This article is written by Juan Cruz Guerrero. If you wish to write for the EYR blog, contact us. Pictures are from Canva.
For this month’s species highlight we decided to focus on a plant you may have come across, but not paid much attention to. Chances are that you’ve had one of these organisms caress your leg as you swam in the warm Mediterranean Sea. Being a plant and living entirely underwater, it is easy to overlook this keystone of Mediterranean ecosystems as just another patch of seaweed. But (as I have learnt myself in writing this blog), this could not be further from the truth. Carbon sink, oxygen source, ecosystem engineer, economic resource and to top it all off living up to to tens of thousands of years; this first iteration of EYR Species Highlight in blog format is dedicated to the underappreciated but invaluable Posidonia oceanica.
With titles like “the underwater forest” or better yet “the lungs of the Mediterranean”, you’d think that the Neptune grass would have a stronger presence in mainstream conservation/rewilding headlines. This not being the case is a testament to the obscurity around this plant in the eyes of the general public. Which is honestly a shame. Because every aspect of this otherwise mundane looking plant has left me more invested in its conservation and restoration. The following blog will discuss the biology, ecology, threats and conservation of this species in order to try and spark more interest in this irreplaceable life form.
What even is a seagrass? Posidonia biology
Seagrass, seaweed, algae. To a layman these terms are interchangeable. Often green, aquatic, majestically swaying in the current. Most of us only pay attention to them when they entangle our fishing lines or when one touches your leg and we can’t help but fear that jaws is out to get us. But there is a distinction between them, and the taxonomy and life history of Posidonia was the first aspect of this species that sparked my interest and is subsequently the first section of this blog.
Taxonomy
Posidonia is not called Neptune grass because it’s a seaweed that happens to resemble a meadow; it is in fact much closer to true grasses than one would assume. Much like dolphins and sea turtles, sea grasses are descendants of terrestrial species that have returned to their roots (pun intended) on the ocean floor. Posidonia is an Angiosperm (flowering plant), specifically a Monocot [1]. Grasses like the wheat, rice and oats our economy runs on are also monocots; as are pineapples, palm trees, coconuts and bananas. Within this highly diverse group, Neptune grass belongs to the order Alismatales [1], making it a relative of some terrestrial species you may have come across, such as the conspicuous red and green fruits of the “Lords and Ladies”, Arum maculatum bursting out of the detritus, or the “Peace lily” of the genus Spathyiphyllum that often decorates flower shops or (in my case) your grandmothers house. There are 9 species within the genus Posidonia, P.oceanica being found exclusively in the Mediterranean while the remaining 8 on the southern coast of Australia [2].
Essentially Posidonia are to pineapples like what manatees are to aardvarks; they’ve seized the opportunity of a vacant niche and returned to the marine environment all life used to once call home. However this peculiar evolutionary history is not what makes Posidonia our species highlight, rather it’s the way a flowering plant can reproduce and grow underwater that makes Neptune grass such a valuable ecological asset.
Reproduction and growth
So how does a flowering plant reproduce under the sea? As incredible as it would be that fish or crustaceans would convergently evolve to become the marine analogs of bees; sexual reproduction in Posidonia is done through hydrophily, which is the release of male gametes into the current that fertilise the female gametes of a neighboring plant [3]. After fertilisation the flower develops into a fruit aptly termed “sea olives”, which get carried by the current to colonise some barren patch of grass or rock. However this form of reproduction is quite sporadic and only occurs when very specific conditions are met, which is between September – November (depending on depth) once every 5 to 10 years [2].
Posidonia’s main reproductive strategy is what makes it such a peculiar species. It does so by vegetative reproduction through rhizom
e elongation and cuttings. In simpler terms, cloning [2]. An individual Posidonia plant contains between 4 and 8 leaves over a meter long. This individual clones itself through plagiotropic (horizontal) and orthotropic (vertical) rhizomes that extend vertically and horizontally through the sediment. Once the clone is fully established with roots and leaves of its own, it is severed from the parent (or I guess you could say from itself). This clonal extension is extremely slow, growing at a rate of approximately 2cm per year (half the rate at which your fingernails grow). The shoots are spaced in intervals of 5-10cm and can live up to 50 years. Which already sounds like a lot, but when accounting for the fact that these shoots are clones of each other, individuals (in the genetic sense) can be much older.
The oldest organism on earth?
A landmark 2012 study took genetic samples across 40 locations of Posidonia meadows in the Mediterranean, in order to test how many of the individuals in the meadows were clones and how many came from sexual reproduction. The results were unprecedented. Clonal colonies as large as 15km long (off the coast of the Balearic islands) were found [4]. This means that a single genetic individual of Posidonia weighed thousands of tons and spans tens of thousands of meters long. As aforementioned, Posidonia’s main form of reproduction through rhizome elongation is notoriously slow. So for a single individual to have grown to such tremendous proportions, that specific genome must have been conceived a long, long, long time ago.
And after extrapolating growth rates to the estimated extent of the meadow, this individual Posidonia was suggested to be between 80,000 – 200,000 years old ! [4] If such astronomical numbers were to be true, it would comfortably make it the oldest organism on earth, surpassing the ~43,600 year old Tasmanian Lomatia tasmanica. Note that these numbers are not peer reviewed and so should not be sold as fact, but it is undeniable that few species can reach the tremendous age of the Neptune grass.
Now that I’ve explained a little about our highlights biology, we have the foundation to understand simply how important it is for the local (and global) ecology.
A (literally) foundational species: Posidonia ecology
Anyone familiar with rewilding will know of keystone species; organisms that have a disproportionally large impact on their local environment by facilitating essential ecological processes. There is no doubt that they are among the most beneficial organisms to rewild and protect. However there is arguably a position even higher in the ecological hierarchy, one that Posidonia holds.
Ecosystem pioneers
Neptune grass is considered a foundational species, because were it not for them large expanses of the Mediterranean coast would remain barren rock and sand [5].
They physically create, modify and maintain habitats for their own survival, and as a consequence the survival of neighboring species. And this is entirely by nature of their unique reproduction and evolutionary history. Being a (once terrestrial) monocot, characterised by having crown roots that grow perpendicular to the primary root, the vascular system of Neptune grass (that seaweeds do not have) is a network of dense, interlocking fibrous root system; in turn keeping the sand and rocks that it grows on tightly packed [6]. This is then exacerbated by its reproduction through rhizome extension; the orthotropic and plagiotropic growth of these shoots capture sediment moved by the current and prevent erosion [6]. The mattes created by the root and rhizome network builds the literal and figurative foundation of much of the Mediterranean coastal ecosystem.
A home for all
20%. That is the estimated proportion of Mediterranean marine life that depends on Posidonia [7]. A multitude of fish, crustaceans, epiphytes and meiofauna rely (to a greater or lesser extent) on the structural diversity and microhabitats created by Neptune grass. A significant proportion of Mediterranean fish species rely on them since they serve as critical nurseries and feeding grounds for the otherwise vulnerable larvae. This is of particular interest to humans since many of these species are commercially significant, such as sea bass, sea bream and cuttlefish. In short Posidonia plays the same role coral reefs fulfill in the rest of the world’s oceans, to the benefit of all animals (humans included).
Atmospheric hero
But the virtues of Posidonia don’t stop at the fishing industry. Certain environments are extremely valuable carbon sinks. Peatlands, salt marshes and rainforests to name some well known examples. According to a number of studies, Posidonia meadows have been calculated to absorb up to 21x more carbon per m2 than tropical rainforests! [10]. “Blue carbon” refers to carbon captured and stored by coastal environments, characterised by higher sequestration rates and longer storage than terrestrial environments. While covering less than 1% of the ocean surface, blue carbon ecosystems account for ~50% of all carbon sequestered in ocean sediments [11]. Posidonia meadows are particularly effective at locking away CO2 due to their slow growth rates, storing the carbon they captured in their root system, creating the “matte”, which is a slowly built carbon rich sediment comparable to terrestrial peatlands that have been radiocarbon dated to be thousands of years old [12]. Posidonia meadows are such effective carbon sinks that they have been estimated to have absorbed between 11%-42% of CO2 emissions released by Mediterranean countries since the industrial revolution [13], an astonishing number and a testament to their value as blue carbon environments.
The air we breathe
Anyone who has done plant biology in high school will also know that in the process of photosynthesis plants release elemental oxygen in the form of O2. Essentially all of the oxygen we breathe reaches our lungs as a consequence of this chemistry. And Neptune grass is of course no exception. Well actually, they kind of are; since even among seagrasses Posidonia releases an enormous amount of oxygen into the water column. On average seagrasses (of other species) release around 10L of O2 per m2 [14], whereas Neptune grass has been calculated to release 14-20L, 40%-100% more than their relatives, making it one of the most oxygenating of the seagrasses [15].
It is clear to me (and hopefully now to you) that Posidonia is one of the species that are disproportionate in how much they do for the biosphere. The Mediterranean would not be the same were it not for this inconspicuous plant. Unfortunately, as is the case for an increasing number of species, our beloved Neptune grass faces a great deal of threats that are putting it (and the species it’s associated with) in great stress.
Decline and a (gradual) comeback
It seems like an unfortunate coincidence that the species that are most valuable to the biosphere (including humans) are the ones facing the harshest anthropogenic pressures. And this is no different in the case of Posidonia. But as is also often the case in the world of conservation, when the resources are provided and the effort is made; even the slowest of growers can start making a rapid recovery.
What is causing the damage
There are a number of individual drivers of decline for Posidonia, all of which are anthropogenic in nature. Coastal urban development, bottom trawling, aquaculture, pollution and in particular boat anchoring have proven to severely impact meadow stability [16]. Mechanical threats fragment the meadow and expose the matte (releasing carbon), which can take decades to centuries to regrow. Sediment kicked up from disturbance and algal blooms caused by eutrophication asphyxiate the plant from light. To top it all off, the Mediterranean being essentially isolated from the rest of the world’s waters is estimated to be warming 20% faster than other bodies of water [17]; the water’s acidity, temperature and sea level are all deviating from what Posidonia has evolved to survive in. The combined force of these factors has dramatically reduced the extent of Neptune grass meadows by 13%-50% over the last 20 years between regions, with a 34% regression across the entire basin [16]. This is not only catastrophic for the thousands of species that rely on Posidonia for habitat, but it has impacted humans directly by worsening coastal erosion, collapsing commercial fish populations and contributing to higher CO2 emissions.
A helping hand
Across the Mediterranean efforts have already been made to bring back this ecological powerhouse. A number of approaches have resulted in mixed success, but the most effective conservation came from removing pressures without the need for transplanting [18]. The best example of this being the project in Prado Bay near Marseille; where simple wastewater treatment and the absence of coastal development resulted in recolonisation of 8.6% per year, with the meadows returning to 94% and 81% of their original cover over the 40 year study [18]. This could not have occurred was it not for the EU Habitats Directive designating Posidonia meadows as a priority habitat for restoration. More aggressive restoration work has also seen success off of Northern Sardinia, where 7,000 patches of 20 cuttings were planted in a large degraded area resulting in 59% survival of the plants in the first year after planting [19]. However it is important to note that even successful transplantation does not instantly create a natural meadow. While the cover of the meadow may increase, if the pressures that caused the decline in the first place (eutrophication, pollution, water clarity) are not addressed, the plants will still exhibit signs of stress and lower productivity, which can cause die-backs.
The returns
When Posidonia returns, so do the animals that depend on them, ensuring food security and supporting tourism in the coastal community. When accounting for the coastal protection, oxygen production and carbon sequestration that Neptune grass provides by nature of its biology it can become a valuable economic asset. Though this value will vary depending on regional differences, the ecosystem services of one meadow along the Italian coast has been estimated to provide €21.660,5 of wealth for every hectare, per year [20]. A very significant number relative to regional budgets and national GDP.
An invaluable plant
When I embarked to write this blog I had never expected it to end up over 2,500 words long. But when it comes to Posidonia there was just so much more than you would expect. I’ve swam in the Mediterranean, I’ve had this inconspicuous plant ever so slightly caress my feet as I snorkeled for fish and octopus. Little did I know that on my search for interesting organisms I was overlooking the species that, were it not for them, there would be no fish or cephalopods to see. More needs to be done for this flowering plant. It may not be the most aesthetic or charismatic, but they are in great part the very essence of the Mediterranean many of us know and love. A humble and ancient spirit of the sea that breathes life (and oxygen) into its surroundings; for the benefit of worms, crabs, lobsters, epiphytes, fish, cuttlefish, octopi, fishermen or people simply enjoying a beach holiday. As the Mediterranean warms, as communities develop along the coast, these underappreciated meadows will face harsher pressures and will be pushed to their limit. But as I’ve explored in the chunk of text above, if we are kind to the Neptune grass, it will reciprocate.