title: Ecophysiology of desiccation/rehydration cycles in mosses and lichens

authors: Green, T.G. Allan

subjects: bryophyte

abstract: Although both lichens and bryophytes are all poikilohydric the groups seem to behave very differently. Bryophytes also show a clear preference for wetter areas and this seems to be a result of the different structures of the organisms. A lichen is algae (or cyanobacteria) suspended in a mycobiont with excess water often having a negative effect on photosynthesis. Bryophytes, in contrast, are true multicellular plants and can construct photosynthetic tissues that can effectively separate their photosynthetic and water storage functions. Under dry atmospheric conditions lichens and bryophytes will desiccate to low water contents and they become dormant. Ability to tolerate desiccation varies considerably both between and within the groups. Somewhat surprisingly, lichens appear to show less ability to tolerate long periods of desiccation than bryophytes, and even some vascular plants. Actual mechanisms of desiccation have been best studied in bryophytes and appear to be constitutive, no protein synthesis is required on rehydration to enable the commencement of metabolism and the necessary protection appears to be always present. Consistently high sucrose levels, for instance are reported from bryophytes. Cellular structure is often maintained when desiccated. Recovery from dryness also differs between the groups with bryophytes generally hydrating more slowly but there are large species differences. In general, rate of recovery may be related to the length of the hydrated activity period, species that hydrate and then dry rapidly, as on rock surfaces, recover rapidly. Species in habitats that remain wet for long periods once hydrated appear to recover more slowly from dryness. In addition to a photosynthetic response to light and temperature, the poikilohydric lichens and bryophytes also have a photosynthetic response to thallus water content. Starting with a dry thallus, addition of water will both increase the thallus water content and also allow photosynthesis and respiration to commence. Both processes increase almost linearly with further hydration at low water contents. Photosynthesis reaches a maximum at an optimal thallus water content (WCopt) that is strongly species dependant. In both groups this photosynthetic optimum represents full cellular turgor. At water contents above this optimum surface or external water can interfere with carbon dioxide uptake and can severely limit photosynthetic rates, especially in lichens. When thallus water contents are normalised to WCopt = 1, then the net photosynthesis (NP) response curves at water contents below WCopt are very similar for liverworts, mosses and higher plants, suggesting a common mechanism in controlling NP. It is suggested that this might be an inhibitor acting on Rubisco activity. In contrast to vascular plants both groups can carry out photosynthesis at lower, suboptimal thallus water contents and very low water potentials but the contribution that this makes to total carbon budget appears to be a major difference between the groups. Bryophytes seem to pass rapidly through this water content range when both drying and hydrating for tens of minutes are often enough. In contrast, it is now apparent that lichens are often active at low thallus water contents. They can not only hydrate from humid air alone, or from dew and fog, but can use these water sources very effectively, often achieving a major part of their annual carbon gain. Information on when the lichens and bryophytes are actually active is only recently starting to appear but, again, the groups seem to differ. Bryophytes strongly prefer wetter habitats and can be active and fully hydrated for long periods and seem to have excellent capacity to tolerate high light and UV radiation when wet. In contrast many lichens, in particular those with green algal symbionts, rarely seem to be hydrated for long periods, especially in high light conditions, and rapidly dry out. Lichens seem to be active mainly under suboptimal conditions one of which is suboptimal water content.

text: Ecophysiology of desiccation/rehydration cycles in mosses and lichens Although both lichens and bryophytes are all poikilohydric the groups seem to behave very differently. Bryophytes also show a clear preference for wetter areas and this seems to be a result of the different structures of the organisms. A lichen is algae (or cyanobacteria) suspended in a mycobiont with excess water often having a negative effect on photosynthesis. Bryophytes, in contrast, are true multicellular plants and can construct photosynthetic tissues that can effectively separate their photosynthetic and water storage functions. Under dry atmospheric conditions lichens and bryophytes will desiccate to low water contents and they become dormant. Ability to tolerate desiccation varies considerably both between and within the groups. Somewhat surprisingly, lichens appear to show less ability to tolerate long periods of desiccation than bryophytes, and even some vascular plants. Actual mechanisms of desiccation have been best studied in bryophytes and appear to be constitutive, no protein synthesis is required on rehydration to enable the commencement of metabolism and the necessary protection appears to be always present. Consistently high sucrose levels, for instance are reported from bryophytes. Cellular structure is often maintained when desiccated. Recovery from dryness also differs between the groups with bryophytes generally hydrating more slowly but there are large species differences. In general, rate of recovery may be related to the length of the hydrated activity period, species that hydrate and then dry rapidly, as on rock surfaces, recover rapidly. Species in habitats that remain wet for long periods once hydrated appear to recover more slowly from dryness. In addition to a photosynthetic response to light and temperature, the poikilohydric lichens and bryophytes also have a photosynthetic response to thallus water content. Starting with a dry thallus, addition of water will both increase the thallus water content and also allow photosynthesis and respiration to commence. Both processes increase almost linearly with further hydration at low water contents. Photosynthesis reaches a maximum at an optimal thallus water content (WCopt) that is strongly species dependant. In both groups this photosynthetic optimum represents full cellular turgor. At water contents above this optimum surface or external water can interfere with carbon dioxide uptake and can severely limit photosynthetic rates, especially in lichens. When thallus water contents are normalised to WCopt = 1, then the net photosynthesis (NP) response curves at water contents below WCopt are very similar for liverworts, mosses and higher plants, suggesting a common mechanism in controlling NP. It is suggested that this might be an inhibitor acting on Rubisco activity. In contrast to vascular plants both groups can carry out photosynthesis at lower, suboptimal thallus water contents and very low water potentials but the contribution that this makes to total carbon budget appears to be a major difference between the groups. Bryophytes seem to pass rapidly through this water content range when both drying and hydrating for tens of minutes are often enough. In contrast, it is now apparent that lichens are often active at low thallus water contents. They can not only hydrate from humid air alone, or from dew and fog, but can use these water sources very effectively, often achieving a major part of their annual carbon gain. Information on when the lichens and bryophytes are actually active is only recently starting to appear but, again, the groups seem to differ. Bryophytes strongly prefer wetter habitats and can be active and fully hydrated for long periods and seem to have excellent capacity to tolerate high light and UV radiation when wet. In contrast many lichens, in particular those with green algal symbionts, rarely seem to be hydrated for long periods, especially in high light conditions, and rapidly dry out. Lichens seem to be active mainly under suboptimal conditions one of which is suboptimal water content.

year: 2011

title: A Sedimentological and Micropaleontological Study of Quaternary Sections of Cores from the New Zealand Sector of the Southern Ocean

authors: Mackie, Jon

abstract: The Southern Ocean lies between the Subtropical Front (STF) at about 45⁰S latitude and the continental landmass of Antarctica. It plays an important role in global climatic and oceanic circulation patterns, and is responsible for production of most water characteristics below the thermocline. The New Zealand sector of the Southern Ocean (NZSSO) is situated in a unique position to study past changes in oceanic circulation and climate from the record preserved in deep-sea sediment cores. Using piston cores collected from water depths of 461-5100 m by USNS Eltanin during 1968-1972, a sedimentological, micropaleontological, and stable oxygen and carbon isotope study, including some radiocarbon dating, has been conducted on the upper 1 m of sediment in 12 cores spanning a latitudinal transect from 52°S to 61°S south of New Zealand. The transect crosses the modem position of the Subantarctic Front (SAF) at ∼56°S and extends as far as the Antarctic Polar Front (AAPF) at ∼63°S.
The study has shown the NZSSO cores must be viewed as two subgroups. The cores are divided at the SAF into a northern subgroup (north of the SAF) and a southern subgroup (south of the SAF) on the basis of sedimentary lithologies, textures, composition and depositional histories. The northern cores are located in the vicinity of the southern margin of Campbell Plateau and are all highly calcareous ( ∼95% ), being composed of planktic and benthic foraminifera and nannofossils with minimal terrigenous material content. The northern cores include two textural facies controlled by the dominant biogenic sediment. Facies A, a muddy sand, is dominated by the Subantarctic foraminiferal assemblage of Globigerina bulloides, Neogloboquadrina pachyderma (sinstral and dextral), Globigerinita spp., Globorotalia injlata and Globorotalia truncatulinoides. Facies B, a sandy mud, is dominated by calcareous nannofossils, especially Emiliania huxley, Coccolithus spp., and Calcidiscus spp.
The southern subgroup of cores lies in the Southwest Pacific Basin and are dominated by biogenic siliceous material and common (11-27%) terrigenous material. They belong to Facies C, a sandy silt, composed mainly of silt-sized frustules of diatomsand nassellaria radiolaria and terrigenous material (quartz, plagioclase feldspars, biotite and muscovite). Where calcareous material exists south of the SAF it is composed of an Antarctic foraminiferal assemblage involving Neogloboquadrina pachyderma (sinstral),Globigerina bulloides and Globorotalia injlata. All foraminifera of the Antarctic assemblage show evidence of dissolution. The northern subgroup of cores lies in an average water depth of 1200 m, while south of the SAF the average water depth of the cores is 4700 m. The lysocline for the NZSSO is determined at ∼3200-4100 m, with a carbonate compensation depth (CCD) of 4800 m. Any calcareous material south of the SAF is actively being affected by dissolution which biases the sedimentary record of the southern subgroup of cores to dissolution resistant organisms (diatoms and radiolaria) and terrigenous material.
The stable isotope records for the northern NZSSO cores show only a 0.8% shift between inferred glacial and interglacial conditions. Smoothing of the record may be due to bioturbation and/or to extensive sediment reworking by bottom currents, forming rather condensed sedimentary records, especially during glacial periods. Oxygen isotope stages back to stage 5 have been suggested in one core (36-42); this places the Last Glacial-Holocene transition at ∼7 cm, stage 2/3 boundary at ∼21 cm, stage 3/4 boundary at ∼65cm, and a possible stage 4/5 boundary at ∼95 cm downcore. The position of the Last Glacial and stage 2/3 boundary are confirmed by radiometric dates obtained from the same northern core: 19,120 +/- 110 yrs BP at 10 cm downcore and 23,900 yrs +/- 200 yrs BP at 20 cm downcore. Faunal changes in the benthic foraminifera occur near the glacialinterglacial transitions from Globocassidulina subglobosa-dominated during glacial periods to common Epistominella exigua during interglacials. Textural facies also show changes near these transitions, with Facies A characterising glacial periods and Facies B during interglacial periods. Associated with changes in texture are colour changes which allow the correlation of the glacial-interglacial transitions across many of the northern cores. The Last Glacial situation cannot be determined uniquely for the southern subgroup of cores as they have, to varying degrees, experienced extensive erosion and reworking during the Late Quaternary or earlier.
Collectively, the data suggest there has not been significant change in oceanic paleoenvironmental conditions north of the SAF, immediately south of New Zealand, between glacial and interglacial periods. This could be because the topographic high marking the southern edge of the Campbell Plateau has essentially fixed the SAF, irrespective of oceanic climatic conditions. The topographic lock would be further enhanced by sea level drops of ∼120 m during glacial periods. In contrast, east and west of New Zealand in open water conditions evidence from other studies indicates major northward shifts in the position of both the SAF and the AAPF. The limited variation downcore in the abundances of sinistrally coiled Neogloboquadrina pachydenna, the small shifts in the stable oxygen isotope record, and the lack of change in faunal assemblages together suggest that ranges in the sea surface temperature change between glacial and interglacial periods at higher latitudes were probably not so severe as at intermediate latitudes. Due to the relatively small temperature changes between glacial and interglacial conditions at higher latitudes the faunal and isotopic indicators may only record extreme events.

text: A Sedimentological and Micropaleontological Study of Quaternary Sections of Cores from the New Zealand Sector of the Southern Ocean The Southern Ocean lies between the Subtropical Front (STF) at about 45⁰S latitude and the continental landmass of Antarctica. It plays an important role in global climatic and oceanic circulation patterns, and is responsible for production of most water characteristics below the thermocline. The New Zealand sector of the Southern Ocean (NZSSO) is situated in a unique position to study past changes in oceanic circulation and climate from the record preserved in deep-sea sediment cores. Using piston cores collected from water depths of 461-5100 m by USNS Eltanin during 1968-1972, a sedimentological, micropaleontological, and stable oxygen and carbon isotope study, including some radiocarbon dating, has been conducted on the upper 1 m of sediment in 12 cores spanning a latitudinal transect from 52°S to 61°S south of New Zealand. The transect crosses the modem position of the Subantarctic Front (SAF) at ∼56°S and extends as far as the Antarctic Polar Front (AAPF) at ∼63°S.
The study has shown the NZSSO cores must be viewed as two subgroups. The cores are divided at the SAF into a northern subgroup (north of the SAF) and a southern subgroup (south of the SAF) on the basis of sedimentary lithologies, textures, composition and depositional histories. The northern cores are located in the vicinity of the southern margin of Campbell Plateau and are all highly calcareous ( ∼95% ), being composed of planktic and benthic foraminifera and nannofossils with minimal terrigenous material content. The northern cores include two textural facies controlled by the dominant biogenic sediment. Facies A, a muddy sand, is dominated by the Subantarctic foraminiferal assemblage of Globigerina bulloides, Neogloboquadrina pachyderma (sinstral and dextral), Globigerinita spp., Globorotalia injlata and Globorotalia truncatulinoides. Facies B, a sandy mud, is dominated by calcareous nannofossils, especially Emiliania huxley, Coccolithus spp., and Calcidiscus spp.
The southern subgroup of cores lies in the Southwest Pacific Basin and are dominated by biogenic siliceous material and common (11-27%) terrigenous material. They belong to Facies C, a sandy silt, composed mainly of silt-sized frustules of diatomsand nassellaria radiolaria and terrigenous material (quartz, plagioclase feldspars, biotite and muscovite). Where calcareous material exists south of the SAF it is composed of an Antarctic foraminiferal assemblage involving Neogloboquadrina pachyderma (sinstral),Globigerina bulloides and Globorotalia injlata. All foraminifera of the Antarctic assemblage show evidence of dissolution. The northern subgroup of cores lies in an average water depth of 1200 m, while south of the SAF the average water depth of the cores is 4700 m. The lysocline for the NZSSO is determined at ∼3200-4100 m, with a carbonate compensation depth (CCD) of 4800 m. Any calcareous material south of the SAF is actively being affected by dissolution which biases the sedimentary record of the southern subgroup of cores to dissolution resistant organisms (diatoms and radiolaria) and terrigenous material.
The stable isotope records for the northern NZSSO cores show only a 0.8% shift between inferred glacial and interglacial conditions. Smoothing of the record may be due to bioturbation and/or to extensive sediment reworking by bottom currents, forming rather condensed sedimentary records, especially during glacial periods. Oxygen isotope stages back to stage 5 have been suggested in one core (36-42); this places the Last Glacial-Holocene transition at ∼7 cm, stage 2/3 boundary at ∼21 cm, stage 3/4 boundary at ∼65cm, and a possible stage 4/5 boundary at ∼95 cm downcore. The position of the Last Glacial and stage 2/3 boundary are confirmed by radiometric dates obtained from the same northern core: 19,120 +/- 110 yrs BP at 10 cm downcore and 23,900 yrs +/- 200 yrs BP at 20 cm downcore. Faunal changes in the benthic foraminifera occur near the glacialinterglacial transitions from Globocassidulina subglobosa-dominated during glacial periods to common Epistominella exigua during interglacials. Textural facies also show changes near these transitions, with Facies A characterising glacial periods and Facies B during interglacial periods. Associated with changes in texture are colour changes which allow the correlation of the glacial-interglacial transitions across many of the northern cores. The Last Glacial situation cannot be determined uniquely for the southern subgroup of cores as they have, to varying degrees, experienced extensive erosion and reworking during the Late Quaternary or earlier.
Collectively, the data suggest there has not been significant change in oceanic paleoenvironmental conditions north of the SAF, immediately south of New Zealand, between glacial and interglacial periods. This could be because the topographic high marking the southern edge of the Campbell Plateau has essentially fixed the SAF, irrespective of oceanic climatic conditions. The topographic lock would be further enhanced by sea level drops of ∼120 m during glacial periods. In contrast, east and west of New Zealand in open water conditions evidence from other studies indicates major northward shifts in the position of both the SAF and the AAPF. The limited variation downcore in the abundances of sinistrally coiled Neogloboquadrina pachydenna, the small shifts in the stable oxygen isotope record, and the lack of change in faunal assemblages together suggest that ranges in the sea surface temperature change between glacial and interglacial periods at higher latitudes were probably not so severe as at intermediate latitudes. Due to the relatively small temperature changes between glacial and interglacial conditions at higher latitudes the faunal and isotopic indicators may only record extreme events.

year: 1998

title: Three Pacific writers from Aotearoa discuss Indigenous languages and visual poetry

authors: Salsano, Marama

abstract: This article draws on several months of conversations, creative workshopping, and writing sessions between Māori writers Ammon Hāwea Apiata and Marama Salsano, and Rotuman writer Mere Taito. Here, we contemplate the presence of Māori and Rotuman languages in our visual poetry. For too long, critical work about English language writing by Indigenous writers from the Pacific has been Eurocentric. Papua New Guinean writer-scholar Steven Winduo suggests the need to unwrite this “imagined Oceania,” while Māori author Keri Hulme writes disparagingly of the lower-cased ‘gods of literature’, and in her ReadNZ lecture, Sāmoan-Māori fantasy writer Lani Wendt Young describes traditional publishing as the “white castle of literature.” While white castled gods of literature have historically ignored the everyday vibrancy of Pacific voices, Indigenous writers from the Pacific continue to unapologetically write, read, experiment, critique, and play with words. Into this complexity, we acknowledge that for many Indigenous writers from the Pacific, English language Eurocentric thought and texts dominate our lives; many of us are second language learners of our languages.

text: Three Pacific writers from Aotearoa discuss Indigenous languages and visual poetry This article draws on several months of conversations, creative workshopping, and writing sessions between Māori writers Ammon Hāwea Apiata and Marama Salsano, and Rotuman writer Mere Taito. Here, we contemplate the presence of Māori and Rotuman languages in our visual poetry. For too long, critical work about English language writing by Indigenous writers from the Pacific has been Eurocentric. Papua New Guinean writer-scholar Steven Winduo suggests the need to unwrite this “imagined Oceania,” while Māori author Keri Hulme writes disparagingly of the lower-cased ‘gods of literature’, and in her ReadNZ lecture, Sāmoan-Māori fantasy writer Lani Wendt Young describes traditional publishing as the “white castle of literature.” While white castled gods of literature have historically ignored the everyday vibrancy of Pacific voices, Indigenous writers from the Pacific continue to unapologetically write, read, experiment, critique, and play with words. Into this complexity, we acknowledge that for many Indigenous writers from the Pacific, English language Eurocentric thought and texts dominate our lives; many of us are second language learners of our languages.

year: 2024-08-24