Kelp


Kelps are large brown algae or seaweeds that make up the order Laminariales. There are about 30 genera. Despite its appearance and use of photosynthesis in chloroplasts, kelp is not a plant but a stramenopile.
Kelp grows from stalks close together in very dense areas like forests under shallow temperate and Arctic oceans. They were previously thought to have appeared in the Miocene, 5 to 23 million years ago based on fossils from California. Kelps were present in the northeastern Pacific Ocean by at least 32 million years ago. These organisms require nutrient-rich water with temperatures between. They are known for their fast growth —the genera Macrocystis and Nereocystis can grow as fast as half a metre a day, ultimately reaching.
Through the 19th century, the word "kelp" was closely associated with seaweeds that could be burned to obtain soda ash. The seaweeds used included species from both the orders Laminariales and Fucales. The word "kelp" was also used directly to refer to these processed ashes.

Description

The thallus consists of flat or leaf-like structures known as lamina that originate from elongated stem-like structures, referred to as the stipes. A root-like structure anchors the kelp to the substrate of the ocean. Gas-filled bladders form at the base of blades of American species, such as Nereocystis lueteana, to hold the kelp blades close to the surface to access light for photosynthesis. The stipes are generally covered with a slimy mucilage layer, rather than a waxy cuticle.

Growth and reproduction

Growth occurs at the base of the meristem, where the blades and stipe meet. Growth may be limited by grazing. Sea urchins, for example, can reduce entire ecosystems to kelp-less wastelands known as urchin barrens. The kelp life cycle involves a diploid sporophyte and haploid gametophyte stage. The haploid phase begins when the mature organism releases many spores, which then germinate to become male or female gametophytes. Sexual reproduction then results in the beginning of the diploid sporophyte stage, which will develop into a mature individual.

Taxonomy

Phylogeny

Seaweed were not generally considered homologues of terrestrial plants, but are only very distantly related to plants, and have evolved plant-like structures through convergent evolution. Where plants have leaves, stems, and reproductive organs, kelp have independently evolved blades, stipes, and sporangia. With radiometric dating and the measure Ma "unequivocal minimum constraint for total group Pinaceae" vascular plants have been measured as having evolved around 419–454 Ma while the ancestors of Laminariales are much younger at 189 Ma. Although these groups are distantly related as well as different in evolutionary age, there are still comparisons that can be made between the structures of terrestrial plants and kelp but in terms of evolutionary history, most of these similarities come from convergent evolution.
Some kelp species including giant kelp, have evolved transport mechanisms for organic as well as inorganic compounds, similar to mechanisms of transport in trees and other vascular plants. In kelp this transportation network uses trumpet-shaped sieve elements. A 2015 study aimed to evaluate the efficiency of giant kelp transport anatomy looked at 6 different laminariales species to see if they had typical vascular plant allometric relationships. Researchers expected to find the kelp's phloem to work similarly to a plant's xylem and therefore display similar allometric trends to minimize pressure gradient. The study found no universal allometric scaling between all tested structures of the laminariales species which implies that the transport network of brown algae is only just beginning to evolve to efficiently fit their current niches.
Apart from undergoing convergent evolution with plants, species of kelp have undergone convergent evolution within their own phylogeny that has led to niche conservatism. This niche conservatism means that some species of kelp have convergently evolved to share similar niches, as opposed to all species diverging into distinct niches through adaptive radiation. A 2020 study looked at functional traits of 14 species of kelp and found that many of these traits evolved convergently across kelp phylogeny. With different species of kelp filling slightly different environmental niches, specifically along a wave disturbance gradient, many of these convergently evolved traits for structural reinforcement also correlate with distribution along that gradient. The wave disturbance gradient that this study refers to is the environments that this kelp inhabit have a varied level of perturbation from the tide and waves that pull at the kelp. It can be assumed from these results that niche partitioning along wave disturbance gradients is a key driver of divergence between closely related kelp.
Due to the often varied and turbulent habitat that kelp populate, plasticity of certain structural traits has been a key for the evolutionary history of the phyla. Plasticity helps with a very important aspect of kelp adaptations to ocean environments, and that is the unusually high levels of morphological homoplasy between lineages. This in fact has made classifying brown algae difficult. Kelp often have similar morphological features to other species within its own area since the roughness of the wave disturbance regime, but can look fairly different from other members of its own species that are found in different wave disturbance regimes. Plasticity in kelps most often involves blade morphology such as the width, ruffle, and thickness of blades. Just one example is the giant bull kelp Nereocystis luetkeana, which have evolved to change blade shape in order to increase drag in water and interception of light when exposed to certain environments. Bull kelp are not unique in this adaptation; many kelp species have evolved a genetic plasticity for blade shapes for different water flow habitats. So individuals of the same species will have differences to other individuals of the same species due to what habitat they grow in. Many species have different morphologies for different wave disturbance regimes but giant kelp Macrocystis integrifolia has been found to have plasticity allowing for 4 distinct types of blade morphology depending on habitat. Where many species only have two or three different blade shapes for maximizing efficiency in only two or three habitats. These different blade shapes were found to decrease breakage and increase ability to photosynthesize. Blade adaptations like these are how kelp have evolved for efficiency in structure in a turbulent ocean environment, to the point where their stability can shape entire habitats. Apart from these structural adaptations, the evolution of dispersal methods relating to structure have been important for the success of kelp as well.
Kelp have had to adapt dispersal methods that can make successful use of ocean currents. Buoyancy of certain kelp structures allows for species to disperse with the flow of water. Certain kelp form kelp rafts, which can travel great distances away from the source population and colonize other areas. The bull kelp genus Durvillaea includes six species, some that have adapted buoyancy and others that have not. Those that have adapted buoyancy have done so thanks to the evolution of a gas filled structure called the pneumatocysts which is an adaptation that allows the kelp to float higher towards the surface to photosynthesize and also aids in dispersal by floating kelp rafts. For Macrocystis pyrifera, adaptation of pneumatocysts and raft forming have made the species dispersal method so successful that the immense spread of coast in which the species can be found has been found to actually be very recently colonized. This can be observed by the low genetic diversity in the subantarctic region. Dispersal by rafts from buoyant species also explains some evolutionary history for non-buoyant species of kelp. Since these rafts commonly have hitchhikers of other diverse species, they provide a mechanism for dispersal for species that lack buoyancy. This mechanism has been recently confirmed to be the cause of some dispersal and evolutionary history for kelp species in a study done with genomic analysis. Studies of kelp structure evolution have helped in the understanding of the adaptations that have allowed for kelp to not only be extremely successful as a group of organisms but also successful as an ecosystem engineer of kelp forests, some of the most diverse and dynamic ecosystems on earth.

Prominent species

Species of Laminaria in the British Isles;
  • Laminaria digitata J.V. Lamouroux
  • Laminaria hyperborea Foslie
  • Laminaria ochroleuca Bachelot de la Pylaie
  • Saccharina latissima J.V.Lamouroux
Species of Laminaria worldwide, listing of species at AlgaeBase:
  • Laminaria agardhii
  • Laminaria bongardina Postels et Ruprecht
  • Laminaria cuneifolia
  • Laminaria dentigera Klellm.
  • Laminaria digitata
  • Setchell
  • Setchell
  • Laminaria nigripes
  • Laminaria ontermedia
  • Laminaria pallida Greville ex J. Agardh
  • Laminaria saccharina Lamouroux, synonym of Saccharina latissima
  • Silva
  • Laminaria sinclairii Farlow, Anderson et Eaton
  • Laminaria stenophylla
Other species in the Laminariales that may be considered as kelp:
  • Alaria esculenta
  • Alaria marginata Post. & Rupr.
  • Saunders
  • J. Agardh
  • Ecklonia maxima Papenfuss
  • Ecklonia radiata J. Agardh
  • Eisenia arborea Aresch.
  • Egregia menziesii Aresch.
  • Setch
  • Macrocystis pyrifera
  • Pleurophycus gardneri Setch. & Saund.
  • Pterygophora californica Rupr.
Non-Laminariales species that may be considered as kelp:
  • Durvillea antarctica, Fucales
  • Durvillea willana, Fucales
  • Durvillaea potatorum Areschoug, Fucales