INTRODUCTION

Diatoms (Greek = "cut in half") are the major group of unicellular, photosynthetic and eukaryotic algae.  They constitute the most speciose group of organisms (worldwide distribution ~ 200,000 species, Bentley et al., 2005) and are found inhabiting a range of habitats from oceans to freshwater systems like rivers, lakes and ponds (Armbrust et al., 2004). Importance of these unique intricate cell patterned organisms, since then has increased manifold in areas of taxonomy, ecology, biomonitoring, biotechnology, etc combining microscopic observation with in situ culturing. It has taken a long time to recognize the significance of the ubiquity of the microscopic life, revealed by Robert Hooke through his compound microscope, despite of the reliance on microorganisms (Ash et al., 2002). Microscope since time immemorial has been used to understand many biological functions in prokaryotes and eukaryotes. Among all the organisms, study of diatoms was started off with microscopic observations i.e., taxonomy (Müller, 1786). Diatom taxonomy is based either on the identification of ribosomal sequences (Medlin et al., 1996) or more classically on the morphology and the shape of frustules, the extracellular silica cell walls (Karthick et al., 2010).  Culturing of diatoms is followed in morphometry and phylogeny (Mann, et al., 2008) and to understand the teratological structures in diatoms (Falasco et al., 2009, Håkansson and Chepurnov, 1999) by herbicidal effects (Debenest et al., 2008), etc., which can be applied in biomonitoring practices (Debenest et al., 2009). Toxicological studies for metal contamination and bioaccumulation of trace metals is also done for biomonitoring applications (Wang and Dei, 2001; Price and Morel, 1990).The community structure (deJong and Admiraal, 1984, Debenest et al., 2009) of diatoms could be understood to unravel  ecological intricacies by culturing them in an artificial media, which mimic the natural condition of diatoms.

Culturing got impetus with Cohn (1850) cultivating unicellular flagellate Haematococcus (Chlorophyceae) in situ. However, these attempts had setback due to the absence of suitable culture media or maintenance (Preisig and Andersen, 2005). Later, Famintzin (1871) cultured algae (Chloroccum infusionum (Schrank) Meneghini and Protococcus viridis Agardh) using a media with a few inorganic salts that was adopted from Knop (1865) used for vascular plants (Preisig and Andersen, 2005).

In situ culturing helps to decipher physiological and biological processes including enzymatic behavior, genetics, etc. affecting growth of an organism in an in vitro environment (except when cultured in outdoor ponds). This requires appropriate culture medium or an agar medium containing essential nutrients (macronutrients, micronutrients, vitamins) and chelator elements, etc., required for the sustained growth of cells.  This is being customized considering the requirement of microorganism (Pelczar et al., 1993).

Culture media can be broadly grouped as marine or freshwater culture media based on the ecology of the diatom species.  Although culturing of algae has a very long history of as old as 1871 (Famintzin 1871), researchers were intrigued with diatom culturing for various reasons. The various fields in which diatom culturing is done to unravel its mystery are illustrated in Figure 1.

Many facets of diatom biology like sexual behavior, chloroplast and protoplast dynamics have been understood with the help of in situ culturing(Edlund and Stoermer, 1991, Mann et al., 1999, Davidovich and Bates, 1998, Chepurnov et al., 2002, Sabbe et al., 2004, Chepurnov et al., 2004).  Various physiological activities (Berland et al., 1973; Lane and Morel, 2000; Reinfelder et al., 2000) and evolution related questions have also been understood by culturing (Armbrust et al., 2004 and Connolly et al., 2006). The concept of bar-coding was introduced to diatom taxonomy (Evans et al., 2007; Kaczmarska et al., 2007) on the premise that the divergence of a small DNA fragment coincides with biological separation of species. This DNA fragment becomes a DNA barcode for species which can be used to flag new species, select optimal taxa for phylogenetic studies, or to signal the geographical extent of divergences in a population (Hajibabaei et al., 2007). DNA bar-coding is used as an initial approach for diverse applications, followed by larger in-depth studies in the respective fields. Different DNA regions within the nuclear, mitochondrial and chloroplast genomes have been considered for testing as a universal DNA barcode for diatoms (Moinz and Kaczmarska, 2009). Culturing helps to isolate the specific diatom and also isolating nuclear, mitochondrial and chloroplast genomes for DNA barcode of a species (Moinz and Kaczmarska, 2009).

Diatoms, in particular, were regarded as useful neutral lipid sources, as liquid fuel precursors, as foods for marine culture of zooplankters (Ahlgren et al., 1990), larval and postlarval shrimp (Chu, 1989), copepods (Bourdier and Amblard, 1989), juvenile oysters (Tsitsa-Tzardis et al., 1993) and as micromachines in nanotechnology (Drum and Gordon, 2003). Many diatoms (Chaetoceros muelleri Schütt, McGinnis et al., 1997; Thalassiosira pseudonana Hasle & Hemidal, Pheodactylum tricornutum Bohlin., Yu et al., 2009; Melosira varians Agardh., Stephanodiscus binderanus (Kütz.) Krieger, Cyclotella meneghiniana Kütz., Sicko-Goad and Andresen, 1991) have been screened through culturing to assess its relevance as prospective biofuel feedstock. Gordon et al., 2005 suggest the need for standardizing and scaling up of diatom in situ culturing to track and prevent diatom malformations associated with culturing. Silica being the component of diatom cell wall, understanding its silicification process through genetic transformation experiments, is essential in the field of diatom nanotechnology.

In the preceding sections, we explain the evolution of the successive marine diatom media, since Miquel (1892-93)’s work. As a result, this deals with primitive to a modernized isolation techniques as it forms a defining step for any species-specific experiments. We then focus on the significance of recipe compositions from 19^th to 21^st century.