Salinity control of benthic microbial mat community production in a Bahamian hypersaline lagoon

Salinity control of benthic microbial mat community production in a Bahamian hypersaline lagoon

Journal of Experimental Marine Biology and Ecology JOURNAL OF EXPERIMENTAL MARINE BIOLOQY AND ECOLOGY 187 (1995) 223-237 Salinity control of benthi...

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Journal of Experimental Marine Biology and Ecology

JOURNAL OF EXPERIMENTAL MARINE BIOLOQY AND ECOLOGY

187 (1995) 223-237

Salinity control of benthic microbial mat community production in a Bahamian hypersaline lagoon J. Pinckney aT*, H. W. Paerla, & B. M. Beboutb a University

of North Carolina at Chapel Hill, Institute of Marine Sciences, 3431 ArendeN Street, Morehead City, NC 28557, USA ’ Max-Planck-Institute fir Marine Mikrobiologie, Fahrenheitstrasse I, W-28359 Bremen, Germany

Received 6 December 1993; revision received 20 April 1994; accepted 24 October 1994

Abstract The purpose of this study was to determine the production and N, fixation responses of a hypersaline mat community following a reduction in salinity and nutrient enrichment. Cyanobateria-dominated microbial mat samples were collected from hypersaline Storr’s Lake and normal seawater salinity Pigeon Creek and preincubated at ambient (90z0) and reduced (45%,) salinities following no nutrient as well as inorganic nutrient (NO;, PO,, trace metals) and dissolved organic carbon (DOC, as mannitol) enrichment. CO1 and N, fixation rates were determined 2 and 4 days later. In addition, DOC (trace concentrations of 3H-labeled glucose and amino acids) uptake was measured in mats under normal and hypersaline conditions. A reduction in salinity from 90 to 45x0 significantly enhanced CO, and N, fixation rates, but inorganic nutrient and DOC additions did not significantly enhance rates compared with the controls. Dissolved organic carbon/dissolved organic nitrogen (DON) uptake was not influenced over the entire range of salinities (45-90%,) used in this study. When salinity-induced osmotic stress was relieved, mats underwent enhanced primary production and nitrogen fixation. Abiotic stress, induced by hypersaline conditions in Bahamian lagoons, results in lower productivity of the microbial mat communities and this stress may outweigh the typical limiting factors regulating phototrophic community primary production. Keywords:

Bahamas;

Cyanobacteria;

Hypersalinity;

Microbial

* Corresponding author. 0022-0981/95/$9.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0022-0981(94)00185-5

mat; Nutrient;

Productivity

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1. Introduction Abiotic stress is an external condition that induces internal physiological changes in an organism to a level below some optimal physiological state (Welden & Slauson, 1986). The degree of abiotic stress to which organisms are exposed influences processes controlling community structure and function. For example, under extreme abiotic stress, predation and competition may be less intense and less important (Welden & Slauson, 1986; Menge & Sutherland, 1987). Understanding the collective growth and functional responses of organisms within a community to changes in abiotic stress provides insight into the biotic mechanisms that communities use for survival and growth in (either anthropogenically or naturally) stressed ecosystems. Abiotic stresses that push organisms to their limits of physiological tolerance should produce profound effects on community structure and function. In most aquatic systems, phototrophic community primary production is limited by the availability of essential nutrients (primarily nitrogen and phosphorus), light, and/or temperature. In contrast, phototrophs in hypersaline lagoons are exposed to salinities over 90x,, which may result in extreme abiotic stress that overrides nutritional limiting factors (nutrient limitation) controlling community growth and function. Benthic microbial mats, composed primarily of filamentous cyanobacteria, diatoms, and a variety of heterotrophic, phototrophic, and chemolithotrophic bacteria, are common features of tropical hypersaline lagoons worldwide (Whitton & Potts, 1982; Moore, 1987; Pentecost, 1989; Black, 1933). Under axenic conditions, cyanobacterial CO2 and N, fixation is significantly reduced by hypersalinity (Miller et al., 1976; Blumwald & Tel-Or, 1983; Mackay et al., 1983; Reddy et al., 1989; Fernandes et al., 1993). Salinityinduced osmotic stress seems to result in a general reduction in rates of physiological processes (Fernandes et al., 1993). These findings suggest that, in hypersaline systems, osmotic stress may displace nutrient availability as the primary limiting factor for cyanobacterial productivity. However, the integrated community responses of benthic microbial mats to hypersalinity have not been experimentally determined. Primary production (CO2 fixation) and the biological conversion of N, to NH, (N2 fixation) are important ecophysiological indicators of the potential for phototrophic community growth (Stal et al., 1985; Paerl et al., 1989; Bebout, 1992). Primary productivity affects the dynamic stability and length of food chains (trophic structure) and is directly related to the ability of ecosystems to respond and recover from perturbations (Rosenzweig, 1973; Oksanen et al., 1981; McNaughton et al., 1989; Moore et al., 1993). Under extreme stress, resources (i.e., energy as ATP) normally used for growth and reproduction are reallocated to stress-compensating mechanisms. This reallocation of resources results in lowered primary productivity. Low productivity ecosystems are more susceptible to perturbations and recover more slowly than high productivity ecosystems (Moore et al., 1993). This suggests that perturbation events in hypersaline lagoons may have long-term effects on community structure, productivity, growth, and function. Diazotrophy (N2 fixation) is a process used by some prokaryotes to avoid chronic N-limitation in oligotrophic marine systems (Webb et al., 1975; Whitton & Potts, 1982; Paerl, 1990). In microbial mat communities, N, fixation may be sufftcient to meet N demands or may serve as a supplementary (rather than exclusive) source of

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“new” nitrogen (Webb et al., 1975; Bebout 1992; Paerl et al., 1993). In both cases, the environmental factors regulating N, fixation may ultimately control mat primary production. The Bahamian Islands have many small hypersaline lagoons that contain welldeveloped benthic microbial mat communities (Black, 1933; Neumann et al., 1970; Monty, 1972, 1976; Hardie, 1977; Pentecost, 1989). Storr’s Lake, on San Salvador Island, Bahamas, is a typical Bahamian hypersaline lagoon that experiences seasonally fluctuating salinities (45-100x,) during wet and dry periods (McNeese, 1988; Mann & Nelson, 1989). The wide distribution of microbial mats in Storr’s Lake suggests that these communities are a productive feature and contribute significantly to the material flux and trophodynamics of the lagoon. Although microbial mats blanket the shallow water sediments in Storr’s Lake, mat phototrophs exhibit low rates of CO, and N, fixation when compared with mats in more temperate zone environments (Bebout et al., 1987; Villbrandt et al., 1990; Bebout, 1992; Canfield & Des Marais, 1993; Paerl et al., 1993). However, rate measurements of COz and N, fixation in Storr’s Lake were determined under in situ hypersaline conditions. To explain the high biomass, low productivity conditions of Storr’s Lake, Bebout (1992) proposed that microbial mat growth occurs during seasonal periods of reduced salinity followed by near-dormancy under hypersaline conditions. The justification for this hypothesis was that under hypersaline conditions, mats are osmotically-stressed, resulting in slowed physiological processes and little growth. To test this hypothesis, it must be shown that mat primary production (as CO2 and N, fixation) is enhanced at reduced salinities. A second hypothesis is that following the mitigation of salinity stress, mat community production will eventually become nutrient-limited. The primary purpose of this study was to determine the production and N, fixation responses of a salinity-stressed microbial mat community following a reduction in salinity and enhanced nutrient availability to assess the effect of hypersaline conditions on community primary production. A second goal was to compare CO2 and N, fixation rates of hypersaline lagoonal mats with a nearby microbial mat community exposed to near-seawater salinities to determine if lagoonal mat productivity is depressed relative to other mat communities.

2. Materials 2.1.

and methods

Study sites

Storr’s Lake (24” 00’ N, 72” 05’ W) is an enclosed, hypersaline lagoon located on the eastern side of San Salvador Island, Bahamas (Fig. 1). The lagoon covers an area of z 1022 ha at an average depth < 2 m. Bottom sediments are composed of a calcareous ooze with a thin, lithified surface layer (Mann & Nelson, 1989). Although the average annual salinity is z 85%,, lagoon salinity varies seasonally, with a maximum as high as 90-loo%, during the dry season (December-April) and a minimum near 40-45%, during the wet season (September-November) (McNeese, 1988). Hydrological inputs to the lagoon occur via tidal conduits and seeps but water levels are not

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ATLANTIC OCEAN 26.

FIELD STATION

KlCDMlTERS

\ STORR’S LAKE

\ Fig. I. Location

map for San Salvador

Island,

PIGEON CREEK LAKE

(ADOR

/SLAbID

Bahamas

and sampling

locations

on the island

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tidally-influenced (Mann & Nelson, 1989). The hypersaline waters in the lagoon are extremely turbid (Kd = 4.4 m-i) and reddish-orange in color (Bebout, 1992; Pinckney, pers. obs.). Shallow water (< 20 cm) sediments in Storr’s Lake are covered by laminated and bulbous calcareous cyanobacterial crusts. The heterocystous filamentous diazotroph Scytonema sp. is the dominant cyanobacterium associated with these crusts, but Gloeocapsa sp., Schizothrix sp., and Entophysalis sp. are also present (Bebout, 1992). Purple, green, and colorless sulfur bacteria are also common community components. We have termed these structures “Scytonema mats”. There are also several small lagoons and embayments on San Salvador Island that are tidally-flushed. Tidal exchange between these lagoons and the sea results in a relatively constant salinity (35-40%,) over daily and annual cycles. Non-calcified, laminated, cyanobacterial mats are commonly found in these shallow water lagoons. These mats, growing at near-seawater salinities, may be considered community functional equivalents of the Storr’s Lake Scytonema mats. Pigeon Creek Lake is a small (= 3 ha), tidally-flushed lagoon at the head of Pigeon Creek. The bottom sediments in the shallow (< 10 cm) lagoon are covered by a mat community composed of Microcoleus chthonoplastes, with a small amount (< 10%) of Lyngbya sp. underlain by a layer of purple sulfur bacteria and covered by a layer of benthic diatoms. In contrast to the Storr’s Lake Scytonema mats, this mat community is composed primarily of non-heterocystous cyanobacterial species. We have termed this microbial community the “Pigeon Creek mat”. All experiments were conducted under natural it-radiance and temperature conditions at the Bahamian Field Station, located near Storr’s Lake, during 6-11 March 1993. Air temperatures during the daytime period ranged from 24.1 to 32.9 “C and water temperatures in the bioassay containers were z 3 o C higher than ambient air temperatures. The water temperature of Storr’s Lake during this period ranged from 27.8 to in the waters of Storr’s lake 28.2 “C. Nutrient (NH,‘, NO;, and PO; concentrations are
Scytonema

mat bioassay

The experiment consisted of incubating the Scytonema mat at reduced (45%,) and ambient (90%,) salinities with a variety of nutrient additions and measuring community CO2 and N, fixation responses to the manipulations (Table 1). Large sections of Scytonema mat (500 cm2 x 1 cm) were collected from Storr’s Lake and transported to the Field Station. Sections were cut into 100 cm2 pieces and placed in 500 ml plastic containers. Half the containers (24) received 300 ml Storr’s Lake water (90%,,) while the other half were filled with 150 ml lake water and 150 ml deionized water (final salinity of 45%,). Nitrate (NaNO,, 5 PM), orthophosphate (H2P04, 2 PM), a trace metal mixture (1 PM Fe and 0.5 PM each Mn, Cu, Zn, MO, Co, and B), and mannito1 (10 mM) were then added to the corresponding treatments (Table 1). Previous nutrient addition experiments on a variety of microbial mat systems suggest that the nutrient enrichments used in this bioassay should be sufficient to elicit a response (Paerl et al., 1993). All nutrient solutions were sterilized (autoclaved) before use. The con-

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Table 1 Shallow Scytonema mat bioassay

experimental

Salinity uoO)

Treatment

90

Control NO, (NaNO,) PO, (KPO,) NO, + PO, Trace Metal + Fe Mannitol

45

Control NO, (NaNO,) PO, (KPO,) NO, + PO4 Trace Metal + Fe Mannitol

design Final concentration

Number

5gM 2pM 5pM+2pM 10% of F/2 media (F/20) 10 mM

4 4 4 4 4 4

5gM 2pM 5 pM+2pM 10% of F/2 media (F/20) 10 mM

4 4 4 4 4 4

of replicates

tainers were placed on a table and exposed to ambient temperature and irradiance. Irradiance during the incubation period was recorded with a LiCor LI-1000 (with model 192 quantum sensor) PAR light meter. Salinities in the containers were measured daily with a refractometer and adjusted (by adding deionized water) to maintain the desired salinity. COz fixation and nitrogenase activity rates were measured on days 2 and 4 to determine the community response to the different treatments. Samples were incubated for 5.5 h on day 2 and 6.5 h on day 4 at ambient irradiance and temperature. 2.3. Pigeon Creek mat bioassay Large squares (500 cm2 x 1 cm) of Pigeon Creek microbial mat were collected, transported to the field station, cut into 100 cm* sections, and placed in 500 ml plastic containers. Each container (except the high salinity treatment) was filled with 300 ml of water collected from Pigeon Creek Lake (40x,). The high salinity treatment containers were filled with 300 ml of Storr’s Lake water (90x0). Nitrate, phosphate, a trace metal mixture, and mannitol were added to the appropriate containers (Table 2) and incubated outside under ambient temperature and irradiance. Salinity in the containers was checked daily and adjusted (by adding deionized water) to maintain the desired salinity. After 2 days, CO2 fixation and nitrogenase activity rates were obtained to determine the community response to the manipulations. The incubation time following t4C and acetylene additions was 5.75 h. 2.4. Dissolved organic carbon uptake by scytonema

mats

The effect of salinity on trace-level (< 1 nM) 3H-labeled dissolved organic carbon (DOC, glucose and mixed amino acids) uptake was determined for Scytonema mats from Storr’s Lake. Mat sections were collected, transported to the field station, and placed in 500 ml plastic containers. Half (3) were filled with 300 ml of Storr’s Lake

J. Pinckney et al. /J. Exp. Mar. Biol. Ecol. 187 (1995) 223-237 Table 2 Pigeon Creek mat bioassay Lake

experimental

design. The high salinity (90%,) water was obtained

Treatment

Final concentration

Control Salinity NO, (NaNO,)

90%0 5pM 2W 5/~M+2pM 10% of F/2 media (F/20) 10 mM

PO4 (KPO,) NO3 + PO4 Trace Metal + Fe Mannitol

229

Number

from Storr’s

of replicates

3 3 3 3 3 3 3

water (9O%J while the other half received 150 ml of lake water and 150 ml of deionized water (45%,). Three Scytonema mat sections (100 cm2) were placed in lake water containers and the procedure repeated for the low salinity containers, giving a total of 6 replicates. The containers were placed outside under ambient temperature and irradiance. After 24-h preincubation, 2 (1.15 cm2) cores were collected from each container and each placed in a 20 ml glass scintillation vial with 10 ml of the incubation water. One vial received 100 ~1 of 3H-glucose (10 PCi; specific activity 240 pCi.prnol-‘) and the other received 100 ~1 of 3H-amino acid mixture (7 PCi; 210 @i*pmol-I). Following isotope additions, samples were incubated for 9.0 h under ambient light and temperature. 2.5. Nitrogenase

activity

Rates of nitrogenase activity (NA) were estimated using the acetylene reduction (AR) method (Stewart et al., 1967 as modified by Bebout et al., 1987). NA provides a relative measure of the rate of N, fixation by the microbial mat community. Cores from the mat were obtained with a cutoff 5 ml plastic syringe (1.15 cm2). Core sections were placed in 15 ml glass serum vials, 6 ml of incubation water added, and capped with rubber serum stoppers. Acetylene (2 ml), generated from calcium carbide, was injected into each sample. Samples were oriented with the top of the mat facing upward and incubated in a flowing water bath covered with one layer of neutral density screen (25 % reduction in PAR) to prevent overexposure to high h-radiances. Dark-incubated samples were wrapped in two layers of aluminum foil and placed in the water bath. After incubation, vials were vigorously shaken for 30 s to liberate ethylene and 5 ml of the gas phase was displaced into evacuated 9 ml vials for later analysis. Ethylene concentrations were quantified by injecting 300 ~1 of the headspace gas into a gas chromatograph (Shimadzu GC9A, flame ionization detector, 2 m Porapak-T column, 80 “C). 2.6.

Carbon (CO,) fixation

Cores of mat material were collected as described in 20 ml glass scintillation vials (with polypropylene

above. Core sections were placed caps) along with 20 ml of the in-

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cubation water and 0.2 ml of NaH14C0, (3.5 PCi; 58 ~Ci*~mol-*). Incubation procedures were the same as described above. Following the incubation, water was removed and the mat plug placed on drying paper. After drying, mat plugs were fumed with concentrated HCl for 6 h in a covered container to remove unincorporated and abiotically-precipitated 14C. Preliminary experiments indicated 6 h was sufficient to remove all inorganic i4C (Paerl, unpubl.). Dried and fumed samples were stored (refrigerated in the dark) in individual vials for z 5 days before counting. The scintillation cocktail (Cyto-Stint, ICN Inc., 5 ml) was added and each sample stored in the dark for 24 h before counting. Counts were obtained using a Beckman TD 5000 Liquid Scintillation Counter and converted to DPM using a quench curve (based on calibrated 14C hexadecane; NEN, Inc.) constructed using mat material from Storr’s Lake. Dissolved inorganic carbon (DIC) in the incubation water was measured by infrared analysis (Beckman model 864 Infrared Analyzer). 2.7. Photopigments Samples of each mat type were collected for photopigment analysis using butyrate core tubes (2.4 cm2). Cores were extruded, the upper 10 mm of mat placed in 20 ml plastic scintillation vials, and frozen until later analysis. At 72 h prior to analysis, 10 ml of extraction solvent (45% methanol, 45% acetone, 10% deionized H,O) was added, samples sonicated for 30 s, and returned to the freezer to allow for slow and complete extraction of photopigments (Pinckney et al., 1994). Photopigments were identified and quantified by high performance liquid chromatography (HPLC) using an in line photodiode array detector (Shimadzu SPD-M6a) (Wright et al., 1991). Photopigments were identified by comparing absorbance spectra with known standards (Wright et al., 1991). 2.8.

Statistical methods

Statistical analyses for most experiments consisted of a mixed model (models I and II) two or three-way analysis of variance procedure. The assumptions of ANOVA were checked before analyses and data transformed where necessary. A posterior? multiple comparisons of means was achieved using the Bonferroni procedure with a = 0.05.

3. Results 3. I. Photopigments The primary photopigment composition of Scytonema mat from Storr’s Lake and the Pigeon Creek Microcoleus mat was determined to compare the relative photosynthetic biomass (as Chlorophyll a, Chl a) of the two mat types. The mean Chl a concentration was not significantly different between the two sites (ANOVA, p = 0.26) and the combined mean for both sites was 11.41 mg Chl a me2 (n = 45, SD = 0.71). The most abundant carotenoid accessory pigments were myxoxanthophyll, zeaxanthin, and can-

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thaxanthin, characteristic of cyanobacteria. On an area1 basis, oxygenic phototrophic biomass was not significantly different for the Storr’s Lake and Pigeon Creek mats. 3.2.

Scytonema

mat bioassay

The effects of salinity, nutrient, and organic substrate additions on CO, fixation and NA rates were determined after incubating samples in the appropriate treatments over a 4-day period (Table 1). For CO* fixation, the data were analyzed using a three-way ANOVA with day (2 levels), salinity (2 levels), and nutrient addition (5 levels) as the three main factors with 4 replicates at each level (n = 80) (Fig. 2). The lower salinity (45x,) treatments exhibited higher rates of CO2 fixation (p
20,

,

EDAY

/

/

N

P

1

I

TIT

MAN

4

ill-lII1 CTRL

N+P

TREATMENT Fig. 2. CO, fixation (production) rates for the Starr’s Lake Scytonema mat bioassay. Incubations were conducted on days 2 (1200 to 1730 local time) and 4 (1000 to 1630 local time) following nutrient and salinity pre-incubations. Abbreviations for treatments are CTRL (control), N (nitrate), P (phosphate), N + P (nitrate and phosphate), TR (trace elements mixture), MAN (mannitol). Values are the mean + 1 SD.

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NA rates were higher on day 2 (lower irradiance) (p <. 0.0s) and barely detectable in the high salinity treatments on day 4 (Fig. 3). The lower salinity (45x,,) significantly enhanced nitrogenase activity on both days (p < 0.00 1). Nutrient additions had no effect on nitrogenase activity at the two salinity levels (p = 0.16) and there were no significant factor interactions. Nitrogenase activity rates were highly variable (shown by the large SD bars in Fig. 3), making it difficult to assess the effects of individual nutrient additions. 3.3. Pigeon creek mat bionssny The effects of salinity and nutrient additions on CO, fixation and NA were also determined for the McrucoLeeus-dominated Pigeon Creek cyanobacterial mat (Fig. 4). CO2 fixation data were analyzed using a one-way ANOVA with nutrient and salinity additions as treatments with 3 replicates for each treatment (n - 24). The phosphate and trace metal additions resulted in CO, rates that were not significantly different from the control (Bonferroni, ptO.05). However, high salinity, nitrate, and nitrate + phosphate additions resulted in lower CO, fixation rates (p~O.05). Nitrogenase activity data were analyzed using a two-way ANOVA with light (i.e. light vs. dark) and treatment (nutrients, DOC, and salinity) as factors with 3 replicates for each factor level (n = 30). NA was higher in the dark compared with the light-exposed

n H.GH SAUN N 1 LOW SALlNlTY

6

2

0

I_.+ L-.--a

CTRL

..N

P

Ni-P

IR

.MAN

TF?EAl’MENT Fig. 3. Nitrogenase activity for the Storr’s Lake Scytunenza mat bioassay. Incubations were conducted on days 2 and 4 following nutrient and salinity pre-incubations. Treatment abbreviations and incubation times are the same as in Fig. 2. Values are the mean 2 1 SD.

J. Pinckney et al. i J. 30

Exp. Mar. I

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Ed.

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I

I

233

I

mUGHI I

DARK

1

I

I

C IRL SAL

N

I N+P

I

I

I

P

TR

MAN

1

TREAl?vlENT Fig. 4. Production (CO, fixation) and nitrogenase activity for the Pigeon Creek Micrucohs mat following nutrient and salinity pre-incubations. Incubations were conducted from 1000 to 1545 local time on day 2. Treatment abbreviations are the same as in Fig. 2 with the addition of S (90s0 S). Nitrogenase activity was not determined (nd) for the N and N + P treatments. Production was not determined for the MAN treatment. VaheS are the mean + 1 SD.

treatments (p < 0.001) (Fig. 4). Phosphate, trace metal, and DOC (mannitol) additions enhanced NA, especially in the dark treatments (Bonferroni, P-Z 0.05). The high salinity and control groups showed significantly lower rates of NA than the other treatments. 3.4. Dissolved organic carbon utilization We also evaluated the rates of DOC utilization under high (907&J and low (45x,) salinity conditions for Storr’s Lake Scyconema mats (Fig. 5). The data were analyzed using a two-way ANOVA with salinity (90z0 vs. 45x,), and substrate addition (3Hglucose, 3H-amino acids mixture) as main factors and 3 replicates for each manipulation (n = 12). The substrate addition factor was included in the analysis to partition the variance associated with differential uptake and activities of the two substrates. The salinity reduction did not significantly influence the uptake of either glucose or amino acids (p = 0.22). 4. Discussion Mat oxygenic phototroph biomass (Chl a) was not significantly different between the Pigeon Creek and Scytonema mats. However, Chl a concentrations were very low

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-

1500

-

187 (1995) 223-237

n

HIGH SAUNITY

H

LOW SAUNITY

1

n I r)

1000

GLUCOSE

AMINO

ACIDS

Fig. 5. The effects of organic substrate additions at high and low salinities on uptake rates for the Starr’s Lake Scytonema mat. Incubation was conducted from 0800 to 1700 local time. Treatment abbreviations are GLU (glucose) and AA (amino acids mixture). Values are the mean + 1 SD.

compared to temperate zone microbial mats. Microbial mats at Bird Shoal (Beaufort, NC) typically have Chl a levels that range from 100 to 400 mg Chl a*me2, which is much higher than the mean (11.4 mg Chl a.mm2) for mats examined in this study (Pinckney, unpubl). This suggests that mat phototrophic biomass is relatively low for Storr’s Lake and Pigeon Creek mats and may partially explain the low photosynthetic rates measured in the bioassays. The Scytonema mat exhibited higher rates of CO, fixation and NA when incubated at a lower salinity, but nutrient and DOC additions (NO, , PO, , trace metals, and mannitol) did not significantly increase rates. Paerl et al. (1993) have shown that nutrient and DOC additions frequently fail to increase CO, fixation in a variety of tropical, subtropical, and temperate microbial mat communities. Rates of CO, fixation by Storr’s Lake mats were low (Z 2-15 mg C.m-*.h-‘) in comparison with microbial mats in North Carolina (Z 50-100 mg C.rnm2.hm’) (Bebout et al., 1993; Paerl et al., 1993) and Tomales Bay, California (Z 60-120 mg C.rnm2 .h-‘) (Paerl et al., 1993). However, rates were comparable to intertidal stromatolitic mats at Stocking Island, Bahamas (lo-25 mg C.m-2.h-‘) (Pinckney et al., submitted). Bioassay results suggest that the hypersaline conditions in Storr’s Lake generally suppress primary productivity and N, fixation. Under lowered salinities, the activity of the Scytonema mats increases while an increase in nutrient availability does not stimulate either CO2 fixation or NA over a 2- or 4-day incubation period. The methodologies used to measure CO, and N, fixation have a variety of limitations. Utilization of the added substrates may have been diffusion-limited and unevenly distributed within the mat. In addition, it has been shown that the 14C method may underestimate production in microbial mats (Revsbech et al., 1981). The incubation times for rate measurements were longer (> 4 h) than times usually used for these types of measurements in other systems. Previous work in this system showed that long incubation times were necessary to obtain measurable rates in these slow-growing mats (Bebout, 1992; Paerl et al., 1993). However, our experimental design incorporated appropriate controls so that we could confidently evaluate the relative differences between the responses of controls and treatments. In Storr’s Lake, salinity interacts with nutrient supply, irradiance, and, sometimes,

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O2 to control lagoonal mat production at the community level (Bebout, 1992). The lack of nutrient stimulation is likely product of salinity-controlled slow growth of mats. Mat growth was too slow to detect or measure nutrient stimulation during the bioassay period (2-4 days). These results support the conclusions of Paerl et al. (1993), which suggested that fast-growing microbial mats in temporal, euhaline (seawater salinity) habitats more readily exhibit nutrient limitation when compared to slower growing hypersaline tropical and subtropical mats. When Microcoleus-dominated Pigeon Creek mat growing at near seawater salinity (40%,) was exposed to hypersaline Storr’s Lake water and nutrient additions, both the nutrient and high salinity treatments reduced CO, fixation rates compared with the control (i.e. normal salinity, no nutrient additions). However, NA was significantly enhanced by PO, , trace metal, and mannitol additions and reduced by the high salinity treatment. Hypersaline conditions seem to have only a minimal effect on CO, fixation but significantly reduced NA by the Pigeon Creek microbial mat. In addition, N, fixation was much higher under dark conditions, suggesting that O2 production by oxygenic photosynthesis may have inhibited N, fixation; a conclusion shared with many prior investigations (Stal et al., 1985; Paerl et al. 1989; Bebout, 1992). The Scytonema mats exhibited DOC (glucose, amino acids) uptake and a reduction in salinity did not enhance DOC uptake. Uptake of dissolved organic nitrogen (DON) and DOC is a common property of benthic microbial mats (Paerl, 1993). Paerl et al. (1993) have suggested that DON/DOC uptake is linked to photoheterotrophy and may represent a supplemental mechanism for carbon and nutrient acquisition, especially in oligotrophic environments. The uptake of DON/DOC by Storr’s Lake Scytonema mats via photoheterotrophy, which does not seem to be diminished at elevated salinities, may partially explain how these mat communities are able to persist under extended periods of hypersalinity. A reduction in salinity from hypersaline (90x0) to reduced (45%,) levels resulted in an increase in CO, and N, fixation rates of Storr’s Lake Scytonema mats, supporting the findings of Bebout (1992). However, the addition of nutrients to Scytonema mats at the lower salinity did not enhance CO2 fixation and NA. In contrast, productivity of Pigeon Creek Microcoleus mats was not inhibited by higher salinities over the 2-day incubation period, but N, fixation was significantly suppressed by the higher salinity. The different responses by the two mat types may be attributed to the different ecophysiological responses of Scytonema and Microcoleus to osmotic stress (Fernandes et al., 1993). Mat primary production in Storr’s Lake appears (at least partially) to be regulated by changes in the salinity of the water in the lagoon. When the restraint of osmotic stress is removed, these mats may experience enhanced growth. Under hypersaline conditions, the mats may be in a “maintenance mode” and exhibit very low production. Direct measurements of CO* and N, fixation rates of Storr’s Lake mats should also be obtained during the rainy season (July to October) when lagoon salinities are reduced to further test the hypothesis of enhanced mat growth at lowered salinities. Abiotic stress, induced by hypersaline conditions within Bahamian lagoons, results in lower productivity of the microbial mat communities and this stress outweighs the typical limiting factors regulating phototrophic community primary production, growth,

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and function. Minor perturbations that cause additional stress, such as higher than normal temperatures, extended periods of reduced h-radiance due to clouds, or low rainfall during the rainy season may contribute to the alteration of community structure within the lagoons. 5. Acknowlegements We thank the Bahamian Field Station, San Salvador Island, Bahamas for providing laboratory space and logistical support, C. Neumann for the invitation to join his class field trip, and M. Fitzpatrick and M. Piehler for sample analyses. Funding for this study was provided by NSF grant OCE 901246 to H.W.P. Partial support for J.L.P. was provided by a Department of Energy Global Change Distinguished Postdoctoral Fellowship.

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