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). \n\nThe aim of this study was to test the hypothesis that increased pCO2 would increase photosynthetic and growth rates to various extents between seagrass species.\n\n\nMethod:\n\nThis 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.\n\nC. 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.\n\nPhotosynthetic 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. \n\nTo 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.\n\nChlorophyll content was determined by acetone extraction of chlorophyll from a young mature leaf from each pot at the end of the experiment. \nTo 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.\n\n\nFormat:\n\nComma Separated File (CSV)\n\n\nData Dictionary:\n\nseagrass_pCO2only.csv:\n\n- pH: Average of the calculated pH of the aquarium over the experimental period.\n- pCO2 (uatm): average pCO2 levels over two weeks.\n- Temp_real (oC): Average of measured temperatures of the aquarium over the experimental period (N=3). These were measured manually.\n- N_sampling: Number of water samples taken for pCO2 and pH determinations\n- Species: ¿CS¿= Cymodocea serrulata, ¿HU¿= Halodule uninervis, ¿TH¿= Thalassia hemprichii\n- Aquaria: ID of the aquarium each pot was kept in.\n- Duplicate_pot: ID of duplicate pot of species in each aquaria \n- Resp (mgO2 g-1DW h-1): dark respiration rates of leaf from haphazardly choosen shoot from pot\n- Productivity (mgO2 g-1DW h-1): photosynthetic rates measured under experimental treatment light level (400 µmol m-2 s-1), measured after 14 days.\n- 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)\n- Ec (µmol m-2 s-1): Light level at which photosynthetic rate is equal to the dark respiration rate.\n- Ek (µmol m-2 s-1): Light level at which photosynthetic rate reaches maximal photosynthetic rate (Pmax).\n- Pmax (mgO2 g-1DW h-1): maximum photosynthetic rates, derived from fitted curve (shown above).\n- Pg:R_ratio: ratio of gross photosynthetic rate (Productivity+Resp) to dark respiration rates.\n- Chl_a (mg g-1FW): chlorophyll a content per gram fresh weight leaf tissue.\n- Chl_b (mg g-1FW): chlorophyll b content per gram fresh weight leaf tissue.\n- 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.\n- 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.\n- Specific leaf area: Total leaf area extended divided by the total biomass of the leaves.\n- SolCarbs (mg g-1DW): Soluble carbohydrates content per unit dry weight of rhizome tissue. Only 4 reps were analysed, i.e. 1 per aquaria. \n- Starch (mg g-1DW): Starch content per unit dry weight of rhizome tissue. Only 4 reps were analysed, i.e. 1 per aquaria.\n- 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.

本数据集包含一份来自为期2周的水族箱实验的数据文件（电子表格），该实验通过调控酸碱度（pCO2）变化，探究3种热带海草（喜盐草（Cymodocea serrulata）、二药藻（Halodule uninervis）及海菖蒲（Thalassia hemprichii））的光合与生长响应。

本研究的目的为验证下述假说：升高的二氧化碳分压（pCO2）会在不同海草物种间以不同程度提升其光合速率与生长速率。

## 实验方法
本实验将3种海草（喜盐草（Cymodocea serrulata）、二药藻（Halodule uninervis）及海菖蒲（Thalassia hemprichii））置于4组二氧化碳分压（pCO2）处理组（440、700、890、1204 µatm）中培养2周，以探究海洋酸化对其生理的影响。所选处理组覆盖了不同末端年代排放情景下预测的2100年左右大气CO2水平范围。每个处理组设置4个水族箱重复，每个水族箱内每个物种设置2个重复培养盆。

二药藻（Halodule uninervis）与喜盐草（Cymodocea serrulata）于2013年3月末采集自大堡礁磁岛考克尔湾的潮间带草甸（南纬19°10.88'，东经146°50.63'）；海菖蒲（Thalassia hemprichii）于2013年4月初采集自大堡礁北部的绿岛（南纬16°45.37'，东经145°58.19'）。二药藻（Halodule uninervis）与沉积物以完整土芯的形式采集，喜盐草（Cymodocea serrulata）与海菖蒲（Thalassia hemprichii）则通过挖掘获取带有相连水平根状茎的完整芽体。实验于采样后2至4周启动。

采用光学氧传感器测定每个培养盆内随机选取芽体的第二年轻叶片的光合速率与呼吸速率。光合速率在一系列光强梯度（10、30、70、110、220、400、510 µmol·m⁻²·s⁻¹）下测定，每个光强梯度持续20分钟。将叶片于60℃烘干48小时后，以干重对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α、饱和光强Ek、补偿光强Ec），将光合-光强（P-E）数据拟合至Jassby与Platt（1976）提出的改进双曲正切模型公式：$P=P_{max}×tanh((αP_{max})/E)$。净生产力（NP）取实验光强（400 µmol·m⁻²·s⁻¹）下测得的光合速率。能量盈余（PG:R）以总光合速率与暗呼吸速率的比值计算得到。

为测定生长速率，实验开始时用针在叶鞘顶部标记所有芽体。14天后收获植株，剪下新生组织，于60℃烘干48小时后称重以获取新生叶生物量。将叶组织生长量以上部生物量标准化，进而计算相对叶生长速率（RGR）。比叶面积（SLA）通过总叶面积除以叶片总生物量计算得到。

叶绿素含量于实验结束时，从每个培养盆的一片成熟幼叶中通过丙酮提取法测定。

为测定非结构性碳水化合物（NSC）含量，将根与根状茎于60℃烘干48小时后均质化。用80%乙醇在80℃下提取可溶性碳水化合物，去除酚类物质并经酸水解后，采用市售葡萄糖氧化酶/过氧化物酶（GOPOD）检测试剂盒（Megazyme公司）测定葡萄糖浓度，于510nm处检测吸光度。可溶性碳水化合物提取后的残渣通过酶（淀粉酶与糖化酶）消化法测定淀粉含量。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
逗号分隔文件（CSV）

## 数据字典
seagrass_pCO2only.csv 各字段说明如下：
- pH：实验期间水族箱平均酸碱度
- pCO2 (µatm)：两周内平均二氧化碳分压水平
- Temp_real (℃)：实验期间水族箱实测平均温度（N=3），采用手动测量
- N_sampling：用于pCO2与酸碱度测定的水样采集数量
- Species：缩写说明："CS"代表喜盐草（Cymodocea serrulata），"HU"代表二药藻（Halodule uninervis），"TH"代表海菖蒲（Thalassia hemprichii）
- Aquaria：每个培养盆所在的水族箱编号
- Duplicate_pot：每个水族箱内同一物种的重复培养盆编号
- Resp (mgO₂·g⁻¹DW·h⁻¹)：随机选取芽体的叶片暗呼吸速率，单位为毫克氧每克干重每小时
- Productivity (mgO₂·g⁻¹DW·h⁻¹)：实验光强（400 µmol·m⁻²·s⁻¹）下测得的光合速率，于实验第14天测定
- alpha：光合效率，由拟合的双曲正切曲线（Jassby与Platt, 1976）的线性斜率计算得出，公式为$P=P_{max}×tanh((αP_{max})/E)$
- Ec (µmol·m⁻²·s⁻¹)：光合速率等于暗呼吸速率时的光强（补偿光强）
- Ek (µmol·m⁻²·s⁻¹)：光合速率达到最大光合速率（Pmax）时的光强（饱和光强）
- Pmax (mgO₂·g⁻¹DW·h⁻¹)：由拟合曲线得到的最大光合速率
- Pg:R_ratio：总光合速率（Productivity+Resp）与暗呼吸速率的比值
- Chl_a (mg·g⁻¹FW)：每克鲜重叶片组织的叶绿素a含量
- Chl_b (mg·g⁻¹FW)：每克鲜重叶片组织的叶绿素b含量
- Growth (mg·sht⁻¹·d⁻¹)：每个培养盆的叶生物量增量，以实验时长（14天）平均后标准化为每株芽的日增量
- RGR (g·g⁻¹·d⁻¹)：相对生长速率，即每个培养盆的叶生物量增量以实验时长（14天）平均后，以培养盆内地上生物量标准化得到
- Specific leaf area：总展开叶面积与叶片总生物量的比值（比叶面积）
- SolCarbs (mg·g⁻¹DW)：根状茎组织单位干重的可溶性碳水化合物含量，仅分析了4个重复，即每个水族箱1个重复
- Starch (mg·g⁻¹DW)：根状茎组织单位干重的淀粉含量，仅分析了4个重复，即每个水族箱1个重复
- NSC (mg·g⁻¹DW)：根状茎组织单位干重的总非结构性碳水化合物含量，仅分析了4个重复，即每个水族箱1个重复