ature and salinity) and dC/dz is the linear gradient of the measured solute. Diffusive fluxes of oxygen and calcium within

Channel N

2km

Heron Island NB

RB Heron Reef

(b) Fig. 1. (a) Location of Heron Island and (b) location of the four investigation sites at Heron Reef. NB, North Beach; RB, reef belt; Ch, Channel station.

Table 1 Sediment characteristics of the investigated stations (sampling depth 7 cm). Apart from grain sizes, data are given as averages standard deviation. NB2 is the near-shore station at North Beach, RB is the reef belt and Ch the channel station. TOC, total organic carbon

Median grain

878 U. Werner et al. / Estuarine, Coastal and Shelf Science 76 (2008) 876e888

the sediment were calculated from the profiles using equation (3) as well as the empirical relations for diffusion coefficients in the sediment:

where Ds,i is the diffusion coefficient in sediments of the species i corrected for temperature and pressure, f is the mea- sured porosity and m ¼ 3( Li and Gregory, 1974; Ullman and Aller, 1982).

(Parte 1 de 5)

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