**5. Effect of climate change on microalgae globally**

The growth of microalgae and the production of microalgal bioactives are significantly driven by different stress conditions. For instance, under oxidative and salinity stress, carotenoids production is upregulated [31]. Similarly, phenolics, antioxidative properties and thiamin biosynthesis are augmented under abiotic stress [32, 33]. In water bodies, microalgal growth is greatly influenced by temperature, nutrients, salinity, the direction of wind and current, light as well as other organisms present in that habitat [34]. Increased CO2 level helps increase nutrient acquisition, photosynthetic activity and growth of microalgae in the freshwater ecosystem [35]. In a freshwater ecosystem, green microalgae, compared to diatom and cyanobacteria, can better adapt to elevated CO2. Still, with increased temperature, cyanobacteria start to proliferate rapidly and become dominant [36]. However, the pitfalls of climate change like elevated temperature, CO2 or UV radiation have severely affected algal growth and productivity [37].

Microalgae produce a myriad of pharmaceutically important secondary metabolites, especially antioxidant and anticancer agents [38]. They are also a reservoir of various biotoxins, and the production of these toxins is more influenced by climate change [34]. Warming in the global climate affects fresh and marine water bodies by the formation of algal blooms of harmful species, which harms other organisms in those aquatic systems as well as human health, food security and the overall economy. Mainly, harmful algal blooms (HAB) in the freshwater system are caused by cyanobacterial species, and in the marine ecosystem, dinoflagellates are responsible for this kind of blooms [39]. Dinoflagellates like *Gambierdiscus* spp., *Fukuyoa* spp. and *Ostreopsis* spp. are well-known HAB species (also known as BHABs) in the benthic region of the ocean. Though microalgae are used as fish feed in mariculture industries and also used as vectors for vaccine delivery for these fishes, HAB species are getting a major threat to aquaculture [40]. Their notorious toxins production (ciguatoxin, palytoxins, ovatoxins) leads to many health risks, such as food poisoning, irritation and respiratory illness. Climate-directed ocean warming is the key factor for the dense and prompt growth of BHABs in many tropical and subtropical marine ecosystems globally. Elevated temperature helps to flourish these toxin-producing BHABs beyond their geographic area and even in an overly populated area where toxicity is a rare concern. Surprisingly, *Gambierdiscus* sp., one of the BHABs, can propagate in the degraded coral environment due to bleaching events [27]. On benthic microalgae, sea warming exhibits a direct positive and strong correlation to whole biomass accrual and growth rate where the presence of abundant mesograzers (gastropods, crustaceans) even cannot deplete their total biomass [41]. Furthermore, pelagic HABs, including *Pseudo-nitzschia*, *Alexandrium catenella* and *Pseudochattonella,* promote toxic blooms which are highly linked to climate change and cause huge mortality in fish farms [42]. Moreover, *Alexandrium catenella* and *Pyrodinium bahamense* can form cysts that can persist as long-dormant phases in warmer conditions have short quiescence and germination phases [43].

Studies have shown that tropical seas, such as the Red, sea often face HAB from different microalgae genres, such as dinoflagellates, raphidophytes, cyanobacteria, diatoms, due to the warmer climate. Currently, five other HABs have been reported in this tropical sea area, including *Noctiluca scintillans*, *P. bahamense, Protoperidinium quinquecorne, Heterosigma akashiwo* and *Trichodesmium erythraeum*. Moreover, cysts of toxic microalgae species have been found on the Red sea coast responsible for further bloom formation [44]. Additionally, microalgae can withstand ocean acidification effects though the capability is different from species to species. A study showed that *Tetraselmis chuii* exhibited better adaptability in terms of metabolic activity in comparison to *Phaeodactylum tricornutum* [45].
