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The two chief types of plants occurring in the marine environment are the algae and sea grasses. These are capable of synthesizing the complex organic substances from the simple inorganic compounds present in sea water. Sea grasses are the seed producing plants which fall into the botanical division of Spermatophyta, whereas marine algae are the primitive group of plants with no true roots, stems and leaves as observed in higher plants and these come under the division Thallophyta. This division also includes certain fungi and bacteria, the latter especially forming an important group in the organic productivity of the sea. Among the algae in the marine habitats, the microscopic and free floating or swimming forms are known as Phytoplankton.The other macroscopic or attached ones, which often grow in the intertidal and subtidal environments, are commonly referred to as Seaweeds.

Most of these marine algae or seaweeds are beautifully coloured and attached to rocks or grow on other plants as epiphytes. A few of them are buried inside the sand and sometimes occur as loose lying communities. Depending upon the type of pigment present in them and other morphological and anatomical characters, macroscopic algae are subdivided into the following four classes: 1.Green Algae (Chlorophyceae) 2. Brown Algae (Phaeophyceae) 3. Red Algae (Rhodophyceae) 4. Blue-green Algae (Myxophyceae) Algae belonging to the first three classes are treated here in detail since they constitute the vast majority of seaweeds of economic importance. Among the blue-green algae, some freshwater species of Nostoc are used as food gelly and many others are capable of fixing nitrogen and maintaining the fertility of the soil. Some information available on the marine flowering plants or sea grasses has also been included in this bulletin.

The pretreatment of seaweed by washing in freshwater is often used in seaweed biofuel research studies. However, the effect of washing seaweed prior to anaerobic digestion (AD) does not appear to have been greatly studied. This study examines washing Sargassum muticum with freshwater and its effect on ultimate and proximate analyses, salt content, methane production from anaerobic digestion, and leachate loss from ensiling. Washing with freshwater significantly (p < 0.01) increased moisture content (unwashed 85.6%, washed 89.1%) but significantly (p < 0.05) reduced ash (unwashed 32.7% dry weight dw, washed 30.6% dw) and salt content (unwashed ash containing 51.5%, washed 42.5%). The dry biomass higher heating value was significantly (p < 0.05) increased by washing due to the lower ash content (11.5 to 12.6 kJ g⁻¹ dw). There was no significant change in the protein or lipid content, although washing increased the nitrogen content (3.85–4.77% dw). Washing significantly (p < 0.05) increased leachate losses during ensiling, with total leachate losses increasing after washing (12.7–25.2%). The methane yield from anaerobic digestion (28 days) was not statistically significantly different (p > 0.05) between unwashed (0.225 L CH4 g⁻¹ VS) and washed samples (0.177 L CH4 g^-1 VS). However, washing delayed biomethane production.

Aquaculture has been the subject of two recent high profile reports. The first, entitled Blue Frontiers, begins by asserting ‘There is a pressing need to elevate the debate on the future of aquaculture and to place this in the context of other animal food production systems, including wild capture fisheries’. The second report made the front cover of Time Magazine and poses the question ‘Can farming save the last wild food?"

The present investigation indicates that G. edulis can be successfully cultivated on commercial scale in the nearshore areas of Gulf of Mannar during the five months period from November to March when the sea is calm. The shallow waters near CMFRI fish farm in Palk Bay are not suitable for G. edulis cultivation as the growth of the plant was affected by various environmental factors mentioned above. The culture experiments of G. edulis conducted earlier by the Central Marine Fisheries Research Institute in 3 - 4 m depth area at Palk Bay near CMFRI fish farm showed good growth of plants as there was less sedimentation, fouling organisms and predators. Hence G. edulis could be cultivated in deep waters in Palk Bay side and attempts may also be made to culture G. edulis in shallow waters at other areas of Palk Bay in order to select the suitable culture sites and period of good growth.