# Laminar _Ulva_ Species: A Multi-Tool for Humankind?
## 2 The Biology of _Ulva_ sp.
### Light Intensity  
The release of reproductive cells in _Ulva_ is reliant on photosynthesis, making light a crucial factor in its spawning process. There exists a positive correlation between the presence of light in the water column and spore biomass. [PERSON] et al. (2020) [4] have reported a linear association between growth rate and light intensity, up to 30 \(\upmu\)mol photons m\({}^{-2}\) s\({}^{-1}\). However, growth reaches saturation levels beyond 70 \(\upmu\)mol photons m\({}^{-2}\) s\({}^{-1}\)[4]. Furthermore, it can be noted that ultraviolet (UV) radiation has a negative effect on growth. Mitigation of UV exposure is achieved by the accumulation of UV-absorbing compounds such as carotenoids, coumarins, phenolic compounds, and mycosporin-like amino acids (MAAs). [PERSON] et al., 2007 [81] found that algal spore biomass increased with the increasing amount of available light, where the higher tested photon flux density (PFD) (90 \(\upmu\)E m\({}^{-2}\)s\({}^{-1}\)) allowed significantly greater algae recruitment when compared to the other two PFDs tested (20 and 40 \(\upmu\)E m\({}^{-2}\)s\({}^{-1}\)) [84]. Another point is the types of endogenous rhythms: ultradian (with periods of several hours or much less), circadian (with periods between 20 and 28 h), and infradian (with periods longer than 28 h). There is evidence that the brief infradian clock controls physiological processes in animals; for example, bivalves, rats, ducks, horses, and humans all have infra-radian rhythms with durations ranging from 2 to 7 days. However, information on these infradian rhythms in marine algae is scarce, and most reports concern reproduction. For example, in _Fucus vesicusus_ (Phaeophyceae) there is a semilunar rhythmicity, and there are reports of a weekly or 5-day reproductive rhythm that occurs in _U. lactuca_, _U. pseudocurata_, and _U. fenestrate_, and an interval of 4 to 5 days between the formation of gametagia occurs in _Derbesia_[4; 85]. However, the question arises whether such rhythms found in algae from different geographic areas will be the same. [PERSON] and [PERSON] 2013 [85] carried out a study comprising two experiments. The first was to analyze the effects of temperature and irradiation on the infradian rhythm of growth rate, where they placed 48 discs with _U. lactuca_ stalk in different combinations of temperature (5, 10, 15, 20 \({}^{\circ}\)C) and irradiance (40 or 60 \(\upmu\)mol m\({}^{-2}\)s\({}^{-1}\) photons) in 12 h of light per day. The second experiment was the analysis of the effects of photoperiod on the infradian rhythm of growth rate, where they placed 81 discs containing male and female gametophytes and sporophytes cultivated in two groups of 8 h light:16 h dark (8:16 L:D) and 12 h light:12 h dark (12:12 L:D). They found that the growth rate, in terms of rate of increase in biomass, exhibited a rhythmic pattern with a major peak every 2 or 3 days, both at a temperature of 5 \({}^{\circ}\)C and photons of 40 \(\upmu\)mol m\({}^{-2}\)s\({}^{-1}\) and at a temperature of 10 \({}^{\circ}\)C and 60 \(\upmu\)mol photons m\({}^{-2}\)s\({}^{-1}\)[85]. Temperatures of 5 and 10 \({}^{\circ}\)C caused an increase in the prevalence of 3-day cycles, keeping _U. lactuca_ in the vegetative growth stage, while fertile tissue appeared mainly at 10 \({}^{\circ}\)C and 60 \(\upmu\)mol photons m\({}^{-2}\)s\({}^{-1}\). Considering references that indicate that higher minimum light (10 \(\upmu\)mol photons m\({}^{-2}\)s\({}^{-1}\)) is required for reproduction versus vegetative growth (5 \(\upmu\)mol photons m\({}^{-2}\)s\({}^{-1}\)) for _Ulva australis_ (as _U. pertusa_) [85]. Exposure to 15 and 20 \({}^{\circ}\)C caused a predominance of 2-day cycles and induced reproduction after 5 days of cultivation, representing 56-90% of all cycles detected. And finally,lengthening the photoperiod from 8 to 12 h altered the amplitude of the growth rate, so that growth accelerated as the light phase increased [85].