(a) Enumerate the beneficial and harmful effects of algae. Add a note on their commercial cultivation. 10+5+5=20 (b) Draw the…

(a) Beneficial and harmful effects of algae; commercial cultivation

Beneficial effects

• Primary production and oxygen: marine phytoplankton, including cyanobacteria and diatoms, carry out about half of global photosynthesis. They are the base of aquatic food chains.
• Food and nutraceuticals: nori (Porphyra/Pyropia), kombu (Laminaria/Saccharina) and wakame (Undaria) are eaten, and Spirulina (Arthrospira) and Chlorella are protein and vitamin supplements. Dunaliella salina yields β-carotene, Haematococcus pluvialis yields astaxanthin, and Crypthecodinium yields DHA (an omega-3 fatty acid).

Phycocolloids:

• Agar from Gracilaria and Gelidium, used in microbiology media, food and pharmaceuticals.
• Carrageenan from Kappaphycus and Eucheuma, used in dairy products and toothpaste.
• Alginates from Sargassum, Laminaria and Macrocystis, used in textiles, ice cream and wound dressings.
• Agriculture: nitrogen-fixing cyanobacteria (Anabaena, Nostoc, Aulosira) and Azolla–Anabaena serve as biofertilisers in rice fields. Seaweed sap is used as a biostimulant (for example from Kappaphycus), and seaweeds serve as manure and fodder.
• Industry and energy: diatomaceous earth is used for filtration and insulation. Oil-rich algae (Botryococcus braunii, Nannochloropsis) are candidates for biodiesel, and algae can capture CO₂ from flue gas.
• Environment and research: algae purify sewage in oxidation ponds, act as indicators of water quality, and serve as model organisms (Chlamydomonas, Chlorella in Calvin's work).

Harmful effects

• Blooms and eutrophication: nutrient pollution produces dense blooms that deplete oxygen at night and on decay, causing fish kills and "dead zones".

Toxins:

• Microcystis produces microcystins (liver toxins), and Anabaena and Aphanizomenon produce anatoxins (nerve toxins).
• Among marine dinoflagellates, Alexandrium causes paralytic shellfish poisoning, Karenia brevis causes red tides, and Gambierdiscus causes ciguatera. Some diatoms (Pseudo-nitzschia) cause amnesic shellfish poisoning.
• Water supply: algae cause bad taste and odour (geosmin), clog filters, and foul intakes, pipes and ship hulls (biofouling).
• Plant and animal disease: Cephaleuros virescens causes red rust of tea and mango in India. Prototheca infects humans and cattle.
• Invasions: Caulerpa taxifolia, the "killer alga", has smothered Mediterranean seagrass beds, and Kappaphycus escapes have been reported on corals in the Gulf of Mannar.

Commercial cultivation

Seaweeds: grown on bamboo rafts, single long-line ropes ("monoline") and tube nets in shallow, clean coastal waters, using vegetative fragments or seedlings from hatcheries.

• In India, Kappaphycus alvarezii (for carrageenan) and Gracilaria (for agar) are farmed in Tamil Nadu (Ramanathapuram, Palk Bay), Gujarat and Lakshadweep, often by women's self-help groups.
• A multipurpose seaweed park in Tamil Nadu and support under the PM Matsya Sampada Yojana aim to expand production.
• China, Indonesia and Korea lead world output.

Microalgae:

• Spirulina is grown in open raceway ponds, paddle-wheel mixed and kept alkaline, for example around Madurai in Tamil Nadu.
• High-value strains such as Haematococcus are grown in closed photobioreactors, which are cleaner and more productive but costlier.
• Harvesting is by filtration or centrifugation, followed by drying.
• Challenges: contamination, the cost of harvesting, disease and "ice-ice" in Kappaphycus, climate stress, and the risk of invasiveness.

(b) Life cycles of homosporous and heterosporous pteridophytes

**Homosporous life cycle (Dryopteris, Pteris)**

• The leafy sporophyte (2n) bears sori of sporangia on the underside of its fronds.
• Spore mother cells undergo meiosis, forming one kind of spore (n).
• A spore germinates on moist soil into a free-living, green, heart-shaped prothallus. It bears both antheridia and archegonia, so it is bisexual (monoecious).
• Sperm swim through water to the archegonium. The zygote (2n) grows into an embryo and then a new sporophyte, which soon becomes independent of the prothallus.

**Heterosporous life cycle (Selaginella; also Marsilea, Salvinia, Azolla, Isoetes)**

• The sporophyte bears microsporangia, containing many small microspores, and megasporangia. In Selaginella each megasporangium usually holds four large megaspores, after all but one megaspore mother cell degenerates.
• Gametophytes are endosporic and unisexual. The microspore divides inside its own wall into a much-reduced male gametophyte (a prothallial cell and an antheridium producing biflagellate sperm). The megaspore develops a female gametophyte within the spore wall, often while still in the megasporangium. The wall cracks along the triradiate ridge to expose the archegonia.
• The female gametophyte uses food stored in the megaspore. After fertilisation, which still needs a film of water, the embryo begins development inside the megaspore. In some species of Selaginella the megaspore is retained on the parent plant.

Differences

• Spores: one kind, all alike, versus two kinds differing in size, number and function.
• Gametophyte: exosporic, free-living and photosynthetic, versus endosporic, reduced and dependent on stored food.
• Sex of the gametophyte: bisexual, versus unisexual (male and female separate).
• Growth and cell number: the prothallus grows slowly and has many cells, versus rapid development of a gametophyte with few cells.
• Embryo: it has to establish itself on the soil, versus starting life protected and provisioned inside the megaspore.

Which is more evolved, and why? Heterospory is advanced.

• It evolved independently several times (in Lycopsids, Marsilea and Salvinia, and fossil progymnosperms such as Archaeopteris). It is the essential first step towards the seed habit.

The sequence of change was:

• Reduction in the number of megaspores to one.
• Retention of the megaspore within the megasporangium.
• Development of the female gametophyte in place.
• Enclosure of the megasporangium by integuments.
• This produced the ovule and, after fertilisation, the seed.
• Endosporic, food-stored gametophytes reduce dependence on the external environment and improve embryo survival. Separate sexes also promote cross-fertilisation.
• Fossil "seed-like" structures in Lepidocarpon and Selaginella rupestris (which retains its megaspore) illustrate this trend.

(c) Hutchinson and Dahlgren classifications

John Hutchinson (Kew)

• ***The Families of Flowering Plants*:** dicots in 1926, monocots in 1934, revised in 1959 and 1973 (about 400 families).
• Phylogenetic basis: based on Bessey's ideas and on 24 principles ("dicta") of primitive and advanced characters, using mainly external morphology.

Dicots:

• Divided into Lignosae, fundamentally woody, beginning with the Magnoliales, and Herbaceae, fundamentally herbaceous, beginning with the Ranales.
• Hutchinson considered woody and herbaceous lines to have evolved in parallel from a common ancestor.
• Monocots: derived from the Ranales. They were divided into Calyciferae, Corolliferae and Glumiflorae.

Rolf Dahlgren (Copenhagen)

• Dates: 1975, 1980 and 1983. He died in 1987.
• Structure: angiosperms form class Magnoliopsida, with subclasses Magnoliidae (dicots) and Liliidae (monocots), divided into superorders ending in -florae (e.g., Magnoliiflorae, Rosiflorae, Corniflorae, Liliiflorae), then orders and families.
• Integrated evidence: it combined morphology with embryology, anatomy, pollen, sieve-tube plastids, seed structure and especially micromolecular chemistry: iridoids, glucosinolates (mustard oils), betalains, ellagic acid and benzylisoquinoline alkaloids.
• The Dahlgrenogram: a two-dimensional "bubble diagram" in which superorders are arranged by similarity. The distribution of any character can be mapped onto it.

Comparison

• Evidence: mainly morphology versus many lines of evidence, including chemistry and ultrastructure.
• Primary division: woody versus herbaceous (Hutchinson) versus Magnoliidae and Liliidae, with superorders (Dahlgren).
• Starting point: both begin with Magnoliales-type groups as primitive. Hutchinson derives monocots from the Ranales; Dahlgren places monocots near the Nymphaeales and Magnoliiflorae.
• Presentation: a linear sequence with keys (Hutchinson) versus a two-dimensional phylogenetic diagram (Dahlgren).

Merits and demerits

Hutchinson: merits.

• It gave clear keys and descriptions useful in herbaria.
• It stated evolutionary principles explicitly.
• It recognised many small, natural families.
• It was widely followed in Asian and African floras.

Hutchinson: demerits.

• The woody/herbaceous split is artificial. It separates closely related families: Araliaceae from Apiaceae, and Verbenaceae from Lamiaceae.
• It relies heavily on a single character, habit, and ignores anatomical, chemical and embryological data.
• Too many small orders make the system cumbersome.

Dahlgren: merits.

• Its groupings are more natural, and many are supported by later DNA work. Glucosinolate-producing families grouped in Capparales correspond to APG Brassicales, and iridoid-containing groups correspond largely to the asterids.
• The Dahlgrenogram is an excellent teaching and research tool for character distribution.

Dahlgren: demerits.

• It is complex, with many superorders.
• It lacks keys for identification, so it is less practical for floras and herbaria.
• Some chemical characters are weighted subjectively.
• It remained partly phenetic.

Both have now been superseded by the molecular APG system (APG IV, 2016). APG confirms some of Dahlgren's groupings but rejects Hutchinson's woody–herbaceous division and the idea that monocots arose from the Ranales. Instead, it places the ANA grade and magnoliids before the monocot and eudicot lineages.