The effects of growth phase and salinity on the hydrogen isotopic composition of alkenones produced by coastal haptophyte algae

The isotopic fractionation of hydrogen during the biosynthesis of alkenones produced by marine haptophyte algae has been shown to depend on salinity and, as such, the hydrogen isotopic composition of alkenones is emerging as a palaeosalinity proxy. The relationship between fractionation and salinity has previously only been determined during exponential growth, whilst it is not yet known in which growth phases natural haptophyte populations predominantly exist. We have therefore determined the relationship between the fractionation factor, alpha alkenones-water, and salinity for C37 alkenones produced in different growth phases of batch cultures of the major alkenone-producing coastal haptophytes Isochrysis galbana (strain CCMP 1323) and Chrysotila lamellosa (strain CCMP 1307) over a range in salinity from ca. 10 to ca. 35. alpha alkenones-water was similar in both species, ranging over 0.841-0.900 for I. galbana and 0.838-0.865 for C. lamellosa. A strong (0.85 <= R**2 <= 0.97; p < 0.0001) relationship between salinity and fractionation factor was observed in both species at all growth phases investigated. This suggests that alkenone dD has the potential to be used as a salinity proxy in coastal areas where haptophyte communities are dominated by these coastal species. However, there was a marked difference in the sensitivity of alpha alkenones-water to salinity between different growth phases: in the exponential growth phase of I. galbana, alpha alkenones-water increased by 0.0019 per salinity unit (S 1), but was less sensitive at 0.0010 S 1 and 0.0008 S 1 during the stationary and decline phases, respectively. Similarly, in C. lamellosa alpha alkenones-water increased by 0.0010 S 1 in the early stationary phase and by 0.0008 S 1 during the late stationary phase. Assuming the shift in sensitivity of alpha alkenones-water to salinity observed at the end of exponential growth in I. galbana is similar in other alkenone-producing species, the predominant growth phase of natural populations of haptophytes will affect the sensitivity of the alkenone salinity proxy. The proxy is likely to be most sensitive to salinity when alkenones are produced in a state similar to exponential growth.

海洋定鞭金藻类（marine haptophyte algae）合成的烯酮（alkenones）过程中发生的氢同位素分馏效应已被证实与盐度相关，因此，烯酮的氢同位素组成正逐渐成为一种古盐度代用指标。此前，该分馏效应与盐度的关系仅在指数生长期得到确定，而目前尚不清楚自然定鞭藻种群主要处于何种生长阶段。
因此，本研究针对两种主要产烯酮的沿岸定鞭金藻——球等鞭金藻（Isochrysis galbana，菌株CCMP 1323）和层状金藻（Chrysotila lamellosa，菌株CCMP 1307）——的分批培养物，在盐度约10至约35的范围内，测定了不同生长阶段下C37烯酮的α_alkenones-water与盐度的关联。两种藻类的α_alkenones-water数值相近，球等鞭金藻的范围为0.841~0.900，层状金藻为0.838~0.865。
在所有检测的生长阶段中，两种藻类的盐度与分馏因子均呈现极强的相关性（0.85 ≤ R² ≤ 0.97；p < 0.0001）。这表明，在以这类沿岸定鞭金藻为优势类群的沿岸海域，烯酮的氢同位素组成具备作为盐度代用指标的潜力。
不过，不同生长阶段下的α_alkenones-water对盐度的敏感性存在显著差异：以球等鞭金藻为例，其指数生长期的α_alkenones-water每盐度单位提升0.0019，但在稳定期和衰亡期的敏感性分别降至0.0010每盐度单位和0.0008每盐度单位。类似地，层状金藻在早期稳定期的α_alkenones-water每盐度单位提升0.0010，晚期稳定期则提升0.0008。
假设其他产烯酮藻类的α_alkenones-water对盐度的敏感性变化规律与球等鞭金藻在指数生长末期的表现一致，那么定鞭藻自然种群的优势生长阶段将会影响烯酮盐度代用指标的敏感性。当烯酮在类似指数生长的状态下合成时，该代用指标对盐度的敏感性大概率最高。