Climate Change and Algal Communities

Abstract

Climate change is one of the major global concerns jeopardizing human health and wildlife. This event is considered a threat to the marine ecosystem as well. Marine algae are the leading producer in the benthic food chain. Therefore, any change in marine algal communities will disrupt the whole ecosystem. Currently, algal species face significant changes in their abundance and distribution worldwide. Toxic species are frequently invading and causing a phenomenon called the harmful algal bloom, which threatens the seafood industry and public health. This chapter will focus on the significant distribution of algal communities worldwide and the impact of climate change on these marine algal species. Besides, this chapter will shed some light on how these changes affect the marine food chain and ultimately affect human health.

1. Introduction

Microalgae are unicellular photosynthetic organisms that can be eukaryotic or prokaryotic (cyanobacteria) and are ubiquitous; they can be found in fresh or marine water, soil and even extreme habitats [1]. Microalgae can transform atmospheric inorganic carbon into organic carbon biomass, and it has been estimated that half of their biomass comprises carbon (w/w) [2]. In addition, they contain approximately 50 % (w/w) carbon in their biomass, which helps them grow better [3]. Surprisingly, some species of marine microalgae can grow at high CO₂ (40 %) concentrations, which is known to help curb greenhouse gases that ultimately have a positive effect on Blue Economy [4]. One of the main benefits of using microalgae in mitigating CO₂ is that they can fix this gas up to 50 times more than terrestrial plants [5].

On the other hand, macroalgae or seaweeds are multicellular and macroscopic algae that contribute as huge biomass producers in the benthic region. Besides their consumption as nutritious food and feed, they are frequently used as fertiliser [6]. Like microalgae, seaweeds also contribute to mitigating climate change through the Blue Carbon strategy. Seaweed can sequester a large amount of CO₂ from the environment that has been predicted that ongoing seaweed farming can capture more than 6 % of CO₂ globally by 2050 [7]. Seaweed aquaculture benefits coastal habitats in different ways. For instance, a study showed that Chinese seaweed farming upgrades the coastal water quality by eliminating nitrogen and phosphate as well as by sequestering carbon, which in turn also helps in decreasing eutrophication. Interestingly, this kind of farming needs no chemical fertiliser pesticides as well saves cultivable lands [8].

Currently, changes in climate patterns give rise to several environmental hazards and the most drastic effect is evident in the marine ecosystem. Consequently, marine microalgae and seaweeds are also experiencing the negative impact of climate change. Studies revealed that changes in habitat conditions make the species migrate or to extinction [9]. Therefore, migration in seaweed species is becoming common. Range shifts in seaweed communities disturb the local distribution pattern and species richness, reported globally [10].

2. Climate change

According to NASA, climate change can be defined as persisting modification in the earth's climate, resulting from anthropogenic activities (industrialisation, burning fossil fuels, deforestation, etc.), which gives rise to greenhouse gases (GHGs) and ultimately contributes to global warming. Climate change is attributed to the increase of greenhouse gases, mainly CO₂ (418 ppm currently), which helps increase the surface temperature of our planet by about 1.01°C, and this trend is on the rise. This phenomenon also engenders warming of the oceans exceeding 0.4°F since the early 70s, a rapid decrease in ice mass (about 147 gigatons/year) in Antarctica, increasing in sea level (3.4 mm/year) and finally gives rise to different natural disasters like drought, heavy rainfall or acidification in the ocean [11].

3. Effect of climate change on living organisms

Changing climate conditions have a profound impact on human health. It can directly affect our health through extreme environmental conditions like drought, heavy rainfall or indirectly through transmissible diseases or malnutrition along with other mental health complications [12]. Severe weather condition helps spread pathogenic microorganisms yielding different waterborne or zoonotic diseases as well as increasing allergens in the air can cause respiratory infections [13, 14]. Climate change is not only impacting human life but also jeopardising wildlife. Many terrestrial and aquatic organisms are at the risk of extinction due to environmental chemical pollutants. Altered environmental conditions can make wildlife more susceptible to chemical toxicants and retard their physiological capability [15]. The impact of climate change on tree mortality is another pressing problem in the forest ecosystem. This temporal rise in tree mortality has been seen in both areas with an increased water shortfall or no such undersupply [16]. Birds, one of the main indicator groups of climate change effects, are more prone to extinction due to severe weather change. It has been predicted that about 900 land bird species will be terminated because of global warming. Moreover, loss of habitat, augmentation of invasive bird species and hunting and spreading of infectious diseases are also contributing factors to bird extinction, which is the outcome of change in climate [17]. Climate change has the most vulnerable effect on agriculture. Changing in climate pattern gives rise to land degradation, desertification or heavy rainfall and floods, which in turn can able to make the soil nutrient deficit, highly saline and less productive. All these stresses are attributable to the risk of global food security [18].

The marine ecosystem is greatly affected by climate change. The oceans absorb 90% of the extra heat of the climate, which would be increased to 5–7 times by 2100 if global warming exceeds 2 °C. Additionally, the ocean absorbs up to 30 % of anthropogenic CO₂ emissions, which renders acidification in the marine system. Ocean acidification and heat cause depletion of nutrients and O₂ supply, which eventually endanger distribution and abundance of marine fishes as well as other organisms, and along with these, climate change also affects demography, calcification and phenology of phytoplankton and zooplankton [19, 20]. It accelerates the bleaching of reef-building corals, which makes great barrier reef (GBR) vulnerable. It has been estimated that a 0.5 °C increase in local temperature may cause rapid degeneration of GBR [21].

4. Global distribution of microalgae and macroalgae

In the marine coastal area, green algae are the major contributor to photosynthesis, especially microalgae from the Mamiellophyceae class are abundant in number while Chlorophyceae are the least. Microalgae from Pyramimonadales and Chlorodendrophyceae are ubiquitous in marine coastal regions [22]. In desert soil, cyanobacterial species are mostly found than other microalgae species because of their greater desiccation tolerating capacity [23]. In the alpine region of New Zealand, microalgae from Chrysophyceae, Xanthophyceae, Chlorophyceae, Trebouxiophyceae, and Charophyceae are more prevalent [24]. A study on the southeast coast of India showed that Isochrysis galbana was the dominant species in that area, while Nannochloropsis oculata was the less common species observed. The dominance of other species found were Chlorella marina > Chromulina freibergensis > Dicrateria inornata > Chaetoceros calcitrans [25]. On the other hand, the coastal region of Indonesia is dominated by diatoms, especially Rhizosolenia spp., Chaetoceros spp. and Coscinodiscus spp. [26]. In the case of benthic harmful algal bloom species (BHABs), two main species, Gambierdiscu and Fukuyoa, are widespread in the Caribbean Sea, the South Pacific, Indian Oceans, North Atlantic Ocean and the Canary Islands. Additionally, they are also reported from the coastal area of Japan, Korea, New Zealand and Australia. On the other hand, Ostreopsis spp. is common in the Mediterranean Sea [27].

A wide variety of seaweed species are distributed on the coasts, and their similarity is dependent on climatic zones. Most of the Rhodophyta belongs to warm temperate Pacific flora. The number of Rhodophyta is more than double in these Pacific florae compared to the cold temperate floras [28]. Seaweed species are vastly available in 30–50° latitude, which covers southern Australia, Japan and the Mediterranean as well as in the Philippines [29]. In recent studies, the distribution of invasive seaweeds has been documented. The highest invasion cases belong to the Mediterranean region (132), followed by the NE Atlantic region, covering the North and Baltic Sea and the eastern Atlantic Islands. Among the Rhodophytes, the Rhodomelaceae and the Ceramiaceae accommodate more invaders, while Chlorophyta, Caulerpa spp. and Codium spp. are recorded as the most successful invader species [30].

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].

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, canopy-forming 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 non-calcifying 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].

7. Impact of HAB on animal and human health

Frequent HAB events in a particular area are reported to endanger and kill marine organisms, especially fish, shellfish, birds, and other marine organisms on a large scale. HAB formation can be associated with either toxic or non-toxic phytoplanktons. Non-toxic species are responsible for discolouration sometimes for fish and shellfish mortality. In this case, oxygen depletion due to dense bloom is accountable for such killing. On the other hand, toxic species cause poisoning in marine fish and bivalves, which renders large-scale fish killing and also severe health hazards for humans [54]. Ciguatera poisoning (CP) is the most common type of HAB-related contamination in fish. Dinoflagellates, Gambierdiscus and Fukuyoa, are attributed to produce ciguatoxins, which ultimately cause CP. Upon consuming contaminated fish, individuals may experience gastrointestinal, cardiovascular, and neurological disturbances [55].

Other HAB-related diseases include amnesic shellfish poisoning, caused mainly by Pseudo-nitzschia and Nitzschia; azaspiracid shellfish poisoning, caused by Azadinium and Amphidoma; diarrhetic shellfish poisoning, caused by Dinophysis, Phalacroma and Prorocentrum; neurotoxic shellfish poisoning, caused by Karenia and Chatonella; paralytic shellfish poisoning, caused by Alexandrium, Gymnodinium and Pyrodinium; and palytoxicosis ovatoxin poisoning, caused by Dinoflagellate, Ostreopsis. All of these poisonings are mainly associated with gastrointestinal, respiratory or neurological disturbances in humans. In severe cases, it may lead to coma or even death. HAB toxins kill marine piscine species and may amalgamate in these marine creatures, which may further transfer to the food web. This may affect other animals as well [56]. For example, birds, snakes and turtles found in HAB affected wetlands are reported to face sub-lithal effects. HAB toxins like microcystin can elevate physiological stress in these organisms, resulting in immune function or reproduction anomalies [57].

Besides direct consumption, exposure to HAB related toxins may also affect human health. Aerosolised brevetoxin from Karenia brevis is responsible for impaired respiratory function. This brevetoxin can spread quickly to the entire water column and maybe end up in the diet of the biota of that area. Not only that, this toxin has a detrimental impact on the larval stages of aquatic creatures [58].

8. HAB mitigation strategies

HABs are the major threats to public health and the economy in benthic areas. HAB formation is not only responsible for destroying aquaculture industries but also severely affecting the tourism industries. Besides fish poisoning, HAB affected beaches become vulnerable due to fouling of water, which may cause skin diseases and other illnesses [59].

Several biological, chemical or physical measures are taken worldwide to mitigate these HAB events. The most popular and effective action is modified clay (MC). By using inorganic and organic substances clay surface is reformed and used to remove the blooms. MC does not damage the water quality and can decrease the eutrophication by absorbing nutrients. Furthermore, MC can quickly reduce algal toxins with a faster degradation rate without affecting other marine organisms [60]. Biological control measures like using lactic acid bacteria and other marine bacteria (e.g. Paracoccus spp.) also effectively reduce HAB toxin [61]. Macroalgae cultivation is another potent way to control HAB events. A study showed macroalgae Saccharina latissima, Chondrus crispus, and Ulva spp. can hinder the growth and toxin production of Alexandrium catenella in aquaculture settings [62].

Besides these control measures, early risk assessment for eutrophication around the fish farming areas may help the related authorities to take precautionary measures. Climate change risk maps also can play a vital role in predicting upcoming HAB events in risk areas [63]. To further minimise economic loss and emotional stress, income diversification and skill development are highly recommended [64].

9. Conclusion

Though seaweed and microalgae farming help in curbing greenhouse gases, these algal communities are now observing threats of extinction and migration. Moreover, toxic microalgae species are becoming a major public health hazard in coastal areas. A proper mitigation plan would help slow down the negative impact of climate change. More research is needed to take necessary and effective mitigation action in terms of the current status of the indigenous seaweed species globally. The actual effect of global warming on the macroalgal communities should be scrutinised first. Moreover, the harmful microalgae bloom events in the coastal regions should be documented and studied profusely, which, in turn, will help the authorities to take actions for preserving the Blue economy.

Acknowledgments

This work is supported by the Higher Institution Centre of Excellence (HICOE) Research Grant (Innovative Vaccines and Therapeutics against Fish Diseases) (Project No. 6369100), and SATREPS (JICA-JST): COSMOS-MOHE G4-B Research Grant (Microalgae for Sustainable Aquaculture Health: Microalgae Vaccine Delivery System) (Project No. 6300866).