There are many resources that need fighting over, and in the sea one of the major ones for photosynthetic organisms is sunlight. Other nutrients such as nitrogen, phospherous and trace metals such as iron and copper can exist in different forms at different levels in the ocean (as shown below), but once you start getting below a certain depth, sunlight quickly becomes a finite and rapidly diminishing resource:
Taken from the reference below.
Different organisms can cope with the lack of sunlight in different ways. Some (especially the larger algae species) have generalised, they contain a whole range of different light capturing pigments which can absorb a range of light wavelengths, including those in the darker depths. But little photosynthesising bacteria like Procholorococcus (which I mentioned in this post) which have the smallest genomes of all photosynthesising organisms, don't have that option. Instead they have to specialise, so that different strains in a species are adapted to different levels of light.
Work done by Rocap (paper reference below) looked at two different strains of Prochlorococcus: MED4 and MIT9313 (which I will just call MED and MIT). The MED strain was found only in the surface waters, while MIT was found much lower down; a phenomenon known as 'vertical niche partitioning'. Despite their genomes differing by only 3%, and despite being technically the same species (although 'species' is an uncertain word in the world of bacteria) they have optimised themselves to completely different levels of not just light but also nutrients, trace metals and virus specificities.
MED (the one near the surface) has a slightly smaller genome than MIT, yet contains twice as many genes dedicated to high-light-inducible proteins, many of which seem to have arisen by gene duplication. It also has genes specialised for the nitrogen sources found near the surface of the water, and organic phosphates (which again are found predominantly on the surface).
MIT on the other hand has fewer genes for ultraviolet damage repair, but more light harvesting genes, for example it possesses two copies of the hight-harvesting chlorophyll binding antenna protein. This helps it to gather as much light as possible, despite being further below the surface. it's also adapted for its specific nitrogen source and increased ability to use orthophosphate, rather than organic phosphates.
Both genomes have lost the ability for photoacclimatisation, that is the ability to change to suit different light conditions. By taking up vertical niche positions, they have forfeited the ability to change their response, meaning that a strict horizontal partition between them must be maintained at all times. Any Prochlorocuccus found at lower levels will be of the MIT variety, while those at higher levels will be MED. It's even thought that there might be further strict niche partitioning; with different ecotypes of MED adapted to use different iron sources, or different temperatures.
For the photosynthetic organisms that inhabit it, seawater is more than just a blue shifting salty mass. It's a whole range of niches and environments, partitioned in three dimensions depending on the surrounding conditions and nutrients.
Rocap G, Larimer FW, Lamerdin J, Malfatti S, Chain P, Ahlgren NA, Arellano A, Coleman M, Hauser L, Hess WR, Johnson ZI, Land M, Lindell D, Post AF, Regala W, Shah M, Shaw SL, Steglich C, Sullivan MB, Ting CS, Tolonen A, Webb EA, Zinser ER, & Chisholm SW (2003). Genome divergence in two Prochlorococcus ecotypes reflects oceanic niche differentiation. Nature, 424 (6952), 1042-7 PMID: 12917642
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