Photosynthetic and growth responses in three tropical seagrass species to pCO2 enrichment (440, 700, 890, 1204 µatm) (NERP TE 5.2, AIMS)

This dataset consists of one data file (spreadsheet) from a 2 week aquarium experiment manipulating pH (pCO2) changes and measuring photosynthetic and growth responses of three tropical seagrass species (Cymodocea serrulata, Halodule uninervis and Thalassia hemprichii).

The aim of this study was to test the hypothesis that increased pCO2 would increase photosynthetic and growth rates to various extents between seagrass species.

Method:

This experiment exposed three seagrass species, Cymodocea serrulata, Halodule uninervis and Thalassia hemprichii, to four pCO2 treatments (440, 700, 890, 1204 µatm) for two weeks to investigate the effects of acidification on their physiology. These treatments were chosen to bracket the range of atmospheric CO2 levels predicted for different end-of-century emission scenarios in the near-future (2100). Each treatment was replicated across four aquaria, each with 2 duplicate pots per species.

C. serrulata and H. uninervis were collected from the intertidal meadow at Cockle Bay, Magnetic Island, Great Barrier Reef (19°10.88’S, 146°50.63’E) in late March 2013. T. hemprichii was collected from Green Island in the Northern Great Barrier Reef (16°45.37’S, 145°58.19’E) in early April 2013. H. uninervis and sediment was collected as intact plugs. C. serrulata and T. hemprichii were collected by excavating intact shoots with connected horizontal rhizomes from the sediment. The experiment started two to four weeks after the collection.

Photosynthetic rates and respiration of the second youngest leaf of a haphazardly chosen shoot from each pot were measured using optical oxygen sensors. Photosynthetic rates were measured over a series of light steps (10, 30, 70, 110, 220, 400, 510 µmol m-2 s-1), with each light step lasting 20 minutes. Rates were normalised to the dry weight of the leaf, after drying leaves at 60°C for 48 hours. To determine photosynthetic parameters (Pmax, alpha, Ek, Ec), photosynthesis versus irradiance (P-E) data plots were fitted to the adapted hyperbolic tangent model equation of Jassby and Platt (1976). Net productivity (NP) was taken to be the photosynthetic rate measured at the experimental light level (400 µmol m-2 s-1). Energetic surplus (PG:R) was calculated as the ratio of gross productivity to dark respiration rate.

To measure growth rates, all shoots were marked at the top of the sheath with a needle at the start of the experiment. After 14 days, the shoots were harvested, the length of new tissue growth were excised, dried at 60°C for 48 hours and weighed for determination of weight of new leaf growth. Leaf tissue growth was normalised to the aboveground biomass to derive relative leaf growth rates (RGR). Specific leaf area (SLA) was calculated by dividing total leaf area with the total biomass of the leaves.

Chlorophyll content was determined by acetone extraction of chlorophyll from a young mature leaf from each pot at the end of the experiment.
To determine non-structural carbohydrates (NSC) content, roots and rhizomes were dried at 60°C for 48 hours, before being homogenised. Soluble carbohydrates were extracted with 80% ethanol at 80°C. The amount of soluble carbohydrates was assayed after removal of phenolic compounds and acid hydrolysis. Starch content was analysed from the residue of the soluble carbohydrate extraction through enzyme (amylase and amyloglucosidase) digestion. The concentration of glucose is measured using a commercially available glucose oxidase/peroxidase (GOPOD) testing reagent (Megazyme). Absorbance was then measured at 510 nm. Total non-structural carbohydrates (NSC) content was the sum of the amount of soluble carbohydrates and starch content.

Format:

Comma Separated File (CSV)

Data Dictionary:

seagrass_pCO2only.csv:

- pH: Average of the calculated pH of the aquarium over the experimental period.
- pCO2 (uatm): average pCO2 levels over two weeks.
- Temp_real (oC): Average of measured temperatures of the aquarium over the experimental period (N=3). These were measured manually.
- N_sampling: Number of water samples taken for pCO2 and pH determinations
- Species: “CS”= Cymodocea serrulata, “HU”= Halodule uninervis, “TH”= Thalassia hemprichii
- Aquaria: ID of the aquarium each pot was kept in.
- Duplicate_pot: ID of duplicate pot of species in each aquaria
- Resp (mgO2 g-1DW h-1): dark respiration rates of leaf from haphazardly choosen shoot from pot
- Productivity (mgO2 g-1DW h-1): photosynthetic rates measured under experimental treatment light level (400 µmol m-2 s-1), measured after 14 days.
- alpha: photosynthetic efficiency of shoot, calculated from the linear slope of fitted hyperbolic tangent curve (Jassby and Platt, 1976) P=P_max×tanh((alpha P_max)/E)
- Ec (µmol m-2 s-1): Light level at which photosynthetic rate is equal to the dark respiration rate.
- Ek (µmol m-2 s-1): Light level at which photosynthetic rate reaches maximal photosynthetic rate (Pmax).
- Pmax (mgO2 g-1DW h-1): maximum photosynthetic rates, derived from fitted curve (shown above).
- Pg:R_ratio: ratio of gross photosynthetic rate (Productivity+Resp) to dark respiration rates.
- Chl_a (mg g-1FW): chlorophyll a content per gram fresh weight leaf tissue.
- Chl_b (mg g-1FW): chlorophyll b content per gram fresh weight leaf tissue.
- Growth (mg sht-1 d-1): increase in leaf biomass per pot, averaged over duration of experiment (14days) and normalise to number of shoots in pot.
- RGR (g g-1 d-1): relative growth rate. Increase in leaf biomass per pot, averaged over duration of experiment (14days) and normalised to aboveground biomass in pot.
- Specific leaf area: Total leaf area extended divided by the total biomass of the leaves.
- SolCarbs (mg g-1DW): Soluble carbohydrates content per unit dry weight of rhizome tissue. Only 4 reps were analysed, i.e. 1 per aquaria.
- Starch (mg g-1DW): Starch content per unit dry weight of rhizome tissue. Only 4 reps were analysed, i.e. 1 per aquaria.
- NSC (mg g-1DW): Total non-structural carbohydrates content per unit dry weight of rhizome tissue. Only 4 reps were analysed, i.e. 1 per aquaria.

本数据集包含一份电子表格数据文件，源自一项为期两周的水族箱实验，该实验通过调控水体酸碱度（pH）与二氧化碳分压（pCO₂）水平变化，探究三种热带海草物种（齿叶丝粉藻*Cymodocea serrulata*、单脉二药藻*Halodule uninervis*、喜盐草*Thalassia hemprichii*）的光合与生长响应。

本研究的核心目标为验证下述假说：二氧化碳分压（pCO₂）升高会提升海草的光合速率与生长速率，但不同海草物种的提升幅度存在差异。

方法：
本实验将三种海草物种（齿叶丝粉藻*Cymodocea serrulata*、单脉二药藻*Halodule uninervis*、喜盐草*Thalassia hemprichii*）置于四种二氧化碳分压（pCO₂）处理组（440、700、890、1204 μatm）中培养两周，以探究海洋酸化对其生理的影响。上述处理组的设置覆盖了2100年（近未来）不同末端排放情景下预测的大气CO₂浓度范围。每个处理组设置4个水族箱重复，每个水族箱内针对每个物种设置2个重复培养盆。

2013年3月下旬，研究人员从大堡礁磁岛蛤蜊湾（19°10.88’S，146°50.63’E）的潮间带海草床采集齿叶丝粉藻与单脉二药藻；2013年4月上旬，从大堡礁北部绿岛（16°45.37’S，145°58.19’E）采集喜盐草。单脉二药藻与沉积物以完整土柱的形式采集，齿叶丝粉藻与喜盐草则通过挖掘沉积物中带有相连横走茎的完整植株进行采集。实验于样本采集后2至4周启动。

研究人员使用光学氧传感器，对每个培养盆中随机选取的植株的倒数第二片幼叶测定其光合速率与呼吸速率。光合速率的测定设置了一系列光照梯度（10、30、70、110、220、400、510 μmol·m⁻²·s⁻¹），每个光照梯度持续20分钟。将叶片置于60℃下烘干48小时后，以叶片干重对光合速率进行标准化处理。为获取光合参数（最大光合速率Pmax、光合效率alpha、饱和光强Ek、补偿光强Ec），研究人员将光-光合（P-E）数据拟合至Jassby与Platt（1976）提出的改良双曲正切模型方程。净生产力（NP）取实验光照水平（400 μmol·m⁻²·s⁻¹）下测得的光合速率。能量盈余（PG:R）以总生产力与暗呼吸速率的比值计算得到。

为测定生长速率，实验开始时使用针在所有植株的叶鞘顶端做标记。14天后收获植株，剪下新生组织，置于60℃下烘干48小时后称重，以获取新生叶片生物量。将叶片组织生长量以上方生物量进行标准化，得到相对叶片生长速率（RGR）。比叶面积（SLA）通过总叶面积除以叶片总生物量计算得到。

实验结束时，从每个培养盆的成熟幼叶中取样，采用丙酮提取法测定叶绿素含量。
为测定非结构性碳水化合物（NSC）含量，首先将根与横走茎置于60℃下烘干48小时后均质化。使用80%乙醇在80℃下提取可溶性碳水化合物，在去除酚类物质并经酸水解后，对可溶性碳水化合物含量进行测定。利用淀粉酶与糖化酶对可溶性碳水化合物提取后的残渣进行酶解，以分析淀粉含量。使用市售葡萄糖氧化酶/过氧化物酶（GOPOD）检测试剂（Megazyme）测定葡萄糖浓度，随后在510 nm波长下检测吸光度。总非结构性碳水化合物（NSC）含量为可溶性碳水化合物与淀粉含量之和。

数据格式：逗号分隔值文件（CSV）

数据字典：seagrass_pCO2only.csv：
- pH：实验周期内水族箱水体计算所得pH的平均值。
- pCO₂（μatm）：两周内的平均二氧化碳分压。
- 实测水温（Temp_real，℃）：实验周期内水族箱水温的手动测量平均值（样本量N=3）。
- 采样次数（N_sampling）：用于测定pCO₂与pH的水体样本采集数量。
- 物种："CS"代表齿叶丝粉藻*Cymodocea serrulata*，"HU"代表单脉二药藻*Halodule uninervis*，"TH"代表喜盐草*Thalassia hemprichii*。
- 水族箱编号：每个培养盆所在的水族箱标识。
- 重复培养盆编号：每个水族箱内对应物种的重复培养盆标识。
- 暗呼吸速率（Resp，mgO₂·g⁻¹干重·h⁻¹）：从培养盆中随机选取植株的叶片暗呼吸速率。
- 生产力（Productivity，mgO₂·g⁻¹干重·h⁻¹）：在实验光照水平（400 μmol·m⁻²·s⁻¹）下测得的光合速率，测定于实验第14天。
- 光合效率（alpha）：植株的光合效率，由拟合的双曲正切曲线（Jassby与Platt，1976）的线性斜率计算得到，拟合方程为P=P_max×tanh((alpha·P_max)/E)。
- 光补偿点（Ec，μmol·m⁻²·s⁻¹）：光合速率等于暗呼吸速率时的光照强度。
- 光饱和点（Ek，μmol·m⁻²·s⁻¹）：光合速率达到最大光合速率（Pmax）时的光照强度。
- 最大光合速率（Pmax，mgO₂·g⁻¹干重·h⁻¹）：由上述拟合曲线推导得到的最大光合速率。
- 总光合/呼吸比（Pg:R_ratio）：总光合速率（生产力+暗呼吸速率）与暗呼吸速率的比值。
- 叶绿素a含量（Chl_a，mg·g⁻¹鲜重）：每克叶片鲜组织中的叶绿素a含量。
- 叶绿素b含量（Chl_b，mg·g⁻¹鲜重）：每克叶片鲜组织中的叶绿素b含量。
- 叶片生长量（Growth，mg·株⁻¹·d⁻¹）：每个培养盆的叶片生物量增长量，以实验周期（14天）取平均值并标准化至培养盆内的植株数量。
- 相对生长速率（RGR，g·g⁻¹·d⁻¹）：相对叶片生长速率，每个培养盆的叶片生物量增长量，以实验周期（14天）取平均值并标准化至培养盆内的地上生物量。
- 比叶面积：总扩展叶面积除以叶片总生物量。
- 可溶性碳水化合物含量（SolCarbs，mg·g⁻¹干重）：横走茎组织单位干重中的可溶性碳水化合物含量。本数据集仅分析了4个重复，即每个水族箱1个重复。
- 淀粉含量（Starch，mg·g⁻¹干重）：横走茎组织单位干重中的淀粉含量。本数据集仅分析了4个重复，即每个水族箱1个重复。
- 总非结构性碳水化合物含量（NSC，mg·g⁻¹干重）：横走茎组织单位干重中的总非结构性碳水化合物含量。本数据集仅分析了4个重复，即每个水族箱1个重复。