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Morphological study of Cyclotella choctawhatcheeanaPrasad (Stephanodiscaceae) from a saline Mexican lake
© Oliva et al; licensee BioMed Central Ltd. 2008
Received: 12 March 2008
Accepted: 08 December 2008
Published: 08 December 2008
Cyclotella choctawhatcheeana Prasad 1990 is a small centric diatom found in the plankton of water bodies with a wide range of salt concentrations. This paper describes the morphological features of the valve of C. choctawhatcheeana, from Alchichica lake, a hyposaline lake located in Central Mexico, and provides information about their ecology with respect to water chemistry and distribution in the water column along the annual cycle. Alchichica, and their neighbor lake Atexcac, are the only Mexican water bodies where C. choctawhatcheeana has been registered.
Morphological differences were found with respect to the original description. The valves of C. choctawhatcheeana from Alchichica exceeded the diameter (5–12 μm) given for the type material (3.0–9.5 μm), and it does not forms or seldom forms short chains (2–3 cells) in contrast of up to 20 cell chains. Other difference was the presence of irregularly distributed small silica granules around the margin of the external view of the valve, meanwhile in Prasad's diagnosis a ring of siliceous granules is present near the valve margin; all other features were within the range of variation of the species. Maximum densities (up to 3877 cells ml-1) of C. choctawhatcheeana were found in Alchichica lake from June to October, along the stratificated period of the lake. Low densities (48 cells ml-1) when the water column was mixed, in January and February. C. choctawhatcheeana of Lake Alchichica was found in an ample depth range from 20 m down to 50 m. Conductivity (K25) ranged between 13.3 and 14.5 mS cm-1 and the pH between 8.8 and 10.0. Water temperature fluctuated between 14.5 and 20°C. Dissolved oxygen ranged from anoxic (non detectable) up to saturation (7 mg l-1).
The morphology of C. choctawhatcheeana from Alchichica corresponded to the original description, with exception of some secondary traits. C. choctawhatcheeana can grow in several different environmental conditions. It can use nutrients along the water column during the mixing period in the lake. But when nutrients are scarce, C. choctawhatcheeana, can be located in very high densities, into a well defined depth layer of the lake, being an important contributor to the depth chlorophyll maximum (DCM). The species seems to be a small size but significant component of the phytoplankton in the saline Mexican lake Alchichica.
Species belonging to genus Cyclotella (Küetzing) Brébisson occur over a wide range of environmental conditions, primarily although freshwater organisms and only eight species (C. caspia, C. choctawhatcheeana, C. cryptica, C. quillensis, C. litoralis, C. meneghiniana, C. striata and C. stylorum) have been found to inhabit saline waters . In recent years the centric diatoms of saline lakes and estuaries have began to receive greater attention, particularly the genus Cyclotella [2, 3]. The taxonomy of Cyclotella is hard to unravel because of the considerable morphological variation among species [4, 5]. Diatoms typically form a significant fraction of the biota in saline lakes . In Lake Alchichica, Puebla Mexico, the diatom assemblage included 10 species out of a total of 19 algae species .
One of them is the centric diatom C. choctawhatcheeana. This species has been previously reported from other inland saline waters [1, 8–12]. So far it covers from Canada (52°19'N) down to Argentina (35°15'S), it also has been found in Africa (20° 30' N), but it was never described from Mexico before (19° 24' N). Information on the presence of C. choctawhatcheeana in low latitude saline waters is scarce, maybe due to tropical inland saline lakes have been less investigated than those in temperate regions. The species is poorly known from Mexico and it has been cited only from two Mexican saline lakes [7, 13, 14], but the morphological description have not been presented.
This paper provides the detailed morphological features of the valve of C. choctawhatcheeana inhabiting the saline waters from crater Lake Alchichica. The detailed (light and scanning electron microscopy) morphological features of the valve and new information about its environmental conditions, abundance and distribution in the water column are provided.
Alchichica is a unique Mexican hyposaline (8.3–9 g l-1; Na-Mg and Cl-HCO3) and alkaline (pH = 8.8–10.0) aquatic system characterized by endemic biota and distinctive features such a tufa towers ring. Among the endemic biota there have been described the atherinid fish Poblana alchichica , the ambystomatid salamander Ambystoma taylorii , the isopod Caecidotea williamsi , and more recently the centric diatom Cyclotella alchichicana .
Sampling and processing of the samples
Sampling took place at mid-day monthly at the central and deepest part of the lake during 2001. In situ profiles of temperature, dissolved oxygen, pH and conductivity (K25) were obtained with a calibrated Hydrolab® DS3/SVR3 multiparameter water-quality data logger and logging system (discrete readings every meter). Ten water samples (depth 2, 5, 10, 15, 20, 25, 30, 40, 50 and 60 m deep) for phytoplankton analysis were obtained with a 6-liter Niskin-type water sampler. Two 500 ml sub-samples from each sampling depth were fixed, one with 4% formaldehyde and the other with Lugol's solution (1%). Phytoplankton were counted in 50 ml settling chamber with a Zeiss inverted microscope D following the Utermöhl method [20, 21]. Valves of C. choctawhatcheeana were counted at a magnification of 806×. Additional material was cleaned through acid oxidation. Aliquots were dried onto cover slips and mounted in Naphrax . Slides were examined by phase-contrast microscopy. Microphotographs were taken with a Nikon Lobophot-2 photomicroscope. For scanning electron microscopy (SEM), cover slips with the dried material were mounted on aluminum stubs and coated with pure silver. We used a JEOL JSM-5200 microscope (working distance 10 mm, accelerating voltage 25 kV). For the description of the valve morphology we followed the terminology in [3, 23, 24].
Morphological traits included for comparison were valve diameter, number and arrangement of the marginal and central fultoportulae (strutted processes), presence of the marginal rimoportula (labiate process), presence of marginal spines, presence of granules, density of striae, and the structure of the central area of the valves.
Description of C. choctawhatcheeanaof the Lake Alchichica
Distribution of C. choctawhatcheeana is wide both latitudinal as well as longitudinal . It is a cosmopolitan species inhabitant of coastal brackish waters and saline lakes. It was first described from brackish-water estuary of the Choctawhatchee Bay, Florida . and after that, from the large estuary of Chesapeake Bay, Maryland an Virginia [10, 25], from the Baltic Sea [26, 27], from the Apalachee Bay, Florida  and recently from a Croatian estuary  and from Brazilian tropical waters [29, 30].
In spite of the species was first discovered inhabiting estuarine waters, there are numerous reports of C. choctawhatcheeana from inland saline lakes of Saskatchewan, Canada (Waldsea Lake, Basin Lake and Deadmoose Lake) , Nevada, USA (Walker Lake and Pyramid Lake) [31–33], California, USA (Salton Sea) , La Pampa, Argentina (Laguna La Amarga) .
There are also reports of fossil material of C. choctawhatcheeana from the North America (Devil's Lake, Medicine Lake, Moon Lake) and North Africa (Adrar Bous, Nigeria)  and San Luis, Argentina (Salinas del Bebedero basin) .
Habitat and environmental notes
C. choctawhatcheeana is able to tolerate water temperatures in the range of 10° to 30°C and wide ranges of salinities . The presence in the Baltic Sea, between 3 and 11 ‰ [26, 27], and the Salton Sea, with a salinity of 40 ‰ . demonstrates that C. choctawhatcheeana is tolerant to wide ranges of salinity fluctuation. Wilson et al.  in an examination of diatom assemblages from 219 saline and freshwater lakes, found a range of salinity tolerance from 5.14 ‰ to 79.80 ‰ for C. choctawhatcheeana. Prasad & Nienow  suggested that salinity in excess of 20 ‰ coupled with temperatures in excess of 25°C might be detrimental to its growth. Recently this species has been found in Apalachee Bay, an oligotrophic bay system in the northeastern Gulf of Mexico , in a karstic estuary of the Zrmanja River, Croatia  and in a tropical coastal lagoon, southeast Brazil [29, 30].
Lake Alchichica environmental characteristics such as its alkaline and saline waters rich in sodium chloride, large amounts of carbonate-bicarbonates, magnesium and sulphates, correspond to the type of habitat described previously for the species .
C. choctawhatcheeana of Lake Alchichica was found in an ample depth range from 20 m down to 50 m. Conductivity (K25) ranged between 13.3 and 14.5 mS cm-1 and the pH between 8.8 and 10.0. Water temperature fluctuated between 14.5 and 20°C. Dissolved oxygen ranged from anoxic (non detectable) up to saturation (7 mg l-1). Alchichica is an oligotrophic lake [7, 35] with low nutrient (N-NH3 between non detectable (n.d.) and 0.98 mg l-1, N-NO2 n.d.-0.007 mg l-1, N-NO3 0.1–1.0 mg l-1, P-PO4 n.d.-0.54 mg l-1) and chlorophyll "a" concentrations (mean < 5 μg l-1).
Morphology of Cyclotella choctawhatcheeana
C. choctawhatcheeana described by Prasad  is often misidentified as C. caspia Grunow [36–39]. Furthermore, Maidana & Romero  stated that C. choctawhatcheeana is related to a group of species widely distributed in saline continental and marine waters named by Hakanson et al.  as the "C. striata complex". Carvalho et al.  found that the species occurring in saline lakes (recent and sub-fossil North American material) were quite different from C. caspia. They studied material closely resembling C. choctawhatcheeana already described . This species had a colliculate external central area and one to several fultoportulae in the central area, whereas C. caspia has a smooth external central area with numerous (13–40) valve-face fultoportulae [27, 8].
C. choctawhatcheeana and C. hakanssoniae are validly described species, however, considered as synonym [3, 26, 27]. The only difference between both taxa is that C. choctawhatcheeana forms chain-like colonies meanwhile C. hakanssoniae is single celled [1, 3, 27].
Comparison of the morphological characteristics of Cyclotella choctawhatcheeana from Alchichica with other authors.
Prasad et al. (1990)
Maidana omero (1995)
Carvalho et al. (1995)
Striae per 5 μm
every 2, 3, 4 costae
(7–14 per valve)
every 3 or 7 costae
(5–14 per valve)
every 3 or 7 costae
every 2, 4 costae
Number of satellite pores of marginal fultoportulae
2, rarely 3
1, rarely 2–3
Number of satellite pores of central fultoportulae
3, occasionally 2
Central area of external view of valve
with tangential undulations and colliculate
with tangential undulations and colliculate
with tangential undulations and colliculate
Silica granules in the marginal area of valve external view
The Alchichica material differed from C. choctawhatcheeana of Prasad's original diagnosis in that in the Lake Alchichica the valves exceeded the diameter (5–12 μm) given for the type material of C. choctawhatcheeana (3.0–9.5 μm), and it does not forms or seldom forms short chains (2–3 cells) in contrast of up to 20 cell chains. The presence of the small silica granules around the margin of the external view of the valve irregularly distributed meanwhile in Prasad's diagnosis a ring of siliceous granules is present near the valve margin; it has been suggested that this differences could be attributed to early stages of speciation, as this widely distributed species could be adapted to local conditions . All other features were within the range of variation described by the authors previously mentioned. The correct identity of this small species is essential, because it could affect the results and conclusions of present and future studies, since it is apparently a very widespread species .
C. choctawhatcheeana inhabits several similar North American water bodies. It has been found in Pyramid and Walker Lakes in Nevada. The ecological traits of both lakes are similar than those of Alchichica: they are hyposaline, alkaline and deep lakes. They are also monomictic lakes where C. choctawhatcheena and the filamentous diatom Chaetoceros elmorei are found together. Remarkably, in the three lakes the filamentous cyanobateria Nodularia spumigena is also an important phytoplankton species, developing blooms along the summer season [31, 32].
In Alchichica, during the 2001 year, the higher densities were observed from September to November, when a thermal stratification was present in the lake and nutrient concentration at the epilimnion was very low. In contrast, Oliva et al.  found the higher C. choctawhatcheeana densities in Alchichica Lake along the 1998 year from January to March, during the mixing season. In other saline lakes, for example in the Walker , diatoms usually are dominant along fall and winter, as was observed in Alchichica. It can use nutrients along the water column during the mixing period, but when nutrients in the upper layer are scarce, C. choctawhatcheeana can be located in very high densities into a well defined depth, the metalimnion, where light intensity is low but nutrient concentrations are high. Due to it small size, the contribution of C. choctawhatcheeana to phytoplankton biomass in lake Alchichica is low, but it could be an important food resource for the lake's zooplankton.
The morphology of C. choctawhatcheeana from Alchichica corresponded to the original description, with exception of the size, chains formation and arrange of the silica granules on the valve. C. choctawhatcheeana can grow in different seasons and with high and low nutrient availiability, being an important contributor to the depth chlorophyll maximum (DCM) present in the stratification period. The species seems to be a small size but significant component of the phytoplankton in the saline Mexican lake Alchichica.
The research on which this paper is based on was supported by Dirección General de Asuntos del Personal Académico, UNAM grant 210806-3, and Consejo Nacional de Ciencia y Tecnología grant 41667-T. We thank Laura Peralta and Luis A. Oseguera (FES Iztacala, UNAM) for field assistance, Jaqueline Cañetas (Institute of Physics, UNAM) for her skilful assistance with the SEM and Nora I. Maidana (Facultad de Ciencias Exactas, Buenos Aires) for her observations on the photographic material from C. choctawhatcheeana. This paper greatly benefited from the comments and critical revision of the manuscript of Sarah J. Davies, University of Edinburgh, Sarah E. Metcalfe, University of Edinburgh and, J. Platt Bradbury. We thank Ann Grant for her valuable comments and correction of the language. Finally, we thank the critical comments of two anonymous reviewers that great improved the manuscript.
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