Picophytoplankton community from tropical marine biofilms
Highlights
► Flow cytometric study of picophytoplankton in natural tropical marine biofilms. ► Picophytoplankton contribution to the total biofilm photosynthetic biomass was > 60%. ► Bacteria and picophytoplankton appeared within 5 min on the substrata. ► Picophytoplankton are the pioneer autotrophic members in the tropical marine biofilms.
Introduction
Diatoms and dinoflagellates are more commonly regarded as the dominant phytoplankters in estuaries and lagoons, reaching bloom proportions in regions with requisite bioavailable nutrients and light (Kennish, 1990, Paerl, 1988). Diatoms are also recognized for their vital role in marine biofilms where they form a major part of the biomass and are considered to be the initial autotrophic colonizers in coastal regions (Callow, 2000, Cooksey et al., 1980). Diatom metabolic activities provide the sole carbon and energy source for heterotrophic bacteria in biofilms where both types of organisms are present (Murray et al., 1986). Both these organisms, due to their higher numbers in the ambient waters have a higher surface encountering probability as a result of which they form important components in the initial phases of biofilm formation (Cooksey et al., 1984). Biofilms have at least three clearly defined roles in marine ecology: i) as major sources of primary production on the shore for microphagous herbivores (Hawkins et al., 1992, Sommer, 1999); ii) as primary attachment sites for macroalgal propagules which settle and germinate on it (Steinberg et al., 2002, Wahl, 1989); and iii) as sources of settlement cues for a variety of marine invertebrates and algae and may promote cellular metamorphosis in the process of biofouling (Dobretsov and Qian, 2006). All these aspects highlight the importance of biofilms in the marine environment.
With epifluorescence microscopy and flow cytometry, unicellular picophytoplankton (cells < 3 μm; Chen et al., 2011), which includes cyanobacteria (Prochlorococcus and Synechococcus) and eukaryotic algae (picoeukaryotes) have been recognized as vital components of the phytoplankton community in oceanic (Campbell et al., 1998, Vaulot et al., 1990) and estuarine ecosystems (Ray et al., 1989, Shang et al., 2007). Their contribution to the total photosynthetic biomass is known to be significantly high in the open oceans and also in the coastal regions (Campbell et al., 1998, Morán, 2007). The higher abundance of picophytoplankton increases their surface encountering capabilities as compared to diatoms. Although, picocyanobacteria have also been observed in mats and biofilms in hot springs (Ferris et al., 1996, Miller and Castenholz, 2000, Ward et al., 1998) and hypersaline ponds (Garcia-Pichel et al., 1998), the role of picophytoplankton community in marine biofilms is not well addressed.
In the process of biofilm formation, within minutes of immersing a clean surface in water it adsorbs a molecular ‘conditioning’ film consisting of organic material (Bakker et al., 2003). For some years, it was held that there was an obligatory order in the succession of organisms that followed the adsorption of the conditioning film (Marzalek et al., 1979). Bacteria were considered to be the initial colonizers (Bhosle et al., 1989, Corpe, 1970, Sieburth, 1979), followed by diatoms, other algae, and invertebrate larvae. However, some studies have shown that although the presence of the bacterial film may facilitate the attachment of diatoms, it is not a prerequisite (Cooksey et al., 1980, Horbund and Freiberger, 1970). These initial biofilm organisms are known to produce exopolymers that assist the process of adhesion and movement (Molino and Wetherbee, 2008). So, the observed pattern of biofilm community development in natural systems (bacteria preceding algae) is more likely to be a reflection of the average abundance and the availability of the different colonizing forms at the moment of immersion of a new substratum (Gawne et al., 1998, Wahl, 1989). In view of this, considering the cell size, cell availability and cell abundance, since picophytoplankton are next to bacteria, it was hypothesized that picophytoplankton are the first autotrophic colonizers in biofilms contributing significantly to the biofilm photosynthetic biomass in the initial stages thereby playing an important role in the biofilm food web dynamics and probably in the production of exopolymers similar to bacteria and diatoms (Patil and Anil, 2005c). This investigation was carried out at the Dona Paula Bay where extensive studies on biofilm diatom communities have been conducted (Mitbavkar and Anil, 2007, Mitbavkar and Anil, 2008, Patil and Anil, 2005a, Patil and Anil, 2005b) but reports on picophytoplankton from the biofilms are lacking. The study addressed the following (1) the succession of organisms in the early phases of biofilm formation and (2) their contribution to the photosynthetic biomass so as to better understand the base of the food web in biofilms which support the higher trophic levels.
Section snippets
Environmental parameters
During each of the experiments, water temperature was monitored at the study site. Salinity was measured every day with an autosal. Samples (500 ml) were collected for nutrient (nitrate, phosphate and silicate) analysis.
Experimental protocol
This study was carried out three times, (1) from 2 May 2009 to 10 May 2009, (2) 15 December 2010 to 22 December 2010 and (3) 3 May to 10 May 2011 at the Dona Paula Bay located at the mouth of the Zuari estuary, Goa, (15° 27.5′ N, 73° 48′ E), on the west coast of India. For
Picophytoplankton groups in the water column and biofilms
Three groups of picophytoplankton were observed, Prochlorococcus-like cells (PRO), Synechococcus (SYN) and picoeukaryotes (PEUK) (Fig. 1). PRO-like cells were identified based on their small side scatter (which is a proxy for cell size) as compared to the other groups, red autofluorescence and lack of orange fluorescence. Based on their larger side scatter and red autofluorescence and lack of orange fluorescence, PEUK were identified. SYN was identified based on their orange pigment
Discussion
It is well known that natural biofilms are usually dominated by diatoms (Brandini et al., 2001). However, the occurrence of picophytoplankton which is the most dominant autotrophic form in the marine environment is less studied in biofilms. Except for a report from a tropical estuary of Singapore where contribution of SYN to the periphytic community ranged from 60 to 80% during a five day incubation period (Nayar et al., 2005), there is no other published literature on picophytoplankton in
Acknowledgments
The authors are grateful to Dr. Satish Shetye, Director, National Institute of Oceanography and Dr. A.C. Anil, scientist, for their support and encouragement. This work was carried out with partial support from the Department of Science and Technology, India. The authors thank the reviewers for their useful comments. This is NIO contribution No. 5185 #. [SS]
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