**6. Effect of climate change on global seaweed communities**

The development and the survival of macroalgae depend on a range of environmental factors, such as temperature, CO2 concentration, nutrients, wave peak, etc., which are also related to climate. Among these parameters, the temperature has an intense influence on their growth and distribution [46]. Elevated oceanic temperature from the result of global warming accelerates the poleward shift of macroalgal species. In a study of Australian seaweed distribution, it has been reported that continued ocean warming is responsible for the abrupt range shift of indigenous seaweed species, which may cause the extinction of 100–350 species within the next few decades. It is noteworthy that over 25% of macroalgal species solely belong to southern Australia, which infers that extinction of these species has a huge impact on total seaweed communities worldwide [47]. Along the North-Atlantic shores, canopyforming seaweeds will show a northward shift due to the ongoing ocean warming incident. *Fucus serratus, Ascophyllum nodosum, Saccharina latissima, Laminaria hyperborea* and *Chondrus crispus* will be eradicated from the warm-temperate region of the North-Atlantic. Moreover, temperate species will be migrated to cooler Southern Arctic areas. With the absence of these seaweeds, Northwest-African and Northwest-Atlantic coasts will be transformed completely [48].

In a study of ocean acidification (OA) impact on the Baltic sea, researchers found that with elevated CO2 levels, the photosynthetic rate of *Furcellaria lumbricalis* and *Coccotylus truncatus* has been augmented. This augmentation rate is much higher in *C. truncates* in comparison to *F. lumbricalis*. However, the effect of CO2 on photosynthesis is also influenced by other environmental factors, especially temperature. Increased water temperature can lessen the photosynthesis activity of *F. lumbricalis* [49]. Though a positive effect is always expected regarding the growth benefit of noncalcifying seaweeds with increased CO2 concentration in coastal water, the opposite result has been found in some seaweeds like *Fucus vesiculosus,* which is frequently found in the North Sea. With increased CO2 in water, *F. vesiculosus* growth rate has been reported to decrease up to 15% [50]. Increased CO2 has a drastic effect on the tissue density of *F. vesiculosus*. It lowers the breaking strength, making this seaweed vulnerable to water waves and storms. Simultaneously, phlorotannin contents may decrease in this phenomenon [51].

OA has an impact on the different developmental stages of seaweeds. *Macrocystis pyrifera* can face a slightly reduced germination rate when OA is at an extreme level, though this seaweed successfully withstands increased OA with even lowered pH [52]. Surprisingly, extract of brown seaweed *Sargassum vulgare* from the acidified ocean exhibited higher bioactivity like anti-oxidant, anti-microbial, anti-lipid peroxidation, as well as anti-cancer activities [53].
