What are phytoplankton? The marvelous little creatures floating silently near the surface of the sea seem to have a presence quite detached from our own; indeed, the typical person may easily overlook the significance of these tiny critters. Nevertheless, this set of organisms that scientists have decided to classify as “phytoplankton” plays an integral role in the marine ecosystem. They affect marine life in ways that silently but surely impact us, sometimes beneficial but, at other times, not so much. In taking a closer look at phytoplankton, their functions, and their potential applications, we may come to terms with their incredible existence.
A Closer Look
The roots phyto (plant) and plankton (to wander) give a brief perspective into what constitutes a phytoplankton. Rather than a phylogenetic classification, what makes an organism a phytoplankton depends simply on two criteria: location and function. Phytoplankton thus consists of a myriad of different species, all of which are microscopic phototrophs that preside in the euphotic zones of aquatic environments. Under photosynthetic conditions (sunlight and specific nutrients), phytoplankton perform an array of important biosynthetic reactions that result in a net primary production of organic materials and oxygen in the oceans.
Phytoplankton have remarkable photosynthetic capacities. In contrast to terrestrial plants, whose major components of biomass (stem and roots) do not contribute to photosynthesis, nearly all of phytoplankton biomass is photosynthetic (1). Furthermore, these microscopic organisms are responsible for nearly half of the biospheric net primary production, despite taking up only 0.2% of the global primary producer biomass (2). This discrepancy derives from the difference in terrestrial plant versus phytoplankton turnover time of plant organic matter: phytoplankton have an average turnover time of 2 to 6 days, whereas the turnover on land lasts an average of 19 years (1). Turnover time refers to the “time required to replace the standing crop of a population or group of populations, calculated as the ratio of standing crop biomass to production” (3).
While this explains the efficiency of phytoplankton primary production, it still leaves a question unanswered: given the vastness of the oceans, why then do phytoplankton not proliferate to consume a larger portion of the global plant biomass?
High Nutrient, Low Chlorophyll: The Iron Hypothesis
For a long time, the absence of fixed nitrogen was thought to be the factor limiting plant growth in the ocean. The consumption of inorganic nitrogen and phosphorus along with carbon is essential for photosynthesis (4). However, many regions of the ocean, such as the North Pacific, Equatorial Pacific, and the Antarctic have an abundance of nutrients (phosphates, nitrates, etc), with concentrations of certain nutrients reaching over 1 μM. Nevertheless, these regions have surprisingly low phytoplankton biomass, with chlorophyll density measuring less than 1 μg/L (5). Because of this seeming paradox, these regions were termed High Nutrient, Low Chlorophyll (HNLC).
In 1943, Hart proposed a hypothesis that attempted to explain this puzzling HNLC phenomenon. Noting that the “observed richness of the neritic [coastal] plankton” coincided with the fact that the “the land [was] regarded as a source of iron,” Hart suggested the absence of iron as the factor limiting phytoplankton production (6). Subsequent studies were conducted to validate Hart’s claims. Hart’s hypothesis, however, proved faulty; the study of iron levels in various parts of the oceans led to the general conclusion that plenty of iron existed for phytoplankton growth and iron could therefore not be limiting factor. Enrichment experiments that compared growth rates with and without iron did not observe any significant difference, either (6).
These studies, however, were overturned in the 1970’s when Dr. Clair Patterson realized that the samples used in studies had iron contaminants that skewed the data; iron in the hulls of the ships and from the measuring equipment itself tainted the samples and invalidated the iron level measurements (7). In recognition of this flaw, the use of “cleaner” techniques had to be adopted to re-evaluate iron levels in oceans, and when good iron data finally came round, the findings aligned consistently with Hart’s speculation, reaffirming that the role of iron in constraining phytoplankton proliferation. Hart, at last, received vindication.
In 1990, John Martin elaborated upon Hart’s 47-year-old speculation and established what we presently acknowledge as the Iron Hypothesis. The Iron Hypothesis realized the climate-influencing potential of phytoplankton activity based on the analysis of glacial cycles. Because of the many dune fields determined to be active during the last glacial period, glacial periods must have experienced much greater wind speeds and dust loads (8,9). This indicated that a much greater load of dust reached the Antarctic then than today, meaning that significantly more iron deposited in the Antarctic during glacial periods (10). When Martin et al. studied these data in relation to glacial-interglacial carbon dioxide data, they found an inverse relationship between dust input (iron input) in the Antarctic and carbon dioxide levels. They reasoned that this relationship explained why carbon dioxide levels were higher during interglacial than glacial periods: the absence of dust input during interglacial periods led to a drop in iron levels that severely hindered phytoplankton growth, thus reducing the capture of carbon dioxide from the atmosphere and raising the CO2 level. Conversely, when more iron entered the oceans, phytoplankton bloomed under conditions of high nutrient and iron concentrations, thus “increasing the power and efficiency of the biological pump, thus contributing to the drawing down of atmospheric CO2 during glacial maxima” (11).
Changing the Climate: The Biological Pump
The biological pump governs “the processes of fixation of inorganic carbon in organic matter during photosynthesis, its transformation by food web processes (trophodynamics), physical mixing, transport and gravitational settling” (12). In other words, the biological pump plays the incredible role of sequestering carbon dioxide from the atmosphere into the surface, followed by the deposition of carbon (in the form of dissolved organic carbons) toward greater ocean depths. Conceivably, the pump happens through two general steps. First, phototrophs in euphoric zones, such as phytoplankton, conduct photosynthesis that fixes atmospheric CO2 into organic carbons, causing a net flow of carbon from the atmosphere into surface waters. Then, several pathways take over that ultimately leads to the net sinking of carbon. Phototrophs secrete carbon in the form of dissolved organic matter that aggregate with other carbon detritus into sinking particulate matter (12). Or they may be eaten by zooplanktons which subsequently either get eaten by organisms located at ocean depths or secrete fecal pellets that traverse further downward. The latter seems to be the major contributor toward the sinking flux of organic carbon; approximately 20-30% of organic carbon ingested by creatures such as zooplanktons becomes secreted as feces (13). Furthermore, these fecal pellets are encapsulated in durable, fast-sinking packets, thus making for very rapid delivery of carbon to greater ocean depths (13). One should note that the mechanism of the biological pump holds true not just for carbon, but more many other essential elements as well, such as nitrogen (14).
Looking back at the mechanisms at play, it seems reasonable to suggest that the introduction of new photosynthetic activity would increase the activity of the biological pump and cause a greater export of organic carbon in a predictable fashion. Richard Eppley and Bruce Peterson were the first to test this. They defined the ratio of new production to total production as the “f-ratio” and successfully determined “f” as a function of total production rate (13). Using the Eppley-Peterson algorithm, one could calculate carbon export rates based on primary production rates.
This relation between photosynthetic rate and biological pump activity holds special importance because of its implications. By somehow increasing phytoplankton primary production, one can turn on the biological pump to more quickly take carbon dioxide out of the atmosphere. It was, in fact, this revelation that raised Martin’s iron hypothesis to prominence: realizing that low iron levels had limited phytoplankton growth, Martin noted that the fertilization of oceans with iron would free phytoplankton growth from restraint, garner greater primary production, and ultimately increase carbon sequestration (7).
The use of iron fertilization for carbon sequestration has been lauded by some as a potential solution for mitigating global warming. The capability of harnessing the power of phytoplankton to take on this challenge seems enticing and some are already trying to put it into action (15). Nevertheless, certain environmental concerns regarding the efficacy and feasibility of iron fertilization as been called into question. Whether iron fertilization will become a commonly used application in the future remains to be seen, and larger studies are presently at work to uncover the risks and benefits of iron fertilization.
Aside from their importance in climate changes, phytoplankton also perform the crucial role of supporting the marine ecosystem. Serving as the basis of the aquatic food web, phytoplankton have extensive influence over all aspects of marine life (16). As a primary producer, phytoplankton is the immediate prey of other small organisms, ranging from zooplanktons to small fish and squid. These in turn support the sustenance of not only other fish, but also birds and mammals, and even the gigantic baleen whales. The great biodiversity and sheer size of an entire ecosystem both seem, incredulously, to lie heavily on the shoulders of phytoplankton, revealing to us just how incredible these creatures are.
But just as easily as they can support life, phytoplankton “can also be the harbingers of death or disease” (16). Under favorable conditions, phytoplankton enter a period of exponential growth that sustains itself until nutrients run out. During this phase of growth, the phytoplankton can quickly produce large masses called “HAB,” or “Harmful Algae Blooms” (16). Of the thousands of phytoplankton species that exist, a handful (<100) of them are known to produce toxins that harm marine life. During HAB events, these harmful phytoplankton species produce dangerous biotoxins that destroy marine life and harm any animal that ingests the contaminated seafood (16, 17). These two faces of phytoplankton—one as a beneficiary to marine life and the other as its destructor—display the dominant role of phytoplankton in the marine ecosystem.
Given the powerful influences that phytoplankton wield, it would be worrisome if these creatures were to gradually decline in population. Indeed, a study led by Boyce et al. raised such a concern in a 2010 paper published in Nature. Since 1979, marine biologists have increasingly used satellites to measure phytoplankton concentration. However, with only three decades worth of data, satellite-derived measurements cannot reliably predict long-term trends in phytoplankton growth. To overcome this shortcoming and analyze phytoplankton growth over the past century, Boyce et al. conducted meta-analysis that compiled modern-day satellite-derived data with measurements of oceanic transparency and in situ chlorophyll observations—measurements that were conducted well before the satellite-era. Estimating the decline rate at 1% of the global median per year, Boyce et al. noted, “The long-term global declines observed here are […] unequivocal” (18). The paper, however, drew considerable criticism; pointing to potential oversights in their mixing of data sets, some scientists questioned the validity of the study’s conclusion. Whether phytoplankton truly exhibit trends of decline is the subject of further studies (19-21). Until new research provide an explanation, the fate of phytoplankton remains a mystery.
Phytoplankton in Our Eyes
Despite being such small organisms, phytoplankton possess an astonishing influence over the world that we live in. They generate a significant portion of the global primary production, fueling life as we know it with the oxygen and organic matter they produce. They provide the basis of most marine life, supporting a complex marine ecology that would otherwise not exist. And they drive the immense biological pumps that tie oceanic activities intimately to climate changes. Although their activities often fall short of our notice, a closer inspection shows the phytoplankton community to carry far more importance than we ever imagined. Expediency thus compels us to pay greater observance to the phytoplankton community, and to recognize them by a worthier title: the most magnificent wanderers of the sea.
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