BIO 2106 Lecture 3: Stoneworts and Desmids PDF

BIO 2106 Lecture 3: Stoneworts and Desmids Stoneworts Stoneworts (also known as charophytes) are a family of complex-structured algae that live in a variety of wetland and freshwater or brackish habitats. These include disused aggregate sites, fenland ditches, dykes, bog pools, lakes, ponds, highl...

BIO 2106 Lecture 3: Stoneworts and Desmids Stoneworts Stoneworts (also known as charophytes) are a family of complex-structured algae that live in a variety of wetland and freshwater or brackish habitats. These include disused aggregate sites, fenland ditches, dykes, bog pools, lakes, ponds, highland lochs and even cattle tracks. Stoneworts are submerged species, generally preferring water depths between one and ten metres. The plants derive their common name from the stony external texture that many of the species acquire as a result of encrustation of their outer surface, mainly with calcium carbonate. Conservation status and distribution Earliest fossil records date back 460 million years, and recent DNA evidence has established stoneworts as close to the precursors of all land plants. Today, globally, there are over 400 species, while the UK supports approximately 30 species. However, since the mid-twentieth century their range has decreased significantly and the number of sites with extant populations has declined by over 60 per cent, due to habitat loss and pollution of both surface and groundwaters. Seventeen species are currently listed as Threatened or Endangered in the Red Data Book of British Stoneworts, while 11 species are currently listed as priority species on the UK Biodiversity Action Plan. There are five genera of stoneworts in the UK, living in the following conditions: ​Chara​: base-rich alkaline waters; species often calcium-encrusted. ​Lamprothamnium​: coastal lagoons. Nitella:​ less alkaline, softer waters (pH 6.5 - 7.5), lacking encrustation, and stem without cortex. ​Nitellopsis​: mesotrophic lakes, lacking encrustation, and stem without cortex. Tolypella​: calcareous ditches and canals. Main threats These include: eutrophication; sedimentation; unmanaged succession, particularly of common reed (​Phragmites australis​); avian grazing, particularly by geese which includes nutrient import. Structure and sensitivities Being algae, stoneworts do not have true leaves or roots. Instead of leaves they have branchlets arranged in whorls around the stem, and they possess fine rhizoids that anchor the plant into the sediment. Stoneworts have very large cells (up to 20 cm in length), with each branchlet, rhizoid wall and internode being one cell thick. While nutrient uptake can be through the rhizoids, gaseous exchange and most nutrient uptake are by direct transport from the water through the outer cells of the branchlets and stems. This makes stoneworts exceptionally sensitive to water quality, particularly the presence of metals. They are also very sensitive to competition from filamentous algae, which are often promoted by nutrient enrichment, particularly nitrates and phosphates from agricultural and urban run-off. Stoneworts of the same species exhibit plasticity of morphology (variation in shape and form) according to the habitat and water quality in which they are found. Although they may persist, often as dominants, for many years, stoneworts are pioneer species and will commonly be the first plant community to colonise newly dug ponds, lakes or ditches. As natural succession takes place and sediments age, recent research has shown that the stonewort community structure is likely to change from species that thrive on new, oxidised sediment such as ​Chara aspera to those which have apparently evolved to inhabit deeper, more hypoxic, benthic conditions such as ​Nitellopsis obtusa or ​Nitella spp. Both of these have been shown to persist in lower light and more mesotrophic conditions. Many stoneworts are spiny (hispid) and it is thought (but not proven) that the spines function for protection and as sites of ion uptake from the water. However many are not. The ​Nitella spp. and ​Nitellopsis spp. are ecorticate (lacking cortex), are not ‘hispid’ and do not deposit calcium salts on the branchlets. This suggests not only a morphological difference but also a metabolic difference between the species. Reproduction Sexual reproduction is by oospores, which form when the eggs within the female oogonia are fertilised by the male antherozoids, which are released from the male antheridia. These reproductive organs (gametangia) are typically situated above (female) and below (male) the branchlet arm. In some cases such as in ​Chara connivens​, male and female parts are on separate plants (dioecious) while in others they are on the same plant (monoecious). Reproduction is stimulated by light and temperature increase and generally occurs when waters reach 14º C. Post fertilisation, the oospores darken and germination can take place within 24 hours of deposition in correct conditions. Burial of the oospores in soft deep sediments has been shown to be a key cause of stonewort community decline. However, disturbance of such buried oospores may also instigate the regeneration of a former stonewort community. In several species reproduction can also be vegetative, by the production of propagules known as bulbils that are formed at rhizoid-bearing joints. These can persist in the substrate and germinate when conditions improve. Water quality A common factor for stoneworts is that, although there appears to be differential tolerance between species, as a group they are highly sensitive to water quality. This fact is possibly due to their algal structure and ancestral development. Recent research at the University of East Anglia has shown that stoneworts are particularly sensitive to a range of freshwater pollutants commonly associated with agricultural run-off and urbanisation. They are particularly sensitive to heavy metals such as copper, and to the effects of competition induced by dissolved nitrates and phosphates. Critical water quality limits Recent research at the University of East Anglia has shown that there is differential tolerance to water quality within and between stonewort species. However, based on a ‘presence or absence’ study of 123 water bodies at 49 important historical stonewort sites, the following approximate concentrations were shown to reduce the probability of stonewort establishment and persistence (Lambert 2007): Nitrogen in the form of nitrate at concentrations above 0.5 mgl-1 in open water. Phosphorus as inorganic phosphate at concentrations above 20 µgl-1 in open water. Copper at concentrations above 50 µgl-1 in the sediment pore water or 100 µgl-1 in open water. In addition, the probability of stoneworts occupying a site is dependent on maximum concentrations of cobalt, manganese and silicate as well as minimum concentrations of calcium, chloride, magnesium, sodium and sulphate. Salinity was also investigated, with growth and photosynthetic function being affected by changes in salinity. Saline ingress as a result of managed retreat and rising sea levels may therefore be another limiting factor in stonewort survival. Stoneworts’ important role in aquatic ecosystems Stoneworts are often the earliest colonisers of newly formed water bodies and perform a number of important roles in aquatic ecosystems. These include: Enhancing water clarity by reducing flow rates and hence aiding sediment deposition. Forming dense mats and hence aiding sediment stabilisation via rhizoid linkages. Providing habitat for both anaerobic and aerobic microbes. Providing daytime shelter for phytoplankton-grazing zooplankton. Providing habitat for mollusc and other invertebrate populations. Providing a substrate for fish spawning and shelter for fish fry. Acting as a ‘nutrient sink’ for organic nutrients, because of their high biomass. Conservation methods Proven restoration methods vary according to the type of habitat and may therefore be site-specific. Dykes, gravel pits, shallow lakes, clay pits, quarries or lochs may all require a different method or combination of methods. The principal aims of conservation/restoration are generally to: i. reduce the annual nutrient budget of the site; ii. remove surface fluid sediments built up through eutrophic die-off; iii. remove or reduce competitive vegetation. The following are some examples of successful conservation methods at a variety of sites. Mechanical clearance At Eye Green in Peterborough, a late-succession site, it was found to be more economical to excavate new pits rather than restore the site by mud-pumping or vegetation removal. GLOSSARY OF SOME TERMS Antheridium: globular usually orange male sexual organ, produced at branchlet nodes. Antherozoid: motile male gamete. Benthic: on or in the bed of a body of water. Bractlet: a single-celled process subtending from the oogonium in females plants of dioecious species of Chara,​ replacing antheridium. Branchlet: the laterals of limited growth produced in whorls at the axial stem nodes. Bulbil: energy-storing agglomeration of starch containing cells developed on the rhizoids and stem nodes of some stoneworts. Propagules for vegetative reproduction. Cortex: the outer covering of longitudinally arranged cells in certain species of ​Chara,​ which gives the axes a striped or ridged appearance. Dioecious: the male and female gametangia are produced on separate male and female plants of the same species. Ecorticate: lacking cortical cells around the stem as in ​Nitella.​ Gametangia: male and female sexual organs. Hypoxic: low oxygen content beneath that considered aerobic. Monoecious: the male and female gametangia are produced on the same plant of a species. Multicorticate: multiple rows of cortical cells making up the stem cortex. Oogonium: the ovoid female sexual organ. This is opaque, usually green (when immature) or brown or black (when fertilised), and develops into an oospore covered with spirally arranged cells. Oospore: diploid spore that is the main agent of dispersal and reproduction. Protonema: juvenile form that emerges from the oospore at germination and gives rise to a new charophyte plant. Rhizoid: colourless hair-like filament growing from the plant base into the substrate, with the dual function of absorption and anchorage. Spine-cells: single-celled processes growing out from the node cells of the stem cortex in some ​Chara species. Stipulode: a single or double ring of single-celled processes growing out from the base of the branchlet whorls. Desmids Desmids are ​green algae​, which most botanists consider to be the earliest-evolved plants. One of the ways that we can tell they are closely related to plants is they share the same photosynthetic pigments -- chlorophylls a and b -- and so have the same familiar green glow. Like other plants, they also build cell walls made of cellulose and store starch and chlorophyll inside double membrane-bound chloroplasts. Desmids are dual life forms. Most cells have two pronounced lobes spanned by an isthmus, as you can see above. Their name derives from this feature; "desmos" is Greek for "bond". Inside the bridge is the nucleus, and in each lobe is a single large chloroplast that may have lobes, plates, or other "processes" that radiate outward. A few species are colonial, occurring in chains instead of pairs. Desmids are creatures of freshwater -- like peat bogs or ponds poor in minerals. Another striking feature of these plants is that they are covered in slime. They secrete mucilaginous sheaths through pores in their walls. These gooey coatings host unusual and possibly symbiotic bacteria.