Phaeocystis globosa has a haplo-diplontic alternative life cycle and is an important causative species of harmful algal blooms. Diploid solitary cells of Phaeocystis can form colonies and bloom in the surface water. However, haploid cells are abundant in deep water rather than surface water. We hypothesize that the haploid cells of Phaeocystis globosa could better adapt to deep dim water than its diploid cells. Haploid and diploid solitary cells of P. globosa were cultured with eutrophic medium (f/2) under moderately low (15 µmol photons m^-2 s^-1), extremely low (5 µmol photons m^-2 s^-1) and normal (60 µmol photons m^-2 s^-1) irradiance conditions. The results showed that irradiances used in this study did not induce reactive oxygen species (ROS) damage in either haploid or diploid cells. Both haploid and diploid solitary cells grew faster at higher irradiance during the initial exponential growth phase. However, the haploid abundances under moderately and extremely low irradiances were higher than that under normal irradiance after the exponential growth phase, but diploid cells formed more colonies at higher irradiances. An increase in the photosynthetic pigments (PSC) ratio combined with a reduction in photoprotective pigments (PPC) ratio were found in both ploidies with decreasing irradiance, but the ratios of PSC and PPC and xanthophyll cycle pigments were significantly higher in haploid cells than in diploid cells. For haploids, the highest potential photochemistry efficiency of photosystems П was found under extremely low irradiance, but for diploids, it was observed under moderately low irradiance. The results suggest that both haploid and diploid solitary cells of P. globosa in eutrophic water can survive under low-light conditions, but haploid cells have an advantage in extremely low irradiance.

Nutrient limits impact diatom sinking in time domain, but response in time-frequency domain is unclear. Studying the response of large diatoms to nutrients exclusively in the time domain fails to fully capture the complete impact of nutrient limitations on sinking behavior due to the absence of crucial information, including period and frequency. Wavelet analysis provides valuable insights into the period and frequency of signals and unveils their positions in the time. This study investigated the sinking behavior response of the large diatom Palmerina hardmaniana to silicate-depleted conditions in the time-frequency domain using wavelet analysis. The results showed that P. hardmaniana was capable of regulating its sinking speed, and this regulation occurred intermittently in time. The predominant frequency of intermittent regulation fell within the range of 0.13-0.50 Hz (equivalent to a period of 2-8 s) for both control and silicate-depleted conditions. The similarity in the frequency range of regulation between the two groups suggests the involvement of shared physiological mechanisms. P. hardmaniana responded to silicate depletion by intensifying the regulation of 0.13-0.50 Hz, which was reflected in the time domain as a change in the proportion of different instantaneous sinking speeds and consequently lead to a significant increase or decrease in the mean sinking speed (p <0.05). Additionally, the regulation of P. hardmaniana sinking behavior was also influenced by the physiological state of the cells. Short-term silicate stress (30 min) enhanced the oscillation power of sinking regulation, while prolonged silicate stress (≥ 3 d) led to a decline in oscillation power.

Epiphytes significantly contribute to the overall primary productivity of seagrass beds. Among them, diatoms are the most diverse and important component of the epiphytic community in seagrass beds. However, studies on this group of diatoms are still limited in China. In this research, we investigated the epiphytic diatoms on Zostera marina Linnaeus from Qingdao Bay on the western coast of the Yellow Sea. A total of 112 taxa belonging to 31 families and 57 genera were morphologically identified, of which 16 taxa were newly recorded in China. Each taxon was illustrated with corresponding light micrographs. The most common genera were Navicula Bory with 12 taxa, Amphora (Ehrenberg) Kützing with 10 taxa, and Nitzschia Hassall with 10 taxa. Notably, species of Cocconeis Ehrenberg were ubiquitous and found in every sample. Based on our observations, the sediment is proposed as a likely source of these epiphytic communities. None of the newly recorded diatoms had been previously reported as epiphytes on seagrasses. Our results improve the understanding of species diversity and distribution of seagrass epiphytic diatoms along the coasts of China.