Nova Hedwigia, Beiheft 135, p. 71-83 February 2009

Epipelic diatoms from an extreme acid environment: Beowulf Spring, Yellowstone National Park, U.S.A.

William 0. Hobbs*l, Alexander P. Wolfe1, William I? Inskeep2, Larry Amskoldl & Kurt 0. Konhauserl

Department of Earth & Atmospheric Sciences, University of Alberta, Edmonton, AB T6G 2E3, Canada Current address: Department of Geosciences, Univ. of Nebraska-Lincoln, NE 68588, USA. Department of Land Resources & Environmental Sciences, Montana State University, Bozeman, MT 59717, USA

* author for correspondence (whobbs2@unl.edu)

With 3 figures and 2 tables

Abstract: A floristic study is presented of the diatom community from Beowulf Spring, Yellowstone Na- tional Park (YNP), U.S.A., a strongly acidic (pH<3) and thermophilous (temperature >50 "C) environment with potentially toxic concentrations of dissolved metals. A sampling transect between regions of active hydrous femc oxide (HFO) precipitation and mats of periphytic red algae (Cyandiaceae) revealed that dia- toms are present mainly in the regions of the spring characterized by the highest temperatures and brown arsenic-rich HFO mats. The diatom flora is dominated by a deme within the Nitzschia thermalis complex, in association with N ovalis, Pinnularia acoricola, and Eunotia exigua. Abiotic silica precipitates were observed on the surfaces of most diatom cells in uncleaned material. Cells of Cyanidiaceae also participate in active silica deposition. These observations not only highlight the tolerance of hot-spring diatoms to extreme environments with respect to temperature, pH, and arsenic (As) concentration, but also illustrate the clear zonation between epipelic algal communities within a hot spring.

Introduction

Although diatoms have been shown to respond to temperature in culture (Patrick 1977, S d & Takahashi 1995), it remains uncertain whether they are directly thermophilous in an eco- physiological sense, or indirectly influenced by environmental conditions that covary with cli- mate (Anderson 2001, Wolfe 2002). The bulk of previous studies on diatoms fiom hot spring environments has focused solely on temperature (Hustedt 1937-1938, Stockner 1967, Brock & Brock 1966), with some attention to the influences of low ambient pH and elevated concentra- tions of potentially toxic elements, such & arsenic (As) and copper (Cu) (Hargreaves et al. 1975, Whitton & Diaz 1981).

In this paper, we present an account of the diatom flora fiom the B e d f Spring in the Noms Geyser Basin, Yellowstone National Park (YNP). This epipelic community inhabits As-rich hy- drous ferric oxide (HFO) mats that coat the sandy substratum of the spring proximal to the vent (Fig. 1). Peripheral to the HFO zone, a green algal mat composed of cyanidiaceous Rhodophyta contains few, if any, diatoms. Light & scanning electron microscopic (SEM) investigations of these

1438-91 34/09/01 35-071 $3.25 O 2009 J. Cramer in der Gebriider Borntraeger Verlagsbuchhandlung, D-70176 Stuttgart

William 0. Hobbs et al.

communities are presented from both uncleaned and oxidized materials. Water chemistry implies that the diatoms are exploiting an ecological niche that may be toxic to other algae. Contrary to Villeneuve & Pienitz's (1998) suggestion that there is no single diatom assemblage typical of ther- mal springs, our study suggests a strong floristic similarity among highly acidic hot springs glo- bally (Hustedt 1937-1938, Carter 1972, Hargreaves et al. 1975, Whitton & Diaz 1981, Cassie & Cooper 1989, Watanabe & Asai 1995, DeNicola 2000, Jordan 2001), implying that indeed there exists a distinct but biogeographically-disjunct flora in these extreme environments.

Methods

Beowulf Hot Spring (44O43'53" N, 1 10°42'41" W) lies within the Norris Geyser Basin, Yellow- stone National Park, Wyoming, U.S.A. (YNP Thermal Inventory #NHSP35)) (Fig. 1A). Surface sediment collections were obtained from both the central portion of the spring, which is coated by HFO, and peripheral green algal mats (Fig. 1B). Samples were either cleaned using 30 % H202, or mounted uncleaned following dlution of the slurry in deionized water. Valves were mounted in Naphrax and observed at lOOOx under oil immersion with differential interference contrast optics (Leica DMLB). For scanning electron microscopy (SEM), slurry was evaporated onto stubs at room temperature in a desiccator, prior to sputter-coating with Au, and examination in a JEOL-630 1 F field-emission SEM. Archives of these samples are kept at 4 OC at the Univer- sity of Alberta. Our results confirm that it is clearly advantageous to examine both cleaned and uncleaned material (Stockner 1967).

Beowulf is considered an acid-sulfate-chloride spring, where the spring discharges extremely acidic (pH<3) and hot (>50 OC) water, with mM concentrations of Na+, K+, C1-, S042- and Si (Fig. 1, Table 1, Inskeep et al. 2004). Water samples were obtained by syringe and filtered through 0.45 pm in-line nylon syringe filters. Samples were further acidified with TraceMetal-grade HNOs and kept refrigerated until analysis by ion chromatography. Temperature and pH were measured in the field using a digital thermometer and an Orion Ross (8165BN) pH meter cali- brated between 2 and 4, respectively.

Results and Discussion

Water chemistry

The Beowulf Spring has an ionic strength of 0.02 M, which is comparable to a marine - brackish estuarine salinity classification (Juggins 1992). Aqueous chemistry from Beowulf Spring has been characterized on multiple occasions and exhibits very little range in concentrations (%lo % or less for most constituents, Inskeep et al. 2004). The water chemistry reveals the presence of

Fig. 1. A: Beowulf Spring, Norris Geyser Basin. Brown areas are hydrous ferric oxides (HFO) and green areas represent cover by rhodophytes including Cyanidium. B: Close-up of the transition between the HFO mat and Cyanidiaceae, with penny for scale. C: SEM photograph of silica casts produced by coccoid Cya- nidiaceae cells. D: Close-up of cells in (C) showing remnants of soft-bodied cells within the silica cast (cc). E-H: SEM micrographs of diatoms from the HFO mat. (E) Nitzschia cf. thermalis var. minor with clean areolae devoid of precipitate. (F) Nitzschia ovalis (No) and Pinnularia acoricola (Pa), also with clean are- olae; the cast of a Cyanidiaceae cell is also visible below h! ovalis. (G) A cell of N ovalis encrusted in a diagenetic silica precipitate that completely sheathes the cell and blocks areolae. (H) A Pinnularia acori- cola cell with diagenetic silica papillae on the apical cell surface; areolae are visible beneath. Scale bars are 10 p, except (H) where it is 1 p.

Epipelic diatoms from an extreme acid environment: Beowulf Spring

William 0. Hobbs et al.

Table 1. Water chemistry of Beowulf Hot Spring, Yellowstone National Park. All concentrations are reported in pM, except where indicated.

Parameter Concentration

Na 11136

*Si 4690

K 1225

Ca 125

Al 124

As 19.6

M g 8.2

Fe 6.2

* P 4.5

Zn 0.9

Mn 0.6

Cu 0.1

anions

*CI- 13770

*Sod2- 1510

*F 147

*No,- 20.0

other

PH 3.0

Temp ("C) 67

Ionic strength (M) 0.02

* data from Inskeep et al. (2004)

potentially toxic metals to algal growth, namely Cu and As (Table 1). In particular, As concentra- tions reach 20 pM, much higher than minimum concentrations required to inhibit growth of dia- toms in culture (Saunders & Riedel 1998). Similarly, Beowulf Cu concentrations are two to three orders of magnitude greater than those used by Saunders & Riedel(1998). Thus, natural concen- trations of As and Cu in the Beowulf Spring approach or exceed those from the most highly impacted acid-mine drainages (Whitton & Diaz 1981, Verb & Vis 2000).

While it is not our intention to speculate as to whether there is active uptake of As by diatoms in Beowulf Spring, As uptake is likely occurring among other microorganisms in YNP hot springs, namely bacteria (Langner et al. 2001, Inskeep et al. 2004). Metabolism of As by marine algae is hypothesized to occur because of the molecular similarity between phosphorous and As (Edmonds & Francesconi 1998). General pathways of metals metabolism by algae include se- questration, complexation with exuded dissolved organic carbon, and redox transformations dur- ing uptake (Saunders & Riedel 1998). Thus, a range of biogeochemical processes exist to allow not only the survival, but indeed the success, of algae inhabiting these extreme environments.

The pH of Beowulf Spring at the time of sampling was 2.26, and the temperature was 67 "C. A number of diatom communities have been documented at pH of <3, although species rich-

Epipelic diatoms from an extreme acid environment: Beowulf Spring 75

ness decreases rapidly below a pH of 4.5 (DeNicola 2000). The temperature of Beowulf Spring exceeds that of most other springs that support diatoms (Brock & Brock 1966, Stockner 1967, Hargreaves et al. 1975, Whitton & Diaz 1981), with rare exceptions (Mann & Schlichting 1967, Compkre & Delmotte 1986).

Green mats

Samples of the green Cyanidiaceae mat lack diatoms (Fig. 1A-B), with the exception of occa- sional frustules that presumably originate from transport. However, significant amorphous silica precipitation occurs in association with Cyanidiaceae cells (Fig. IC-D). Silica biomineralization under highly acidic conditions is not well-documented, with the exception of Asada & Tazaki's (2001) study of Cyaniditim caldarizim in a Japanese spring at pH<2. They proposed passive mod- els including nucleation of silica on cell walls, polymerization of silicic acid, and adhesion of colloidal silica gel. These models, however, contradict general observations that spontaneous silica polymerization and sinter formation typically occur at circumneutral pH (Konhauser et al. 2004), and that excess H+ inhibits silica polymerization (Fournier 1985). Nonetheless, the end product, smooth cyst-like features that frequently coalesce between adjacent cyanidiacean cells (Asada & Tazaki 2001), is identical to our observations from the Beowulf material (Fig. 1 C-D).

Brown mats

The HFO mat contained abundant diatoms that occur as solitary epipelic forms. All observed specimens are biraphid taxa, hence capable of motile displacement within the mat. Several spec- imens preserved intact chloroplasts when viewed in LM. SEM revealed diatom surfaces both completely free of secondary silica precipitates and with clean areolae (Fig. 1E & F), as well as thoroughly Si-encrusted specimens (Fig. 1G & H). In the latter instances, inorganic Si precipita- tion has formed a continuous botryoidal coating, comprised of coalescent individual papillae of approximately 100 nm diameter. We hypothesize that living diatoms have cleaner valve surfaces than dead ones, and perhaps that inorganic silica precipitation may at times overwhelm living cells. Inskeep et al. (2004) have provided a complete characterization of the brown mat, revealing a variety of SiOl crystalline polymorphs and microbial communities encrusted with As-rich HFO. It is unclear to us at this time what aspect of the HFO mat contributes to the success of the diatoms living there, in sharp contrast to their absence in the green mat. Given that Cyanidiaceae are virtually absent in the HFO mats, it is also possible that resource competition plays a role in delimiting these algae.

Floristic description

The epipelic diatom community in Beowulf Spring consisted of five species (Table 1). The dominant diatom taxon was Nitzschia cf. thermalis var. minor Hilse (56 %), followed in de- creasing order of relative abundance by Pinnularia acoricola Hustedt (21 %), Eunotia exigua (de Brebisson ex Kiitzing) Rabenhorst (1 5 %), Nitzschia ovalis Arnott (7 %) and Encyonema rnintitum (Hilse ex Rabenhorst) D.G. Mann (<1 %). We discuss the four dominant diatoms further below, and provide LM and SEM micrographs of each. We do not consider E. nlintltllm further because its abundance is insufficient to ascertain with confidence that it was truly liv- ing in the spring.

76 William 0. Hobbs et al

Nitzschia cf. therttialis var. minor (Figs. I E , 2F-H & 3A-E, Table 2)

Length: 15-28 pm Width: 2 4 pm Striae / 10 pm: 23-27

Fibulae 1 10 pm: 8-1 1 We have identified specimens from the Beowulf Spring that appear to conform with N. ther-r?ialis var. nlinor Hilsc according to sketches by Grunow in Van Heurck (1 880-1 885; Plate 59, Fig. 22)

of the isotype material collected by Rabenhorst (no. 1266). Lange-Bertalot ( 1 978) considered Nitzschia thernzalis var. ininor senstc Hustedt (Simonsen 1987, Plate 346, Fig. 9-1 1) to actually represent a small specimen of N. ~rnihonata (Ehrenberg) Lange-Bertalot. He then proposed a new

taxon, N. honihurgiensis Lange-Bertalot, for the N. ther.rna1i.s complex, based on fibula morphol- ogy and the central constriction of the cell margin. N. urnbonatu was combined from Ehrenberg's basionym Navic~rla zrmbonata and is a synonym for Nitzschia thermalis sensu Grunow (Lange- Bertalot 1978). Of the two species N. ~~mbona ta and N. homhurgiensis, the Beowulf specimens more closely conform to the latter (Table 2). Both our specimens and N. homh~lrgieiisis are lin-

ear with apiculate ends, transapical striae composed of fine areolae, and a constricted central

Fig. 2. Light micrographs of Beowulf Spring diatoms from cleaned san~ples (1000x magnification). A-E:

Eunotia e-~igl~a. (A-D) Valve views showing variable morphology. (E) Girdle view showing terminal raphe nodules at the apices (arrows). F-H: Ni~zschia cf. /her-malis var. minor: (F-G) Valve \liew; central constl-ic- tion of valve margin and interruption of raphe and fibulae; fine striae (H) girdle view. I-J: Nitz.vckin ovulis valve view; dense fibulae and striae not resolvable under LM. K-M: Pinnz~loria ocoricola ( K - L ) valve view. (M) girdle view. Scale bar is 10 ym.

L I

I Epipelic diatoms from an extreme acid environment: Beowulf Spring 77

I Table 2. A comparison of the morphology, ultrastructure and ecology of Nitzschia cf. thennalis var. minor from Beowulf Spring, Yellowstone National Park, U.S.A. and similar species.

Nitzschia cf. ther- N hombergiensis N. umbonata N steynii ' N rimosa

malis var. minor a

Length (pm) 15-28 32-52 22- 125 27-54.5 60

I Width (pm) 2-4 5-6 6-9 4-5 7 I

1 Striae / 1 Opm 23-27 3 4 4 0 24-30 24-26 25-28

1 Fibulae / l o p ) 8-10 9-15 7-10 1&12 7-9

Fibulae notes 0.5-1 pm thick Irregular & Short, narrow, Prominent Robust on valve margin; widely spaced; regularly spaced; & evenly irregularly spaced can extend to convergence of spaced

centre of valve 2-3 straie I

Central area yes Yes slight variable no constricted

Habitat Thermal springs Circumneutral; un- Thermal springs; Thermal Thermal polluted waters eutrophic waters; springs springs

acid mine drainage

a. Description of specimens from Beowulf Spring, Yellowstone Nat. Park, U.S.A (this paper). b. Lange-Bertalot (1978) and Krammer & Lange-Bertalot (1988) c. Schoeman & Archibald (1988) d. Bourelly & Manguin (1952)

margin (Figs. 3A-E). REM of N homburgiensis and the isotype material of N. thermalis var. minor Hilse reveal that the fibulae are irregular, widely spaced, and can extend to the center of the transapical axis (Plate 39 of Lange-Bertalot 1978). Comparatively, the fibulae from Beowulf specimens range in thickness from 0.5 to 1.0 pm along the apical axis (Fig. 2C) and are confined to the margin of the valve. In addition the raphe of our specimens is more eccentric than that of N. homburgiensis (Fig. 3B). Cassie (1989) illustrated N. umbonata from a shallow thermal pool (Tokannu, New Zealand) with similar fibulae and raphe structures to Beowulf specimens, but associated with significantly larger valves (3645 ym). Morphometric comparisons of N. cf thermalis var. minoi; N. hombergiensis, N. unibonata, N steynii Cholnoky emend. (Schoeman &

Archibald 1988), andN. rimosa Manguin (Bourelly & Manguin 1952) indicate that each of these taxa has a distinctive morphology (Table 2).

Further questions arise in considering the ecologies of these diatoms. Lange-Bertalot (1 978) associates N. homburgiensis with circumneutral, unpolluted freshwaters, including the Arctic, whereas N. umbonata inhabits both polluted localities and moreover, hot springs. From a strictly ecological standpoint, specimens from Beowulf have far closer affinity to Lange-Berlatot's (1978) N. umbonata. Because the combination thermalis var. minor predates all of the poten- tially equivalent taxa (andlor closely-allied forms), we have identified the Beowulf specimens as Nitzschia cf. thermalis var. minor Hilse. At this point, and pending further analysis, we feel that proposing a new name would only further confuse nomenclatural issues within the 'N. thermalis' complex. There appear to be no similar specimens held in the collections of either the Canadian Museum of Nature or the Philadelphia Academy of Natural Sciences (PB. Hamilton & M.B. Edlund pers. comm.).

Nonetheless, it is clear that diatoms of the N. umbonata-thermalis cluster occur in a number of locations in Europe and N. America that have extreme water chemistries. For example, Whitton & Diaz (1981) recorded N. thernialis at acid mine drainage sites with pH between 3 and 4 in the U.K. and Belgium. Stockner (1967) noted the presence of N. thermalis in alkaline hot springs

78 William 0. Hobbs et al.

(>35OC) situated in Mount Rainier, Washington, USA, but not in similar springs from the Upper Geyser Basin, YNP, suggesting that this species is not necessarily acidobiontic. It should be noted that N thermalis is larger and has no central interruption of the fibulae, in contrast to our specimens and N. thermalis var. minor sensu Hilse (Van Heurck 1880-1 885), though both nom- inate and varietal forms seem to occur in similar environments. Weed (1 889) found specimens of Denticula tliermalis Kutz. in hot springs of the Pelican Creek area in YNP, which he may have confused with any number of Nitzschia. From our extensive literature survey, it appears that Nitzschia cf. thermalis var. minor is not widespread in hot springs (Copeland 1936, Stockner 1967, Whitton & Diaz 1981, Villeneuve & Pienitz 1998, DeNicola 2000). This may suggest that N. cf. thermalis var. minor may be restricted to only the hottest springs of high ionic strength and greatest acidity.

Pinnularia acoricola Hustedt (Figs. 2K-M & 3G-I) Length: 1 1-17.5 ym Width: 2.5-3.5 ym Striae/ 10 pm: 13-17 l? acoricola has a lanceolate to slightly oval valve morphology, with broadly rounded apices. The striae are strongly convergent at the poles and radial in the central area. The central area is either rounded or interrupts striae extending to the valve margin. The external distal ends of the raphe are hooked in the opposite direction to the prox~mal ends. DeNicola (2000) discusses the misidentification of I? acoricola as P obscura, and similarities to P chapmanianu. However, neither of these species can be considered synonyms to l? acoricola. Our specimens, examples of which are shown both in LM (Figs. 2K-M) and SEM (Figs. 3G-I), are consistent with detailed descriptions and light micrographs offered by Carter (1972), Krammer & Lange-Bertalot (1986) and Watanabe & Asai (1 995).

I? acoricola has been observed in a number of acidic waters globally (Hustedt 1937-1938, Carter 1972, Hargreaves et al. 1975, Whitton & Diaz 1981, Cassie & Cooper 1989, Watanabe &

Asai 1995, DeNicola 2000, Jordan 2001). From the literature, it seems clear that this species is endemic to highly acidic environments, namely hot springs and acid mine drainage. The pH of waters in which it has been observed ranges from < I to 4, and temperatures up to 50°C. Whitton & Diaz (1981) present one of the few studies which considers the direct influence of dissolved heavy metals on the success of diatoms in acidic waters. The authors use 59 records of P acori- cola in an attempt to elucidate any relationships between pH and Cu and Zn, they present upper limits for this diatom of 10 mg Cu I-' (0.16 mM) and 100 mg Zn I-' (1.53 mM). Comparatively, aqueous concentrations of Cu and Zn in Beowulf Spring are four orders of magnitude lower (Table 1). It is also noted that P acoricola has been documented in close association with Cya-

nidium cakdarizrm elsewhere, in both hot springs (Whitton & Diaz 198 1) and endolithic habitats (Hernandez-Chavarria & Sittenfeld 2006).

Fig. 3. SEM photographs of diatoms from cleaned samples of Reowulf Spring. A-E: Nitzsclzia cf. rhernii~iis v. tninot: ( B ) Shows the external view of the central nodule (cn) with the eccentric raphe. (C) An internal view showing the fibulae on the valve margin with the interior of central nodule (cn) visible. (D) Apiculate distal end of the valve showing fibulae and distal raphe end terminating in a helictoglossa (h). (E) Apiculate end with eccentric raphe F: Nitzschia ovali.~ cell. G-1: Pinnularia acoricola. (G) Hypovalve view showing the strongly convergent striae at the poles and radial pattern in the central area. (H) Girdle view (I) distal raphe end deflected; striae appear as groups of individual puncta in costa-like areas. J-L: Ellnotin c~iguo. ( J ) Valve view with the curved raphe terminus on the valve face. (K) Raphe structure on the valve mantle with helictoglossa evident. (L) Close-up of the internal raphe terminus showing the helictoglossa (h) and rimoportulae (r). White scale bars are 1 pm, except A, G, H, J, & K where it is 10 ym.

Epipelic diatoms from an extreme acid environment: Beowulf Spring 79

80 William 0. Hobbs et al.

Eirnotia exigua (de Brebisson ex. Kiitzing) Rabenhorst 1864 (Figs. 2A-E & 3J-L) Length: 8-3 1 pm Width: 3-5 pm Striae / I0 pm: 16-25 Valves are crescent-shaped and isopolar, with ends capitate to rostrate, the ventral margin is slightly concave. Striae are punctate and transapical. Raphe is short, located on the ventral mar- gin of the mantle, and occasionally migrate to the valve face distally with a curved terminal fis- sure (Fig. 3J), where it terminates internally in a helictoglossa (Fig. 3L). A single rimoportula occurs at one pole of each valve. There is considerable variability in the morphology of Erirzotia exigua, which has led to the reference of the species as a complex or 'E. exigzra sensu lato' (Kramrner & Lange-Bertalot 1991, DeNicola 2000).

Etrnotia exigua is truly a cosmopolitan species, present in lakes, ponds, bogs and hot springs from tropical to arctic regions (Scherer 1981, Whitton & Diaz 198 1, Round 199 1, DeNicola 2000, Joynt & Wolfe 2001). E. exigua is an acidobiontic taxa occurring almost exclusively in acidic environments, and having a pH optimum <5 (Charles 1985). Whitton & Diaz (1981) ob- served a pH range of 2-4 in 44 samples from hot springs and acid mine drainage. Furthermore, concentrations of Cu and Zn >I00 mg 1-I did not impede the diatom's viability.

Nitzschia ovalis Arnott (Fjgs 21-J & 3F) Length: 12-20.5 pm Width: 3.5-5 pm Fibulae / 10 pm: 12-14 Valves are elliptical with rounded apices. Striae are fine and generally unresolvable in light mi- croscopy (Figs. 21-3) and require SEM to reveal their morphology (Fig. 3F). There is no central area or nodule, and fibulae are regularly spaced on the valve margin with no central interruption. The raphe structure is peripheral and not visible in LM. Therc is potential for misidentification of N. ovalis as N. communis Rabenhorst, and vice versa (DeNicola 2000). The morphology of N commzrnis is slightly narrower, and striae are more evident under LM. Nitzschic~ ovalis also shares features with N. ptuifla Grunow emcnd. Lange-Bertalot and N. aurar.iae Cholnoky, both of which are narrower and, in the case of N. pusilla, exhibits slightly rostrate apices (Krammer & Lange-Bertalot 1988, DeNicola 2000). The Beowulf specimens of N. ovalis are consistent with both type material in Van Heurck (1880-1885) and European specimens described and il- lustrated by Krammer & Lange-Bertalot (1988).

Very few records of N. ovalis exist for freshwater environments despite its original description as a continental species. However, there are a number of marine accounts for this diatom, and it has been referred to as an estuarine benthic taxon (Saks 1982). Laboratory work showed that optimal growth occurred in waters of high ionic strength (i.e., 0.03 M) and the cultures tolerated temperatures up to 36OC (Saks 1982). Its presence in Beowulf Spring most likely relates to the high ionic strength of the water, coupled to broad tolerances with respect to pH. Other reported occurrences of N. ovalis in freshwaters seem to be restricted to acid mine drainage in the U.K. (Hargreaves et al. 1975).

Conclusions

The extreme conditions of Beowulf Spring reduce algal diversity to taxa with tolerance of low pH, high ionic strength, and elevated concentrations of aqueous metals that are inhibitory or toxic to other groups. Clear zonation between cyanidiacean (green) and diatom (brown) mats within the Beowulf Spring show that the habitat is further subdivided spatially (Fig. 1). The pos- sibility of competitive exclusion between the habitats occupied by either group cannot be dis- missed, all the more as both groups appear to metabolize silica actively. Epipelic diatoms from

Epipelic diatoms from a n extreme acid environment: Beowulf Spring 8 1

the brown mats in Beowulf Spring, and their close affinities with other hot-spnng assemblages, raise several interesting questions concerning the ecophysiology and biogeography o f diatoms. I t appears that the algae in the Beowulf system are not merely surviving extreme environmental

conditions beyond the tolerances o f other forms, but potentially deriving benefits from certain

aspects of this habltat. Low diatom diversity and hence reduced inter-specific competition, cou-

pled to the absence o f predators, are obvious factors that may contribute to the success of this

community. The potential metabolism of toxic metals by these diatoms must also be considered, and this merits further investigation. The apparent active silica metabolism o f Cyanidiaceae may

provide considerable phylogenetic insight, given the relationship between diatoms and red algae

(Keellng et al. 2004). In terms of biogeography, two o f the diatoms, Pinnula~.ia crco~.icolcr and Nitzschia cf. ther-nru1i.s var. nzinor-, appear almost exclusively associated with hot-springs. where-

as another, N. ovcrlis, is halophilous, and a fourth, Ezrrzoticr e.~iglru, is an acidobiont that inhabits

acid-mine drainages and naturally-acidic environments with equal facility. It remains a mystery

how such disjunct associations have emerged. Together, our observations pro\lide strong incen- tives to more thoroughly catalog the diatom floras o f Y N P springs.

Acknowledgments

We thank the National Park Service (Department o f the Interior) for permission to conduct rc-

search in Yellowstone National Park. We also thank Stefan Lalonde for sampling and laboratory support. Drs. Paul Hamilton and Mark Edlund graciously confirmed the lack o f Nitzschia cf.

tkernlalis var. nrinor- in the herbariutns o f the Canadian Museum of Nature and the Philadelphia

Academy o f Natural Sciences, respectively. Research was supported by the Natural Sciences and

Engineering Research Council o f Canada (NSERC) and the National Science Foundation (NSF

Microbial Observatory). This paper is dedicated to Dr. Eugene Stoermer for his enormous con- tribution to diatom research.

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