Floristics of epiphytic diatoms on Thalassia testudinum from the southern Gulf of Mexico

Florística de diatomeas epífitas de Thalassia testudinum del sur del golfo de México

Daniela López-Mejía¹, David Alfaro Siqueiros-Beltrones^2*, Francisco Omar López-Fuerte³ & Francisco Gutiérrez-Mendieta¹

¹ Universidad Autónoma Metropolitana – Iztapalapa, Laboratorio de ecosistemas costeros.
Av. San Rafael Atlixco 186, Leyes de Reforma 1ra. Secc. 09340 Ciudad de México, CDMX.
² Instituto Politécnico Nacional, Centro Interdisciplinario de Ciencias Marinas, Dpto. Plancton y Ecología Marina.
Av. Instituto Politécnico Nacional S/N, Col. Playa Palo de Santa Rita, La Paz, BCS. CP 23096.
³ Departamento Académico de Ciencias Marinas y Costeras,
Universidad Autónoma de Baja California Sur, Km 5.5, Carretera al Sur, 23091 La Paz, Baja California Sur, México.

Corresponding author: dsiquei@gmail.com

D. López-Mejia, D.A. Siqueiros Beltrones, F.O. López-Fuerte & F. Gutiérrez-Mendieta. 2021. Floristics of epiphytic diatoms on Thalassia testudinum form the southern Gulf of Mexico. Cymbella 7 (1): 18-35. http://cymbella.mx

Introduction.

Benthic diatoms play an important role within the microbial communities that characterize certain habitats (Siqueiros-Beltrones 2002). They are fast colonizers of both non-living surfaces (MacLulich 1986) and live substrata (Sieburth & Thomas 1973) where they may cover a great part of the available surfaces of their hosts, such as macroalgae (Siqueiros-Beltrones et al. 2004; Siqueiros Beltrones et al. 2016) or seagrasses (Siqueiros-Beltrones & Ibarra-Obando 1985).

Although studies on epiphytic diatoms may be considered lacking worldwide, mainly because of the enormous scientific research potential around them, sufficient theoretical basis exists to address and investigate diverse roles of epiphytic diatoms, weather on their taxonomy including floristics, their ecology, biogeography, or eco-physiological interactions with their hosts. However, there is still much to do concerning floristics alone (Siqueiros-Beltrones & Martínez-Hernández 2017).

For the Mexican coasts little research exist and has been done on diatom floristics, most related to epiphytic diatoms from macroalgal hosts. Evidence from these studies suggests that macroalgae provide more than just attaching surfaces for diatoms (Hernández-Almeida & Siqueiros-Beltrones 2008, 2012; Siqueiros-Beltrones & Hernández-Almeida 2006). On the other hand, pioneer research that can be traced to the first floristic study on diatoms found living on blades of sea grass (Zostera marina L.) in NW Mexico shows that seagrasses also provide vast surfaces for colonization by as many as 215 species of diatoms, either free living or colonial forms (Siqueiros-Beltrones & Ibarra-Obando 1985). Elsewhere, in Caribbean waters (Grand Cayman) Corlett and Jones (2007) recorded 61 diatom species on T. testudinum blades, while López-Fuerte et al. (2013) identified 107 taxa and infra-species from Laguna de Yalahau for the Mexican Caribbean. Likewise, along the Atlantic coasts (Florida Bay) Reyes-Vazquez (1970) described 42 species of epiphytic diatoms on T. testudinum, while Sullivan (1979) recorded 32 species. More recently, Frankovich et al. (2006) listed up to 255 diatom taxa (species and infra-species), the highest species richness of epiphytic diatoms ever observed on T. testudinum blades exposing the floristic potential of epiphytic diatoms on this substrate. In contrast, besides the Mastogloia species list published by Siqueiros-Beltrones et al. (2020) no other formal studies on epiphytic diatoms of seagrasses have been carried out hitherto for the Mexican Atlantic, where turtle-grass Thalassia testudinum K. D. Koenig is widely distributed at low depths (Spalding et al. 2003).

The extension and resilience of T. testudinum beds in the southern Gulf of Mexico (GM) offers an excellent opportunity to study the epiphytic diatoms that use them as hosts, also providing a floristic frame for the Mastogloia species that characterize this particular ecosystem (Siqueiros-Beltrones et al. 2020). Thus, our objective was to describe the species composition of epiphytic diatom living on T. testudinum blades from the southern GM off the coast of Campeche, Mexico. Because our approach was mainly floristics, we proposed the hypothesis that the species richness would be higher than in the above studies and would include taxa that are new additions to the current floristic list for Mexican littorals (López-Fuerte & Siqueiros-Beltrones 2016). We must consider that references such as presence/absence of taxa are used to determine biogeographical distributions based on their relations with various environmental factors. In this way the effort dedicated to floristics may help to trim such interpretations down to a more precise dimension. Albeit relative abundances were estimated also to gain a preview of the more or less conspicuous taxa for later ecological and distribution analyses. And as it turned out, a previous report of this study widened the expectations on any unusual characteristic of the epiphytic diatom taxocoenosis, exemplified by the many recorded species of Mastogloia (Siqueiros-Beltrones et al. 2020).

Study area.

The study area is, located within the southern GM and comprises the north coast of the State of Campeche, between the municipalities of Champotón and Calkiní (19° 19’ 43.90’’ and 20° 20’ 23.30’’ N, 90° 47’48. 90’’ and 90° 129’ 43.30’’ W). There, three sites were surveyed: 1) Champotón, located in front of the Champotón river, considered the most important estuary in the state (Villalobos-Zapata & Mendoza-Vega 2010); 2) Petenes, located in the northernmost coast of the state within the Biosphere Reserve Los Petenes (DOF, 24, May 1999) that covers an extension of 282,857 ha; and 3) Costa, located on the coast of the city of San Francisco de Campeche (Fig. 1).

Methods.

Sampling was carried out in 2013 on April 8 - 11 in Petenes, from April 29 - May 2 in Champotón, and from August 17 - 20 in Costa, in a total of 23 sampling sites: six in Champotón, seven in Costa, and 10 in Petenes. Sites were chosen according to the occurrence and extension of T. testudinum beds. In each site three bundles of the seagrass were collected and stored in 1 L plastic jars. At the laboratory all the blades from the bundles were scraped with a spatula to separate the epiphytic diatoms. The gathered material was oxidized in flasks to eliminate all organic matter using nitric acid and commercial alcohol (Siqueiros-Beltrones 2002). The cleaned diatom frustules were mounted on permanent slides using synthetic resin Pleurax (R.I. = 1.7).

The permanent mounts were observed under an Axioscope 40 Zeiss compound microscope with plan-apochromatic optics. Diatoms were identified on the basis of frustule morphology using classic references: Cleve-Euler (1952, 1953a, 1953b, 1955), Hustedt (1930,1959, 1966), Peragallo & Peragallo (1908), Schmidt (1874-1959); as well as more recent literature: Foged (1975,1978,1984), Krammer & Lange-Bertalot (1991), Lange-Bertalot & Krammer (1987), Lange-Bertalot et al. (1996), Lobban et al. (2012), Loir & Novarino (2013), López-Fuerte et al. (2010), López-Fuerte et al. (2013), Metzeltin and García-Rodrígez (2003), Moreno et al. (1996), Novelo et al. (2007), Siqueiros-Beltrones (2002), Siqueiros-Beltrones et al. (2004), Stidolph et al. (2012), Tomas (1997), Witkowski et al. (2000). Taxonomic nomenclature was updated using Guiry & Guiry (2020) and Round et al. (1990).

Quantitatively, taxa were classified according to their relative abundances (N=500) into rare (15<X≤1), frequent (100<X≤15), abundant (300<X≤100), and very abundant (>300). Similarity between the surveyed localities was measured using Jaccard presence/absence index. The resulting floristic list was contrasted with the recent available floristic lists both for the GM and the Mexican coasts overall (Krayesky et al. 2009; Licea et al. 2016; López-Fuerte & Siqueiros-Beltrones 2016; López-Fuerte et al. 2013; Siqueiros-Beltrones & Martínez-Hernández 2017). Images of representative taxa including several new records were captured using a CMOS Konus digital ocular lens microscope at 1000×.

Results.

A total of 238 epiphytic diatom taxa from 66 genera were identified living on the blades of T. testudinum from the northern coast of the State of Campeche (Table 1). The most diverse genus was Mastogloia with 51 species and varieties, with 11 taxa found in a single locality; following were Nitzschia (22taxa), Navicula (17), Amphora (13), Cocconeis (13) and Halamphora (12). Only two taxa on the list were not assigned to a taxon at species level, while 63 other potential species not included in the list remained unidentified, including 25 Mastogloia.

Overall, 19 taxa were not accounted for during the quantitative phase. Diatom species distribution was as follows: 100 taxa (42 %) were present in all three localities. In Champotón, 38 exclusive taxa were recorded. Whilst, in Costa and Petenes, 21 and 14 exclusive taxa were recorded, respectively. The diatom assemblages from Champotón yielded the highest species richness with 176 taxa and 54 genera; there, the most diverse genera were Mastogloia (43), Nitzschia (18) and Navicula (15). In Costa 141 taxa and 41 genera were found: 39 Mastogloia, 15 Nitzschia, 9 Navicula, Cocconeis and Halamphora taxa. In Petenes 136 species were recorded; there, Mastogloia comprised 34 species, Nitzschia 10, Cocconeis 10, and Amphora 4. On the basis of their quantitative classification diatom taxa were distributed in a similar way among the three localities, depicting a typical benthic diatom assemblage with few abundant and very abundant taxa, and many rare and frequent taxa (Table 2).

According to the Jaccard similarity index (presence/absence of taxa), two groups were formed separating the Costa from the Petenes and Champotón assemblages (Figure 2).

New records. Out of the above taxonomic list, 52 taxa are new records for the Mexican coasts and 25 are new records for the GM (Table 2); several are shown in figures 3-6. Three of these taxa were very abundant, particularly Cocconeis lineata, with the higher overall abundance (10.78 %) and Seminavis obtosiuscula (2.19 %); these two are new for the GM, and Mastogloia urveae (1.64 %), recently recorded for the Mexican coasts overall. For any of the three localities, two of the new records were abundant, three for Mexican coasts and three for the GM; 30 taxa were frequent, 21 for Mexican coasts and nine for the GM; and, 41 new records were rare taxa, 28 for Mexican waters and 13 for the GM. Thus, by locality, Champotón showed the higher number of new records (38 taxa), 16 for Mexico and 7 for GM. In Costa nine new records, six overall for Mexico and three for the GM. The Petenes locality had the lower number of new records (seven taxa), six for Mexico and one for GM.

Discussion.

Although the Champotón locality yielded the highest species richness and Petenes the lowest, these were grouped together, segregated from the Costa locality in the similarity analysis. The former two were surveyed during the dry season and the latter in the rainy season, suggesting that species composition of the diatom assemblages varied according to the corresponding seasonal environmental conditions. Besides the import of fresh-water taxa or seasonal exclusion of strict marine forms, the seasonal distribution of other coastal marine taxa requires investigation, along with further taxonomic issues.

It is but surprising that this report on the epiphytic diatoms of Thalassia testudinum is only the fourth formal floristic study on seagrasses (the third was derived from the same project, Siqueiros-Beltrones et al. 2020) that serve as hosts for diatoms in Mexican waters after Siqueiros-Beltrones and Ibarra-Obando (1985) for Zostera marina, a second one for T. testudinum (López-Fuerte et al. 2013). Although the floristic account in this study (238 taxa) is one of the richest for Mexican littorals, it is surpassed by the epipelic assemblages (325 taxa) observed in Bahía Magdalena, BCS, Mexico (López-Fuerte & Siqueiros-Beltrones 2006), and by the epiphytic diatom assemblages from multi-specific macroalgae hosts (271 taxa) surveyed by Hernández-Almeida & Siqueiros-Beltrones (2012).

Albeit, compared to similar studies, the species richness in this study surpasses the 215 taxa recorded for Zostera marina in NW Mexico (Siqueiros-Beltrones & Ibarra-Obando 1985), being very similar to the 232 epipelic diatom taxa recorded at Guerrero Negro lagoon, also in the NW region (Siqueiros-Beltrones et al. 2017). And, although it falls short from the 255 taxa recorded by Frankovich et al. (2006) in the Florida (USA) T. testudinum beds, more than 60 taxa remain unidentified for the Campeche samples. As it is, it more than doubles the 107 taxa recorded recently by López-Fuerte et al. (2013) for the same species host in the Caribbean.

Our ex professo qualitative outlook for diatom species living on T. testudinum blades was influential in supporting part of our hypothesis; the high species richness may well be a combination of this and the tropical habitat, which could be reflected on the numerous species observed of Mastogloia (Siqueiros-Beltrones et al. 2020), although only 51 identified to species level. This agrees with the taxonomic diversity of this genus (>80 taxa) recorded for the Caribbean by Loir and Novarino (2013) albeit in sediments. It would thus seem more related to a biogeographical factor than to substrate affinity, both for this genus and rest of the recorded taxa.

Similar comparisons to the above are now possible based on this floristic account of epiphytic diatoms of T. testudinum for the southern Gulf of Mexico. Besides the biogeographic implications, this taxonomic reference may aid in ecological studies, e.g., trophic relations with various species of grazers such as snails, sea-turtles, and manatees that forage on these seagrass beds. However, much formal taxonomic work is needed on the benthic diatoms of Mexican littorals in order to adequately complement the much-needed floristic studies. This is supported also by the high abundance of six new records.

Notwithstanding, both the species richness and the observed species composition of these epiphytic diatom assemblages may serve as reference in assessing environmental impact as to how undisturbed habitat conditions may reflect on the structure of these diatom assemblages.

Acknowledgements.

This study was supported by project “Indicadores del estado de las comunidades de pastos marinos en la zona costera del Golfo de México: Fase II”, (Margarita E. Gallegos Martinez). It is part of the Master in Biology thesis of D. López Mejía who received a scholarship from CONACYT at Universidad Autónoma Metropolitana – Iztapalapa. The second author (DASB) is fellow of Comisión de Operación y Fomento de Actividades Académicas (COFAA), and Estímulo al Desempeño de los Investigadores (EDI) IPN. We acknowledge the precise observations by three anonymous referees to a former manuscript.

References

Sometido: 3 de abril de 2021
Revisado: 24 de junio de 2021
Corregido: 28 de julio de 2021
Aceptado: 1 de agosto de 2021