Plant Records: When Land Doesn’t Mean Land…

Name: Common eelgrass
Scientific name: Zostera marina *
Known for: Stabilizing the seabed, providing food and habitat for other marine organisms, maintaining water quality, supporting local economies…
Record broken: occupant of the most extreme habitat for a land plant

Contenders for this record include plants in California’s Death Valley – which is extremely arid and returns one of the highest land surface temperatures on Earth at c. 60 ^oC. Or, those that inhabit the highest mountains with their thin atmospheres, even thinner nutrient-poor soils, and excess of DNA-damaging ultra violet irradiation. But, my nominee is found in a habitat even more extreme than either of those. It lives on the ocean floor, and is one of the angiosperms collectively known as seagrasses.

Now, the temptation of the reader to cry ‘foul!’ and protest that the seabed is not land and so the entry is inadmissible is understandable, but so wrong. Certainly, the Plant Kingdom – ‘the land plants’ – is arguably all about the struggle in which aquatic photosynthesisers overcame problems inherent in adapting to a terrestrial existence, such as developing plant parts specialised for: anchorage in, and abstraction of water from, the soil (the roots); converting energy from a  physical form to a chemical one via photosynthesis (leaves); supporting the leaves so they’re closer to the life-giving sun (stems); connecting the specialised parts with internal, low-resistance, long-distance transport pathways (xylem and phloem); providing the aerial parts with a desiccation-resistant cuticle, and then creating controllable-orificed stomata which not only allow for appropriate gas exchange for photosynthesis and respiration but also prevent excess uncontrolled water loss; creating multi-varied organs for reproduction (flowers); and dispersal of the next generation (fruits and seeds). That catalogue of evolutionary achievements is most impressive, and it’s no surprise therefore that a land-dwelling plant is expected as the holder of this record. But, there’s more to land plants than even that enviable track record of adaptation and innovation suggests.

So, imagine a plant that, having completed this tremendous evolutionary journey that gave rise to the flowering plants, decided that it had had enough of the life terrestrial and wanted an even greater challenge. A challenge that includes: being entirely surrounded by water (a medium which presents a major limitation to gas exchange between the plant and the external environment in that life-giving, and -sustaining gases such as O₂ and CO₂ diffuse at one-ten thousandth of the speed in water compared to the atmosphere – William Armstrong, 1979); having roots in a permanently waterlogged ‘soil’ (with all the attendant problems of that undesirable situation, such as extremely low oxygen status); and with the not insignificant problem of abstracting water from a bathing medium of saline seawater (with its water potential that is so much lower than the plant’s). What a worthy winner that would be.

Well, you don’t have to imagine such plants, they exist. They are the seagrasses, and they get my vote for “occupant of the most extreme habitat for a land plant”.

The successful reconquest of a fully aquatic – and marine! – existence is a remarkable evolutionary turn-about requiring many alterations to anatomy and physiology [e.g. Kuo and den Hartog, 2006] – not least of which is loss of stomata (which structures are redundant in a plant that has no concerns about water loss and which can absorb gases directly from the seawater into its photosynthetic epidermis to which tissue chloroplasts have been relocated from the more usual, but deeper-seated, mesophyll layers of the leaves of ‘proper’ land-dwelling plants).

If you’re impressed by the conquest of dry land by the plant kingdom (and who wouldn’t be?!), how much more impressed should you be by flowering plants that have left their ancestral, terrestrial home – the crucible of plant life – and adapted to a fully submarine existence.

As a reminder of their land origin, seagrasses are often so abundant that they form large stands referred to as meadows. And, as befits comparison with grass-dominated meadows of the terrestrial world, these too are grazed by cows – sea cows [or dugongs].

Image credits: Zostera (and Mr Watts hands) at Studland [P Cuttings’ archive]

Thalassia hemprichii [Hemprich F.G. & Ehrenberbg C.G./Wikimedia]

Dugong [Julien Willem/Wikimedia]

* Any of the >70 species of seagrasses, e.g. Thalassia testudinum [Brigitta van Tussenbroek et al., 2016], or Posidonia oceanica, could have been chosen as this category’s award-winner. Zostera marina (or common eelgrass as it’s known in the UK) has been selected as the emblematic representative of this group because it was the first marine angiosperm to have had its genome fully sequenced  (Jeanine Olsen et al., 2016) – during the year covered by this blog’s record-breaking plant collection.