Phytoplankton cell size is well known as an essential functional trait, but its control factors are still unclear. Considering light provides the necessary energy for phytoplankton survival, we hypothesized that photosynthetic light energy utilization could influence phytoplankton cell size control. Several scenarios were conducted to understand the relationship between Fv /Fm and cell size for phytoplankton interspecies, and metatranscriptome in the field and transcriptome in the laboratory were used to understand relevant molecular mechanisms. The results indicated that there was a universal significant positive relationship between Fv /Fm and cell volume in general. The molecular evidence demonstrated that light utilization by phytoplankton regulates their cell size by harmonizing the generation and allocation of chemical energy and fixed carbon in the cell. Phytoplankton cell size would cease to enlarge once the increased light energy conversion and subsequent fixed carbon could no longer satisfy the increasing demand of size enlargement. This unity of energy and matter in shaping phytoplankton size results in cell size being an important functional trait. This study is the first to discover the above molecular mechanisms and is helpful to deepen the understanding on the cell size control of phytoplankton.
Redfield first reported a carbon: nitrogen (C:N) ratio of approximately 6.6 in marine phytoplankton. However, recent studies show that phytoplankton C:N ratio has a large range (marine: 6.5-9.9; freshwater: 7.8-10.5) and is species-specific.These studies pose a great challenge to phytoplankton stoichiometric homeostasis, which traditionally refers to their ability to maintain relatively stable elemental composition with the variation in external nutrient availability. The underlying mechanisms of the interaction between phytoplankton stoichiometric homeostasis and nutrient availability need further clarification. Therefore, in the field seven reservoirs in Tianjin, North China, were investigated to understand their phytoplankton C:N ratios and the influencing factors, and in the laboratory, Chlamydomonas reinhardtii, as a model organism, was used to investigate its C and N metabolism and relevant physiological parameters under different C and N availability. Transcriptome sequencing, nano-scale secondary ion mass spectrometry, and C stable isotope analysis were used to understand cellular C-N metabolism at the molecular level, cellular C-N compartmentation, and C utilization strategy, respectively, in the culture experiment. The main aim of this study was to understand how C-N availability affects the C:N ratio of freshwater phytoplankton at the molecular level.The results indicated that CO2 limitation had no significant effect on the phytoplankton C:N ratio in either scene, whereas limitation of dissolved inorganic N induced the ratio to be a 35% higher in the field and a 138% higher in the laboratory, respectively. Under CO2 limitation, algal CO2-concentrating mechanisms were operated to ensure a C supply, and coupled C-N molecular regulation remained the cellular C:N ratio stable. Under nitrate limitation, differentially expressed gene-regulated intensities increase enormously, and their increasing proportion was comparable to that of the algal C:N ratio; cellular metabolism was reorganized to form a “subhealthy” C-N stoichiometric state with high C:N ratios. In addition, the N transport system had a specific role under CO2 and nitrate limitations. This study implies that algal stoichiometric homeostasis depends on the involved limitation element and will help to deepen the understanding of C-N stoichiometric homeostasis in freshwater phytoplankton.
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