Early stage chlorosis can be due to nutrient deficiencies (e.g., nitrogen, iron, or magnesium) or improper light intensity. Ensure that the light levels are appropriate for Arabidopsis. Later-stage chlorosis could also be linked to temperature fluctuations, humidity issues, or even photoperiod mismatches. Double-check the chamber settings to ensure consistency.

Regarding the algae issue, it's quite common in growth chambers due to the humid environment and nutrient-rich substrate. Algae can indeed compete with seedlings. Ensure pots or trays have proper drainage to avoid waterlogging and water from the bottom rather than the top to keep the surface dry. You may also cover the surface a thin layer of sterile sand.

It would be a bit unusual to have nutrient deficiency in very young seedlings in a standard growing soil. Looks like you have a good mix, perlite etc. Plus, the algae are growing really well, which indicates lots of nutrients and water.

My best guess would be lighting conditions, maybe too bright or too long of time. There doesn't seem to be any etiolation so it's definitely bright enough.

I agree with Kaiser Wani on the waterlogging issue. Once the plants are established you can let the soil dry a bit between waterings (which will help).

Commenting on the algae issue, I agree with Mr. Wani about watering from the bottom. However, water ONLY when the soil is completely dry and before the seedlings wilt. After watering, do not leave your pots in standing water, drain the excess water immediately. In addition to algae problems, you might end up with gnat infestations as their larvae feed on the algae growing on the soil surface.

Kaiser Wani, Amy Klocko and Sonia Irigoyen

Thank you for your response! I appreciate it