title: Increasing Microcystis cell density enhances microcystin synthesis: a mesocosm study

authors: Wood, Susanna A.

subjects: cell abundance

abstract: An experimental protocol using mesocosms was established to study the effect of Microcystis sp. cell abundance on microcystin production. The mesocosms (55 L) were set up in a shallow eutrophic lake and received either no (control), low (to simulate a moderate surface accumulation), or high (to simulate a dense surface scum) concentrations of Microcystis sp. cells collected from the lake water adjacent to the mesocosms. In the low- and high-cell addition mesocosms (2 replicates of each), the initial addition of Microcystis sp. cells doubled the starting cell abundance from 500 000 to 1 000 000 cells mL⁻¹, but there was no detectable effect on microcystin quotas. Two further cell additions were made to the high-cell addition mesocosms after 60 and 120 min, increasing densities to 2 900 000 and 7 000 000 cells mL-1, respectively. Both additions resulted in marked increases in microcystin quotas from 0.1 pg cell-1 to 0.60 and 1.38 pg cell⁻¹, respectively, over the 240 min period. Extracellular microcystins accounted for <12% of the total microcystin load throughout the whole experiment. The results of this study indicate a relationship between Microcystis cell abundance and/or mutually correlated environmental parameters and microcystin synthesis.

text: Increasing Microcystis cell density enhances microcystin synthesis: a mesocosm study An experimental protocol using mesocosms was established to study the effect of Microcystis sp. cell abundance on microcystin production. The mesocosms (55 L) were set up in a shallow eutrophic lake and received either no (control), low (to simulate a moderate surface accumulation), or high (to simulate a dense surface scum) concentrations of Microcystis sp. cells collected from the lake water adjacent to the mesocosms. In the low- and high-cell addition mesocosms (2 replicates of each), the initial addition of Microcystis sp. cells doubled the starting cell abundance from 500 000 to 1 000 000 cells mL⁻¹, but there was no detectable effect on microcystin quotas. Two further cell additions were made to the high-cell addition mesocosms after 60 and 120 min, increasing densities to 2 900 000 and 7 000 000 cells mL-1, respectively. Both additions resulted in marked increases in microcystin quotas from 0.1 pg cell-1 to 0.60 and 1.38 pg cell⁻¹, respectively, over the 240 min period. Extracellular microcystins accounted for <12% of the total microcystin load throughout the whole experiment. The results of this study indicate a relationship between Microcystis cell abundance and/or mutually correlated environmental parameters and microcystin synthesis.

year: 2012

title: Nutritional niche separation in coexisting bog species demonstrated by ¹⁵N-enriched simulated rainfall

authors: Clarkson, Beverley R.

subjects: niche separation

abstract: Empodisma minus and Sporadanthus ferrugineus (both Restionaceae) coexist in New Zealand raised bogs, yet Sporadanthus have significantly more depleted ¹⁵N natural abundance signatures than coexisting Empodisma. Their root systems are spatially separated with Empodisma having a thick surface layer of about 50 mm of cluster roots overlying the deeper Sporadanthus roots. We hypothesized this root displacement allows Empodisma to preferentially access the primary N input from rainfall, thus establishing niche separation, and tested this using tracer stable isotopes. We aerially applied 1.6 mmol m⁻² of ¹⁵N as (NH₄)₂SO₄ chased by deionized water to simulate a rainfall event of 34 L m⁻². Root/peat matrix cores were harvested after 5 h and analysed for ¹⁵N uptake. Approximately 80% of the tracer applied was recovered in the cores, with 90% of this recovered in the upper Empodisma cluster root layer. Seven weeks after application, young shoots of Empodisma were significantly enriched (mean δ15N = +7.21‰; reference = −0.42‰), whereas those of coexisting Sporadanthus were not (mean δ15N = −2.76‰; reference = −4.24‰). However, we were unable to quantify the ¹⁵N uptake because of the dilution effect of the large biomass. We calculated the contribution of biological nitrogen fixation as a possible alternative source of N in achieving niche separation. The acetylene reduction assay showed minor amounts of nitrogenase activity are associated with Empodisma and Sporadanthus roots (equivalent to 0.045 ± 0.019 and 0.104 ± 0.017 kg N ha⁻¹ year⁻¹respectively). Our results suggest that the species acquire nutrients from different rooting zones, with Empodisma accessing nutrients at the surface from rainfall and Sporadanthus accessing nutrients from mineralization in deeper peat layers. Such niche differentiation probably facilitates species coexistence and may provide a mechanism for slowing the rate of competitive displacement during long-term succession.

text: Nutritional niche separation in coexisting bog species demonstrated by ¹⁵N-enriched simulated rainfall Empodisma minus and Sporadanthus ferrugineus (both Restionaceae) coexist in New Zealand raised bogs, yet Sporadanthus have significantly more depleted ¹⁵N natural abundance signatures than coexisting Empodisma. Their root systems are spatially separated with Empodisma having a thick surface layer of about 50 mm of cluster roots overlying the deeper Sporadanthus roots. We hypothesized this root displacement allows Empodisma to preferentially access the primary N input from rainfall, thus establishing niche separation, and tested this using tracer stable isotopes. We aerially applied 1.6 mmol m⁻² of ¹⁵N as (NH₄)₂SO₄ chased by deionized water to simulate a rainfall event of 34 L m⁻². Root/peat matrix cores were harvested after 5 h and analysed for ¹⁵N uptake. Approximately 80% of the tracer applied was recovered in the cores, with 90% of this recovered in the upper Empodisma cluster root layer. Seven weeks after application, young shoots of Empodisma were significantly enriched (mean δ15N = +7.21‰; reference = −0.42‰), whereas those of coexisting Sporadanthus were not (mean δ15N = −2.76‰; reference = −4.24‰). However, we were unable to quantify the ¹⁵N uptake because of the dilution effect of the large biomass. We calculated the contribution of biological nitrogen fixation as a possible alternative source of N in achieving niche separation. The acetylene reduction assay showed minor amounts of nitrogenase activity are associated with Empodisma and Sporadanthus roots (equivalent to 0.045 ± 0.019 and 0.104 ± 0.017 kg N ha⁻¹ year⁻¹respectively). Our results suggest that the species acquire nutrients from different rooting zones, with Empodisma accessing nutrients at the surface from rainfall and Sporadanthus accessing nutrients from mineralization in deeper peat layers. Such niche differentiation probably facilitates species coexistence and may provide a mechanism for slowing the rate of competitive displacement during long-term succession.

year: 2009

title: Applying the Ka Awatea model to Māori students’ experiences at university

authors: Derby, Melissa

abstract: Understanding key factors that contribute to (or negatively impact on) Māori student success can: 
• Promote the development of better policy that supports Māori students to achieve educational success; 
• Aid in targeting resources to support success; 
• Encourage a move away from deficit insinuation that the challenges some Māori face in education are based on ethnicity; and 
• Promote better understanding of contextual factors and the myriad of things that contribute to the outcomes we experience.

text: Applying the Ka Awatea model to Māori students’ experiences at university Understanding key factors that contribute to (or negatively impact on) Māori student success can: 
• Promote the development of better policy that supports Māori students to achieve educational success; 
• Aid in targeting resources to support success; 
• Encourage a move away from deficit insinuation that the challenges some Māori face in education are based on ethnicity; and 
• Promote better understanding of contextual factors and the myriad of things that contribute to the outcomes we experience.

year: 2022-11-16