Introduction

Benthic diatoms are highly sensitive to hydrogeological and environmental changes in the water body. Diatom community structure generally varies concerning seasons and land-based discharges with associated nutrient fluctuations to the system in the form of run-offs and point-source pollutions. Understanding diatoms and its community structure have been useful in aquatic health assessment and cost-effective eco-system monitoring studies during the last few decades. Diatoms have aided as bioindicators of environmental changes like eutrophication, metal contamination, salinization, acidification, and catchment alterations with land-use changes (Dixit et al., 1992). Seasonal variability at the regional scale and level of anthropogenic inputs influences community structure in lotic eco-systems, especially the estuaries (Saranya et al., 2018). Anthropogenic inputs include land-based discharges that alter nutrient levels of the aquatic eco-systems in its immediate vicinity. Benthic diatoms are sensitive to such impulsive habitat changes in terms of physico-chemical and hydrological parameters. Hence understanding the variations in community structure at different locations with varied anthropogenic stress levels across seasons would give insights on species preference to a particular substratum and its resilience to fluctuating environmental conditions.

Screening based on this criterion would help in the selection of season tolerant strains for biofuel production. In recent decades, there has been a greater convergence towards sustainable bioresource utilization especially for biofuel production given addressing the country’s energy shortages. Dwindling stock of fossil fuels, fluctuating prices in the world oil market, and concurrence among sensible nations to reduce CO₂ levels necessitates focus towards renewable energy. Global biofuel production has risen by 2.6% in 2016, which is higher by 0.4% than 2015’s average (BP World Renewable Energy Statistics report 2016). As per the IEA statistical report (2016), the world has relied up to 4% on bioenergy derived liquid transportation fuels and this Figure is expected to increase by 4.3% in 2020. There are numerous global endeavors, evident from the annual publication of 550 research papers after 2005 (Azadi et al., 2017). These endeavors highlight the interest in attaining energy self-reliance and energy security.

Fossil fuel-dependent electric power generation and petroleum products driven transportation sectors especially in India significantly contribute to the greenhouse gas footprint (Ramachandra and Shwetmala, 2012; Ramachandra and Shwetmala, 2009; Ramachandra and Hegde, 2015). Bioethanol and biodiesel are two primary liquid biofuels that are gaining momentum in the country’s energy scenario. Lignocellulosic biomasses and seaweeds are considered for bioethanol production, while microalgae are considered as significant lipid resources for biodiesel production. The algae constitute third-generation biofuel feedstock. Figure 1 represents the share of different algal species (family-wise) experimented so far for biofuel production. A major share in the exploitation of algae as lipid precursors is from green microalgae than diatoms to produce biofuel. Diatoms belonging to the class Bacillariophyceae have a prominent advantage in terms of its ubiquitous presence and are benthic with substratum attachment characteristics, which would considerably lower the cost associated with microalgae harvesting.

Diatoms have proven biofuel characteristics, evident from prioritizing 40 diatom strains (out of 50 microalgae strains) with higher lipid potential among the total 3000 screened strains by Aquatic Species Program hosted by the US Department of Energy (Sheehan et al., 1998). However, the economic viability of microalgae-based biofuel production systems is a major challenge in commercializing diatoms-based biofuels. Hence it is a rationale to choose a diatom strain or a consortium of strains that are consistently productive during all seasons under fluctuating environmental conditions, then a strain that gives higher productivity only under optimal conditions (Hildebrand et al., 2012). Prioritizing potential strains entails understanding respective strains/species’ ecological preference under varying field conditions.

Figure 1 Share of Different Microalgae as Renewable Energy Feedstock

LITERATURE REVIEWS

Estuaries are transition zones or biologically productive ecotones with a constant confluence of fresh and saline waters (Kennish, 2002). They act as filters/buffer zones that trap natural and anthropogenic pollution causing materials between continents and pelagic environments (Qiu et al., 2010). They are dynamic water bodies characterized by strong gradients of salinity and nutrients due to land-based discharges (Lohrenz et al., 1990). Estuaries experience seasonal variations in irradiance, temperature, nutrients, and freshwater inflow, which influences the microalgal community structure (Seguro et al., 2015). Notable contributions to benthic diatom community structure as a part of aquatic health assessment programs in eco-system monitoring investigations are now available (Bere, 2016; Dalu and Froneman, 2016; Hering et al., 2006; Kelly et al., 1998; Reid et al., 1995; Reid, 2005; Schoeman, 1979; Tan et al., 2017; Townsend and Gell, 2005). Diatoms are highly resilient group of micro-organisms (Ramachandra et al., 2009; Ramachandra et al., 2011) with higher productivity, that dominates the world oceans with approximately 40% of the marine primary productivity and 20% of global carbon fixation (Hildebrand et al., 2012; Leblanc et al., 2018; Levitan et al., 2014). Nearly one-fifth of world’s photosynthesis is carried out by diatoms (Armbrust, 2009; Nelson et al., 1995). Diatoms comprise of the lowest level marine food web and are responsible for export of carbon from surface to deeper oceans (Tréguer et al., 2018). Moreover, Diatoms gain importance at myriad dimensions as nano-fabricators of biogenic silica, progenitor elements of present day’s crude petroleum reserves (Levitan et al., 2014), biofuel precursors, live feed for aquaculture, bio-active compound synthesizers, bio-based silicon semiconductors for solar photovoltaics (Jeffryes et al., 2011; Zhu et al., 2013) eminent source of carotenoids and essential fatty acids, subject of clue in the field of criminology and forensic sciences (Sun et al., 2015). Diatoms not only are the sources of lipids but also has potential scope for value added products due to the presence of high value bio active compounds such as carotenoids, pigments, neuro-excitatory amino acid derivative toxin – domoic acid and Naviculan – a sulphated polysaccharide having antiviral property (Prestegard et al., 2009).

RESEARCH PROBLEM

The study focusses on understanding the habitat wise ecological characteristics and diatom community structure. This not only aids in the selection of promising diatom species but also helps in understanding respective environmental conditions (which helps in the design of sustainable bioreactors). Environmental gradients and seasonal fluxes in water and biological parameters were monitored monthly for a period of one year to understand the influence of physico-chemical parameter and nutrient dynamics on benthic diatom community structure.

This study also intended to capture the seasonal variability with respect to substrata and habitat specificity of benthic diatoms and its relation to community structure. Insights into ecological perspectives would also lower the costs involved in the optimization of growth parameters. Seasons and environmental gradients play a pivotal role in richness, abundance, and community structure of diatoms especially in benthic diatoms - the most predominant group of all micro-organisms in the intertidal zones of the estuary. As benthic diatoms adhere to substrata and form biofilms with the help of exo-cellular polysaccharide secretion, understanding substratum specificity could give additional insights on the type of community structure and the habitat preferences with substratum specificity of sensitive and tolerant species.

AIM OF THE STUDY

The objectives of the present study are to:

1. Understand the effect of seasonal fluctuations of water quality on diatom species composition;
2. Investigate the variations in benthic diatom community structure in terms of tolerance and sensitivity through statistical analyses; and
3. Estimate the biomass and lipid potential of enriched diatom from a particular substratum that had shown the highest species composition during ecological monitoring studies.