LAUSR

Diatom cells utilize a variety of metabolic pathways to cope with internal energy imbalances caused by stressful environmental conditions. In this study, the model diatom species, Thalassiosira pseudonana, was grown in nutrient replete and nitrate (NO3−)- and dissolved silicate (Si)-depleted media at three growth temperatures (4, 17, 28 °C) to determine how nutrient enrichment and temperature affects diatom growth, photosynthetic efficiency, nitrate reductase (NR) enzyme activity, biogenic silica (bSiO2) deposition, and NR gene expression. Growth rates for nutrient-replete cultures were highest at 17 °C. Across all nutrient treatments, the cells grown at 17 °C had an average Fv/Fm of 0.44 ± 0.006, while the cells were grown at 4 °C and 28 °C had an average Fv/Fm of 0.37 ± 0.004 and 0.38 ± 0.01, respectively. Activity of NR was variable across treatments with no significant effect of temperature. The relative expression of the targeted NR gene was, on average, ~ 10 times higher in the 4 °C cultures and ~ 4 times higher in the 28 °C than in the 17 °C cultures, while the activity of the NR enzyme was generally highest in the cultures grown at 17 °C that were enriched with NO3−. Cells grown under nutrient-replete conditions had significantly higher bSiO2 deposition rates at 4 °C than cells grown at 17 and 28 °C. These data support the notion that cold, nutrient-replete conditions lead to increases in diatom silicification and that NR activity may be regulated downstream of mRNA transcription under specific environmental conditions.