Before the Easter holidays, I had spent a week on the island of Pag, in Croatia, freediving, taking long walks around the area, picking up herbs and observing what surrounded me. One day while walking along the coastal rocky shore, I noticed many parts with differently distributed algae, which appeared just like horizontal belts. Some of these belts were green, some yellowish and then they got darker as they sank towards the depths. What I saw were basically different zones of algal species and this is essentially called ZONATION of macroalgal communities.
This occurrence is defined by disturbance and stress factors. The former is represented by winds and wave motion, for example; while the latter is associated to light exposure/disponibility and immersion regimes. If disturbance and stress decrease, competition for space, and consequently light availability, takes place.
Some algae grow better at higher light intensities, therefore they were named photophilus (“like the light”) and are generally found in the euphotic zone. Sciaphilus algae, in contrast, can grow in the sub-euphotic depths, down to -200 meters, where the light intensity is less than 0,01% of that on the surface. In 1833, Engelmann thought that it was possible to distribute algal groups in different layers and that these would remain only there. So his idea was that green algae would stay only near the surface, brown algae at intermediate levels and red algae in the depths. This idea of course was wrong: red and brown algae live at all levels because they are able to produce compounds to protect themselves from too much UV exposure; they accumulate pigments and accessory pigments, or even change genetically their structure in order to get the maximum amount of light. INGENIOUS, right?
Eurythermal algae tolerate wider ranges of temperature, stenothermal algae live restricted to small changes, but the stages most vulnerable to temperature variations are the early life stages. Temperature is actually the principal factor that determines the algal biogeographical distribution.
Algae did not only find a way to live in the best way possible for what concerns light, but they have also adapted to increased salinity, which would otherwise cause plasmolysis. Some are able to absorb ions, lose water and synthetase osmotically active compounds, such as mannitol and succharose, which we so happily consume through foods.
Tides determine the emersion/immersion of intertidal algae species, and currents are necessary for the dispersion of the species. The wave motion removes sessile animals (mussels, oysters, etc.) that compete with macroalgal communities for space, continuously mixes the surrounding water and maintains a high level of available nutrients, as well as reduces the shading by moving the algal thalli (bodies). Although wave motion benefits the growth, when it is too strong, it does not allow the attachment of spores and zygotes to the substrate or damages and removes the thalli.
The factors above, plus the age, affect the uptake of nutrients which are important for the optimal growth rate and adaptation to extreme environments. Carbon, nitrogen, sulfur and phosphorus are essential, along with iron, cobalt and manganese. These elements either serve during photosynthesis, are key components of various photosynthetic compartments (e.g. manganese in the D1 domain of the photosystem Ⅱ), work as reserve pools, or contribute to the synthesis of amino acids.
You would think that the list of stressors and disturbance factors is over, but no… Grazers are also present, and algae have found great strategies to avoid them: they have changed their morphology either by producing spines and other curious structures or calcifying themselves, and some even produce secondary metabolites like terpenoids, alkaloids and polyunsaturated fatty acids (PUFAs). Come on now, you have to admit that algae are ahead, right?
Moving on to the juicy part of this blogpost, there are many ways to define the vertical zones, such as artificial, biological or tidal. For a better understanding, I will focus on the last model of zonation that divides a rocky shore according to tidal excursion, and each of these zones is characterized by a typical biocenose (situation in which organisms live together in mutual dependence).
Littoral zone is the highest one above the sea level and it is independently affected by the sea. The salty air penetrates halophytes, plants that tolerate high concentrations of salt, via capillarity.
Supralittoral zone or the “splash zone” extends between the highest level where salt water drops collide with the substratum and the level of the high tide. Being the only zone of the marine phytal system constantly emerged, it is colonised by marine populations that are extremely tolerant to prolonged emersion periods. Usually cyanobacteria and lichens are found among this zone, colouring the substrate in black in case of cyanobacterial populations.
Intertidal zone is, as the name in itself describes, the zone between the high tide and the low tide. It is different in each region around the world because it depends on the amplitude of tidal excursions. For instance, the intertidal zone near the UK where the tide extends to almost 15 meters is definitely different than that of the Mediterranean Sea where the tidal excursion reaches 0,9 meters. Where the tides are quite narrow and the substrate rocky (parts of the Med), this zone is divided into horizons. The superior one gets wet by the waves and is typically colonised by cyanobacteria and some pluricellular algae. The inferior, on the other hand, is almost always immersed and characterized by denser and continuous algal communities. Interestingly, there are structures called “trottoirs,” up to 1 meter thick, constructed by Lithophyllum byssoides and other calcareous algae.
Infralittoral zone extends from the lowest level of the low tide to the lowest depth where photophilic plants can live, generally corresponding to 1% of light intensity of that on the water surface. The depth, therefore, depends on water transparency. It is the most diversified zone not only for the flora but fauna, too. Biotopes and biocenosis here are a real “hotspot” for studying the ecological relations. In the Med, the rocky bottoms are mostly colonised by macroalgae, such as the extremely important engineer and photophilic Cystoseira sp. and the sciaphilus species Peyssonnelia squamaria; whereas the soft bottoms are populated mainly by seagrasses like Posidonia oceanica and Cymodocea nodosa, with endemic and invasive Caulerpa sp.. In the temperate-cold regions, rocky bottoms are characterized by the giant kelps of the order of Laminariales that represent a very important habitat for the associated fauna; while Zostera marina and Amphibolis antarctica are found among soft bottoms.
Circalittoral zone is the last zone of our interest, where sciaphilus algal communities are found. It starts where the infralittoral zone ends and reaches the depth at which photosynthesis cannot occur any longer. Again, it depends on the transparency of the water – in some regions it can be right under the surface, in crystal clear waters, however, can end at 120 – 150 meters depth. The temperature range is very restricted, the bottom is dominated by faunal communities and the typical algal species generally belong to the phylum Rhodophyta (“rhodon” meaning rose, “phyton” meaning plant from Ancient Greek) that have incrusted thalli. Natural constructions found in this last zone are the coralligenous, banks and rims.
From what you have read, you probably must think I am nuts – to like organisms with such superpowers, yet highly discriminated by our preferences for moving organisms, for the “pretty” and colourful ones. My aim was to surprise you, to stress out why the air you are breathing is so important. Without all these adaptations, life on Earth would not be possible, or it would be for species different than mankind.