Selecting for Chlamydomonas reinhardtii fitness in the KSC Veggie Unit

The Space Algae Experiment Verification Test completed a competitive growth selection on mutagenized microalgae in a series of batch cultures that were passaged for three growth cycles. The experiment was conducted with ground control conditions similar to those actually used in spaceflight. Cultures were limited in growth rate due to gas permeable membranes to provide oxygen and carbon dioxide exchange and a lack of agitation to mix the cells throughout the liquid media. Two strains were grown and three biological replicates were completed for each strain. Specific variables (factors) tested were: 1) The effect of UVC (short-wave ultraviolet light) mutagenesis was tested by sampling the algae cultures prior to mutagenesis. 2) The sensitivity of different strains was tested by conducting the experiment with a wild-type and cell wall mutant (cw15) strain. 3) The effect of competitive growth was tested by sampling each biological replicate experiment at the end of each growth cycle. 4) The effect of live culture storage in the dark was tested by sampling each growth cycle twice. At the time of passage, a sample of cells was pelleted and frozen. The cultures were then stored in the dark in a Cargo Transport Bag (CTB) to simulate storage on the ISS and return of cultures. This factor tested whether cultures could be stored alive in the dark or if samples needed to be fixed at the time of passage in order to get an accurate representation of the genetic variation in each cycle of growth. Paired-end, whole genome sequencing was completed for 38 samples: 2 strains pre mutagenesis and 2 strains X 3 biological replicates X 3 growth cycles X 2 storage conditions.

太空藻类实验验证测试针对经诱变处理的微藻，在历经3轮生长循环传代的分批式培养体系中完成了竞争性生长筛选。本实验采用与实际太空飞行场景一致的地面模拟对照条件开展。由于采用透气膜实现氧气与二氧化碳交换，且未配备搅拌装置以实现培养液内细胞的均匀混合，培养体系的生长速率受到限制。实验共使用2株微藻，每株菌株设置3次生物学重复。
本次实验测试的核心变量（因子）如下：
1）UVC（短波紫外线，short-wave ultraviolet light）诱变效应：通过采集诱变处理前的藻类培养物样本完成验证；
2）不同菌株的敏感性：通过使用野生型菌株与细胞壁缺陷突变株（cw15）开展实验完成测试；
3）竞争性生长效应：通过在每一轮生长周期结束时采集各生物学重复实验的样本完成验证；
4）活体培养物暗存储存效应：通过对每一轮生长周期进行两次取样完成测试。
在传代环节，采集细胞样本并离心沉淀后进行冷冻保存；随后将培养物置于货运运输袋（Cargo Transport Bag, CTB）的黑暗环境中储存，以模拟国际空间站（International Space Station, ISS）上的储存及样本返回流程。该变量旨在验证：培养物是否可在黑暗环境中保持活体储存状态，或是需要在传代时即时固定样本，才能准确反映每一轮生长周期内的遗传变异情况。
最终对38份样本完成了双端全基因组测序：其中包括2株诱变前的菌株样本，以及2株菌株×3次生物学重复×3轮生长周期×2种储存条件的组合样本。